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Corrosion Basic

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                                     Inspector Knowledge Series 03-0

                                      An Introduction to Corrosion




                                      材料基础-腐蚀 图文简易教材


                                      Descriptive approach- Corrosion Basic
       Mok Chek Min 莫泽民




           This Ebook are meant to be read with internet connection hook-on.
Online interactive material, videos and animations will assist you in the understanding of
                 corrosion basic. Video contents are highlighted by icons




                       此册为多媒体互动书本-请链接互联网阅读
                        (在线阅读,视频播放,外部链接,书本下载)




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REVISION HISTORY




   01         01.10.2008         For Approval            Charlie C. CM Mok
  Rev        Date (dd.mm.yyyy)        Reason for issue   Prep      Check     Appr




CHANGE DESCRIPTION

Revision              Change description

        01           For Approval




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Content:
Chapter 1: Corrosion Fundamentals

            1.1 Why Metals Corrode
            1.2 Electrochemistry Fundamentals
                1.2.1 The Nature of Matter
                1.2.2 Electrochemical Cells
            1.3 Basic Corrosion Theory
                1.3.1 Standard EMF / Galvanic Series
                1.3.2 Why Corrosion Cells Form
                     1.3.2.1 Metallurgical factors.
                     1.3.2.2 Environmental factors
                             O2.
                             CO2.
                             H2S.
                             Microbial Influenced MIC.


Chapter 2: Forms of Corrosion

            Uniform Corrosion
            Galvanic Corrosion
            Concentration Cell Corrosion
            Pitting Corrosion
            Crevice Corrosion
            Filiform Corrosion
            Intergranular Corrosion.
            Leaching, Selective attack.
            Stress Corrosion Cracking
            Corrosion Fatigue
            Fretting Corrosion
            Erosion Corrosion
            De-alloying
            Hydrogen Damage
                Environmental assist HIC
                Blistering
                HTHA and Welds related hydrogen corrosion
            Corrosion in Concrete
            Microbial Corrosion
            Cavitation.
            Liquid Metal Embrittlement.
            Exfoliation Corrosion

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 Chapter 3 Corrosion Control
               Design
               Materials Selection
               Protective Coatings
               Inhibitors and Other Means of Environmental Alteration
               Corrosion Allowances
               Cathodic Protection / Anodic Protection
 Chapter 4: Sources of Additional Information
 Chapter 5: Online Books
 Appendix:
               Pourbaix Diagram.
               Hydrogen Damages
               Degrading Mechanisms of the Oil & Gas Industries
               Corrosion Testing Standards
               Online Courses




Recommended corrosion forum:




Recommended download:




http://university.arabsbook.com/forum25/thread37770.html




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Chapter 1:
Corrosion Fundamentals                                       Corr.Overview




                                                      Corrosion can be defined as the degradation of a
                                                      material due to a reaction with its environment.

                                                      Degradation implies deterioration of physical properties of
                                                      the material. This can be a weakening of the material due
                                                      to a loss of cross-sectional area, it can be the shattering
                                                      of a metal due to hydrogen embitterment, or it can be the
                                                      cracking of a polymer due to sunlight exposure.

                                                   Materials can be metals, polymers (plastics, rubbers,
                                                   etc.), ceramics (concrete, brick, etc.) or composites-
                                                   mechanical mixtures of two or more materials with
                                                   different properties. Because metals are the most used
                                                   type of structural materials most of this book will be
devoted to the corrosion of metals. Most corrosion of metals is electrochemical in nature. Corrosion can be
broadly classified into wet aqueous and dry high temperature corrosion.This study material deals only on wet
corrosion.



1.1 Why Metals Corrode

                 Metals corrode because we use them in environments where they are chemically unstable.

                 All metals exhibit a tendency to be oxidized, some more easily than others. The driving force
                 that causes metals to corrode is a natural consequence of their temporary existence in
                 metallic form. To reach this metallic state from their occurrence in nature in the form of various
                 chemical compounds (ores), it is necessary for them to absorb energy by smelting, refining
                 processes. These stored up energy later return by corrosion, the energy required to release
                 the metals from their original compounds.

Only copper and the precious metals (gold, silver, platinum, etc.) are found in nature in their metallic state. All
other metals, to include iron-the metal most commonly used-are processed from minerals or ores into metals
which are inherently unstable in their environments.

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This golden statue in Bangkok, Thailand, is made of the only metal which is thermodynamically stable in room
temperature air. All other metals are unstable and have a tendency to revert to their more stable mineral forms.
Some metals form protective ceramic films (passive films) on their surfaces and these prevent, or slow down,
their corrosion process. The woman in the picture above is wearing anodized titanium earrings. The thickness
of the titanium oxide on the metal surface refracts the light and causes the rainbow colors on her earrings. Her
husband is wearing stainless steel eyeglasses. The passive film that formed on his eyeglasses is only about a
dozen atoms thick, but this passive film is so protective that his eyeglasses are protected from corrosion. We
can prevent corrosion by using metals that form naturally protective passive films, but these alloys are usually
expensive, so we have developed other means of corrosion control.




                             →                           →




Energy was added in during the processing of iron ores into iron, on rusting energy was released. See the similarity of the
color initial and final corroded product.




                                                                             Statue of liberty rusting nose




It may be also matters of life and death.



Before we go further, a basic understanding of chemistry is necessary. Following are very interesting links to
learn chemistry:
http://preparatorychemistry.com/Bishop_animations.htm
You may then study further with this links;
http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html
http://www.chem.ox.ac.uk/vrchemistry/foundation.html
If you get excited with chemistry you may even get deeper;
http://www.shodor.org/unchem/basic/nomen/index.html


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Uncontrolled corrosion may lead to disastrous consequences.

1.2 Electrochemistry Fundamentals
The following brief introduction to chemistry and electrochemistry is intended to give the user of this book a
basic understanding of corrosion.




   Pourbaix Dig.              /    BASIC PRINCIPLES OF CORROSION

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1.2.1 The Nature of Matters

Atoms:
All matter is made of atoms composed of protons, neutrons, and electrons. The center, or nucleus, of the atom
is composed of positively charge protons and neutral neutrons. The outside of the atom has negatively
charged electrons in various orbits. This is shown schematically in the picture to the right where the electrons
are shown orbiting the center, or nucleus, of the atom in much the same way that the planets orbit the sun in
our solar system.

All atoms have the same number of protons (positively charged) and electrons (negatively charged). Therefore
all atoms have a neutral charge (the positive and negative charges cancel each other). Most atoms have
                                                  approximately the same number of neutrons as they do
                                                  protons or electrons, although this is not necessary, and
                                                  the number of neutrons does not affect the identity of the
                                                  element.

                                                    The number of protons (atomic number) in an atom
                                                    determines which kind of atom we have, and the atomic
                                                    mass (weight) of the atom is determined by the number of
                                                    protons and neutrons in the nucleus (the electrons are so
                                                    small as to be almost weightless).

                                                    There are over 100 different elements that have been
                                                    discovered. These are shown in the Periodic Table of the
                                                    Elements below. The letter symbols for the elements come
                                                    from their Latin names, so for example, H stands for
                                                    hydrogen, C for Carbon, O for oxygen, while Fe stands for
                                                    iron and Cu stands for copper.




Atomic number Z = Numbers of protons in the nucleus.

Mass number A = Numbers of protons and neutron in the nucleus.


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Table: Subatomic particles important in chemistry.


particle symbol charge mass, kg               mass, daltons
electron e-         -1       9.10953×10-31 0.000548
proton    p+        +1       1.67265×10-27 1.007276
neutron n           0        1.67495×10-27 1.008665


Ions: Charged atoms or molecules are call ions.

Ions are formed when atoms, or groups of atoms, lose or gain electrons and become charged. Metals lose
                                                                +2   +3   +2
some of their electrons to form positively charged ions, e.g. Fe , Al , Cu , etc. Nonmetals gain electrons and
                                      -   -2 -2
form negatively charged ions, e.g. Cl , O , S etc.

An ion is an atom or molecule which has lost or gained one or more valence electrons, giving it a positive or
negative electrical charge. A negatively charged ion, which has more electrons in its electron shells than it has
protons in its nuclei, is known as an anion. Conversely, a positively-charged ion, which has fewer electrons
than protons, is known as a cation.

Anion – Negative charged ion, it is attracted to the Positive Anode (+ve).

Cation – Positive charged ion, it is attracted to the Negative Cathode (-ve).

An ion consisting of a single atom is called a monatomic ion, but if it consists of two or more atoms, it is a
polyatomic ion. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called oxyanions.

Ions are denoted in the same way as electrically neutral atoms and molecules except for the presence of a
superscript indicating the sign of the net electric charge and the number of electrons lost or gained, if more
                           +          2−
than one. For example: H and SO4 .




More reading:
   http://csep10.phys.utk.edu/astr162/lect/light/bohr.html
   http://chemmovies.unl.edu/ChemAnime/atomic_orbits.htm
   http://www.chemguide.co.uk/atoms/properties/atomorbs.html


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Atomic Orbitals




Models of the Atom




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Formation of polyatomic and molecular ions

                                                                           Polyatomic and molecular ions are
                                                                           often formed by the combination of
                                                                           elemental ions such as H+ with
                                                                           neutral molecules or by the gain of
                                                                           such elemental ions from neutral
                                                                           molecules. A simple example of this
                                                                           is the ammonium ion NH4+ which
                                                                           can be formed by ammonia NH3
                                                                           accepting a proton, H+. Ammonia
                                                                           and ammonium have the same
                                                                           number of electrons in essentially
                                                                           the same electronic configuration
                                                                           but differ in protons. The charge has
                                                                           been added by the addition of a
                                                                           proton (H+) not the addition or
                                                                           removal of electrons. The distinction
                                                                           between this and the removal of an
                                                                           electron from the whole molecule is
important in large systems because it usually results in much more stable ions with complete electron shells.
For example NH3·+ is not stable because of an incomplete valence shell around nitrogen and is in fact a radical
ion.

(NH3 was oxidized to NH4+ and HCl was reduced to Cl-)

                             The ammonia NH3 molecule has a trigonal pyramidal shape, as predicted by
                             VSEPR theory. The nitrogen atom in the molecule has a lone electron pair, and
                             ammonia acts as a base, a proton acceptor. This shape gives the molecule a
                             dipole moment and makes it polar so that ammonia readily dissolves in water.




Ionization potential
The ionization potential, ionization energy or EI of an atom or molecule is the energy required to remove an
electron from the isolated atom or ion. More generally, the nth ionization energy is the energy required to strip
it of the nth electron after the first n − 1 electrons have been removed. It is considered a measure of the
"reluctance" of an atom or ion to surrender an electron, or the "strength" by which the electron is bound; the
greater the ionization energy, the more difficult it is to remove an electron. The ionization potential is an
indicator of the reactivity of an element. Elements with low ionization energy tend to be reducing agents and to
form salts.
Ions
    •     Anions are negatively charged ions, formed when an atom gains electrons in a reaction. Anions are
          negatively charged because there are more electrons associated with them than there are protons in
          their nuclei.

    •     Cations are positively charged ions, formed when an atom loses electrons in a reaction, forming an
          'electron hole'. Cations are the opposite of anions, since cations have fewer electrons than protons.

    •     Radical ions: radical ions are ions that contain an odd number of electrons and are mostly very
          reactive and unstable.



In chemistry, radicals (often referred to as free radicals) are atoms, molecules or ions with unpaired electrons
on an otherwise open shell configuration. These unpaired electrons are usually highly reactive.


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In written chemical equations, free radicals are frequently denoted by a dot placed immediately to the right of
the atomic symbol or molecular formula as follows:


Chlorine gas can be broken down by ultraviolet light to form atomic chlorine radicals.


Molecules:
Compounds are groups of metals and nonmetals that form distinct chemicals. Most of us are familiar with the
formula H2O, which indicates that each water molecule is made of two hydrogen atoms and one oxygen atom.
Many molecules are formed by sharing electrons between adjacent atoms. A water molecule has adjacent
hydrogen and oxygen atoms sharing some of their electrons.




Note: The color distribution indicates dipole property of water molecule.

Acids and bases:
Water is the most common chemical on the face of the earth. It is made of three different constituents,
hydrogen ions, hydroxide ions, and covalently bonded (shared electron) water molecules. Most of water is
                                                                    +           -
composed of water molecules, but it also has low concentrations of H ions and OH ions.
                                             +                -                                +
Neutral water has an equal number of H ions and OH ions. When water has an excess of H ions, we call the
                                              -
resultant liquid an acid. If water has more OH ions, then we call it a base.

We measure the strength of an acid or a base on the pH scale. pH is defined by the following equation:
             +
pH = -log [H ]

It is sufficient to note that some metals (e.g. zinc and aluminum) will corrode at faster rates in acids or bases
than in neutral environments. Other metals, e.g. steel, will corrode at relatively high rates in acids but have
lower corrosion rates in most neutral and basic environments.
                                                                   th
Even a strong acid, with a pH of 0, will be less than 1/1000 by weight hydrogen ions. Neutral water, at a pH of
                        +
7, is less than 1 part H in 10 million parts covalently bonded water molecules.

pH is the negative logarithm of the effective hydrogen ion concentration in moles per liter of solution (more
                                                      +                  +
exactly the activity), or algebraically pH = −log10 [H ] or pH= log101/[H ].


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Exercise:
  +
[H ] of 0.00000001, pH= -log [0.00000001], pH=8
  +
[H ] of 0.001, pH= -log [0.001], pH=3
  +
[H ] of 0.1, pH= -log [0.1], pH=?

Mnemonic device: Acids have low numbers (less than 7), bases have high numbers (greater than 7). Neutral
waters have pH near 7 and tend to be relatively non-corrosive to many materials.
                                    +
pH 1 has 10 times more active H pH 2



Galvanic cell




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1.2.2 The Electrochemical Cell

                                                           The following brief introduction to chemistry and
                                                           electrochemistry is intended to give the user of this
                                                           book a basic understanding of corrosion.

                                                           Oxidation and Reduction:
                                                           Metals are elements that tend to lose electrons
                                                           when they are involved in chemical reactions, and
                                                           nonmetals are those elements that tend to gain
                                                           electrons.

                                                           Sometimes these elements form ions, charged
                                                           elements or groups of elements. Metallic ions,
                                                           because they are formed from atoms that have lost
                                                           electrons, are positively charged (the nucleus is
                                                           unchanged). When an atom or ion loses electrons it
                                                           is said to have been oxidized.

                                                        A common oxidation reaction in corrosion is the
                                                        oxidation of neutral iron atoms to positively charged
                                                        iron ions:
                                                                  +2          -
                                                        Fe » Fe        + 2e
                                                        The electrons lost from a metal must go somewhere,
                                                        and they usually end up on a nonmetallic atom forming
                                                        a negatively charged nonmetallic ion. Because the
                                                        charge of these ions has become smaller (more
                                                        negative charges) the ion or atom which has gained
                                                        the electron(s) is said to have been reduced.
                                                           +             -
                                                        4H +O2 + 4e » 2H2O or
                                                                   -
                                                        2H+ +2e » H2
                                                        While other reduction reactions are possible, the
                                                        reduction of oxygen is involved in well over 90% of all
                                                        corrosion reactions. Thus the amount of oxygen
                                                        present in an environment, and its ability to absorb
                                                        electrons, is an important factor in determining the
                                                        amount of oxidation, or corrosion, of metal that occurs.

Electrochemical Reactions:
The two metal strips shown below are exposed to the same acid.

Both metals undergo similar oxidation reactions:
            +2          -
Cu → Cu          + 2e

            +2              -
Zn → Zn          + 2e
The electrons freed by the oxidation reactions are consumed by reduction reactions.
On the copper the reduction reaction is:
  +               -
4H +O2 +4e → 2H2O
The corrosion rate of the copper is limited by the amount of dissolved oxygen in acid.
On the zinc the reduction reaction is:
  +     -
2H +2e → H2

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                                                                                 The hydrogen ions are
                                                                                 converted to hydrogen gas
                                                                                 molecules and can actually
                                                                                 be seen bubbling off from
                                                                                 the acid.
                                                                                 If we now connect the two
                                                                                 metal samples with a wire
                                                                                 and measure the electricity
                                                                                 through the connecting wire,
                                                                                 we find that one of the
                                                                                 electrodes becomes different
                                                                                 in potential than the other
                                                                                 and that the corrosion rate of
                                                                                 the copper decreases while
                                                                                 the corrosion rate of the zinc
                                                                                 increases. By connecting the
                                                                                 two metals, we have made
                                                                                 the copper a cathode in an
                                                                                 electrochemical cell, and the
                                                                                 zinc has become an anode.
                                                                                 The accelerated corrosion of
                                                                                 the zinc may be so much
that all of the oxidation of the copper stops and it becomes protected from corrosion. We call this method of
corrosion control cathodic protection.

The reaction at the copper (cathode) becomes:
  +     -
2H +2e → H2
                                                              The voltage of the copper shifts to a point where
                                                              hydrogen ion reduction can occur at the copper
                                                              surface. The oxidation (corrosion) of the copper
                                                              cathode may completely stop due to the
                                                              electrical connection to the zinc anode.

                                                              The reaction at the zinc (anode) remains the
                                                              same,
                                                                        +2          -
                                                              Zn » Zn        + 2e

                                                              But the reaction rate increases due to the fact
                                                              that the surface area of the clean (un-corroding)
                                                              copper surface can now support a reduction
                                                              reaction at a high rate.


                                                              Thus connecting these two metals virtually
                                                              stopped the corrosion of the copper and
                                                              increased the corrosion rate of the zinc. We say
                                                              that the zinc cathodically protected the copper
                                                              from corrosion. Cathodic protection is a common
                                                              means of corrosion control.


Mnemonic device: Anodes oxidize; cathodes reduce.




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Oxidation and Reduction (electrons)




Acronyms for oxidation and reduction:
   •     Oxidation is losing electron or gaining Proton H+

   •     Reduction is gaining electrons or losing H+

   •     Electron loss means oxidation:

   •     Losing electrons oxidation, gaining electrons reduction:


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More on oxidation and reduction.


    •     Oxidation describes the loss of electrons by a molecule, atom or ion

    •     Reduction describes the gain of electrons by a molecule, atom or ion


Oxidizing and reducing agents

Substances that have the ability to oxidize other substances are said to be oxidative and are known as
oxidizing agents, oxidants or oxidizers. Put another way, the oxidant removes electrons from another
substance, and is thus reduced itself. And because it "accepts" electrons it is also called an electron acceptor.

The chemical way to look at redox processes is that the

           Reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent
           loses electrons and is oxidized

           Oxidant or oxidizing agent gains electrons and is reduced.

The pair of an oxidizing and reducing agent that are involved in a particular reaction is called a redox pair.
Mnemonic device:

To be oxidized other has to be reduced and vice versa. If you get oxidized you are a reducing agent, if
you get reduced you are an oxidizing agent.


Examples of redox reactions


A good example is the reaction between hydrogen and fluorine:


We can write this overall reaction as two half-reactions: the oxidation reaction

                                      H2 was oxidized by losing electrons, it was a reducing agent.
and the reduction reaction:

                                      F2 was reduced by gaining electron, it was an oxidizing agent.
Analyzing each half-reaction in isolation can often make the overall chemical process clearer. Because
there is no net change in charge during a redox reaction, the number of electrons in excess in the oxidation
reaction must equal the number consumed by the reduction reaction (as shown above).

Elements, even in molecular form, always have an oxidation number of zero. In the first half reaction,
hydrogen is oxidized from an oxidation number of zero to an oxidation number of +1. In the second half
reaction, fluorine is reduced from an oxidation number of zero to an oxidation number of −1.

When adding the reactions together the electrons cancel:




And the ions combine to form hydrogen fluoride:




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Displacement reactions
Redox occurs in single displacement reactions or substitution reactions. The redox component of this type
of reaction is the change of oxidation state (charge) on certain atoms, not the actual exchange of atoms in
the compounds.

For example, in the reaction between iron and copper(II) sulphate solution:


The ionic equation for this reaction is:


As two half-equations, it is seen that the iron is oxidized:


And the copper is reduced:



Other examples
    •     iron(II) oxidizes to iron(III):
          Fe2+ → Fe3+ + e−
    •     hydrogen peroxide reduces to hydroxide in the presence of an acid:
          H2O2 + 2 e− → 2 OH−


Overall equation for the above:

          2Fe2+ + H2O2 + 2H+ → 2Fe3+ + 2H2O
          4Fe + 3O2 → 2 Fe2O3




Example: Fe0 + Cu++SO4 --> Cu0 + Fe++SO4
                          --                  --



Copper is more electrochemically noble than iron (Fe) and will displace iron from the surface, i.e., cause iron to dissolve
into solution so it can come out as a metal.



Click here to see interactive materials on Redox Reactions and Electrochemical Reactions.



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Rusting of iron is oxidation-reduction reaction, where iron is oxidized, Fe → Fe2+ with loss of 2 electron and iron
in this case a reductant.



More reading:
Oxidation-Reduction
The following is a brief overview of the basics.

 Oxidation-reduction reactions involve the transfer of electrons between substances. They take place
simultaneously, which makes sense because if one substance loses electrons, another must gain them. Many
of the reactions we’ve encountered thus far fall into this category. For example, all single-replacement
reactions are redox reactions. Terms you’ll need to be familiar with.

Electrochemistry: The study of the interchange of chemical and electrical energy.

Oxidation: The loss of electrons. Since electrons are negative, this will appear as an increase in the charge
(e.g., Zn loses two electrons; its charge goes from 0 to +2). Metals are oxidized.

Oxidizing agent (OA): The species that is reduced and thus causes oxidation.

Reduction: The gain of electrons. When an element gains electrons, the charge on the element appears to
decrease, so we say it has a reduction of charge (e.g., Cl gains one electron and goes from an oxidation
number of 0 to -1). Nonmetals are reduced.

Reducing agent (RA): The species that is oxidized and thus causes reduction.

Oxidation number: The assigned charge on an atom. You’ve been using these numbers to balance formulas.
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Half-reaction: An equation that shows either oxidation or reduction alone.

Example

When powdered zinc metal is mixed with iodine crystals and a drop of water is added, the resulting reaction
produces a great deal of energy. The mixture bursts into flames, and a purple smoke made up of I2 vapor is
produced from the excess iodine. The equation for the reaction is


                                        Zn(s) + I2(s)   ZnI2(s) + energy

Identify the elements that are oxidized and reduced, and determine the oxidizing and reducing agents.

Voltaic (or Galvanic) Cells

Redox reactions release energy, and this energy can be used to do work if the reactions take place in a voltaic
cell. In a voltaic cell (sometimes called a galvanic cell), the transfer of electrons occurs through an external
pathway instead of directly between the two elements. The figure below shows a typical voltaic cell (this one
contains the redox reaction between zinc and copper):




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Standard Reduction Potentials


The potential of a voltaic cell as a whole will depend on the half-cells that are involved. Each half-cell has a
known potential, called its standard reduction potential (Eº). The cell potential is a measure of the difference
between the two electrode potentials, and the potential at each electrode is calculated as the potential for
reduction at the electrode. That’s why they’re standard reduction potentials, not standard oxidation potentials.
On this reduction potential chart, the elements that have the most positive reduction potentials are easily
reduced and would be good oxidizing agents (in general, the nonmetals), while the elements that have the
least positive reduction potentials are easily oxidized and would be good reducing agents (in general, metals).


Electrolytic Cells

While voltaic cells harness the energy from redox reactions, electrolytic cells can be used to drive non-
spontaneous redox reactions, which are also called electrolysis reactions. Electrolytic cells are used to
produce pure forms of an element; for example, they’re used to separate ores, in electroplating metals (such
as applying gold to a less expensive metal), and to charge batteries (such as car batteries). These types of
cells rely on a battery or any DC source—in other words, whereas the voltaic cell is a battery, the electrolytic
cell needs a battery. Also unlike voltaic cells, which are made up of two containers, electrolytic cells have just
one container. However, like in voltaic cells, in electrolytic cells electrons still flow from the anode to the
cathode. An electrolytic cell is shown below.




More reading: Electrochemistry

http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrochem.html   Physic and Chemistry (College Level)

http://www.ionode.com.au/Techorp.html Redox Theory

http://www6.grafton.k12.wi.us/ghs/teacher/mstaude/ Chemistry Basic

http://www.tannerm.com/electrochem.htm General Chemistry

http://www.chem1.com/acad/pdf/c1xElchem.pdf Electrolysis

http://www.chem1.com/acad/webtext/elchem/ec4.html     All about Nernst Equation.




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1.3 Basic Corrosion Theory
Corrosion of metal is mostly electrochemical reaction composed of two half cell reactions, an anodic reaction
and a cathodic reaction. The anodic reaction releases electrons, while the cathodic reaction consumes
electrons. There are three common cathodic reactions, oxygen reduction (fast), hydrogen evolution from
neutral water (slow), and hydrogen evolution from acid (fast).
The corrosion cell




The corrosion cell can be represented as follows:

                                                                                         Anodic reaction:
                                                                                         M → Mn+ + ne-

                                                                                         M stands for a metal and n
                                                                                         stands for the number of
                                                                                         electrons that an atom of
                                                                                         the metal will easily
                                                                                         release.
                                                                                         i.e. for iron and steel: Fe
                                                                                         → Fe2+ + 2e-
                                                                                         Cathodic reactions:

                                                                                         O2 + 4 H+ + 4e- → 2H2O
                                                                                         (oxygen reduction in acidic
                                                                                         solution)
                                                                                         1/2 O2 + H2O + 2e- → 2
                                                                                         OH- (oxygen reduction in
neutral or basic solution)
2 H+ + 2e- → H2 (hydrogen evolution from acidic solution)
2 H2O + 2e- → H2 + 2 OH- (hydrogen evolution from neutral water)

Each half-cell reaction has an electrical potential, known as the half-cell electrode potential. The anodic
reaction potential, Ea, plus the cathodic reaction potential, Ec, adds up to E, the cell potential. If the overall cell
potential is positive, the reaction will proceed spontaneously.




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Every metal or alloy has a unique corrosion potential in a defined environment. When the reactants and
products are at an arbitrarily defined standard state, the half-cell electrode potentials are designated Eo. These
standard potentials are measured with respect to the standard hydrogen electrode (SHE). A listing of standard
half-cell electrode potentials is given in Table 1.

Selected half-cell reduction potentials are given in Table 1. To determine oxidation potentials, reverse the
direction of the arrow and reverse the sign of the standard potential. For a given cathodic reaction, those
anodic (reversed) reactions below it in the table will go spontaneously, while those above it will not. Thus any
metal below the hydrogen evolution reaction will corrode (oxidize) in acidic solutions.

e.g., Cathodic reaction: 2H+ + 2e- → H2 (hydrogen evolution)

Two possible anodic reactions:

Cu → Cu2+ + 2e- (above cathodic reaction in table - will not corrode)
Zn → Zn2+ + 2e- (below cathodic reaction in table - spontaneous corrosion)
Thus, in the presence of H+ ions, Zinc (Zn) will spontaneously corrode while copper (Cu) will not.



1.3.1 Oxidation-reduction electromotive-force potentials /
galvanic series.
There has been some confusion regarding oxidation-reduction electromotive-force potentials and the
galvanic series. While there are similarities between the galvanic series and standard reduction potentials,
there are also some fundamental differences. While standard potentials can provide an indication of the
stability of a metal, as it is done with E-pH or Pourbaix diagrams, galvanic series are used to predict whether or
not galvanic corrosion will occur, and if so, which of the two coupled metals will exhibit increased corrosion.
Thus, these two tabulations have entirely different uses and should therefore not be confused.

Table1. Standard Electromotive Force Potentials

Cathodic Reactions                       Standard Potential, eo (volts vs. SHE)

Au3+ + 3e- → Au                          +1.498 (Most Noble)
O2 + 4H+ + 4e- → 2H2O                    +1.229 (in acidic solution)
Pt2+ + 2e- → Pt                          +1.118
NO3- + 4H+ + 3e- → NO + 2H2O             +0.957
Ag+ + e- → Ag                            +0.799
O2 + 2H2O + 4e- → 4OH-                   +0.401 (in neutral or basic solution)
Cu2+ + 2e- → Cu                          +0.337
2H+ + 2e- → H2                           0.000
Pb2+ + 2e- → Pb                          -0.126
Sn2+ + 2e- → Sn                          -0.138
Ni2+ + 2e- → Ni                          -0.250

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Co2+ + 2e- → Co                            -0.277
Cd2+ + 2e- → Cd                            -0.403
Fe2+ + 2e- → Fe                            -0.447
Cr3+ + 3e- → Cr                            -0.744
Zn2+ + 2e- → Zn                            -0.762
2H2O + 2e- → H2 + 2OH-                     -0.828 (pH = 14)
Al3+ + 3e- → Al                            -1.662
Mg2+ + 2e- → Mg                            -2.372
Na+ + e- → Na                              -2.71
K+ + e- → K                                -2.931 (Most Active)
Source: Handbook of Chemistry and Physics, 71st ed, CRC Press, 1991

Table 1 can be used to show that copper will corrode in nitric acid solutions (oxidizing) and aerated water. Similarly,
aluminum (Al), magnesium (Mg), sodium (Na) and potassium (K) will react spontaneously with water in neutral or basic
solutions.



Galvanic series (nobler higher)
The following is the galvanic series for stagnant (that is, low oxygen content) seawater. The
order may change in different environments.

    •     Graphite

    •     Palladium

    •     Platinum

    •     Gold

    •     Silver

    •     Titanium

    •     Stainless steel (316 passive)

    •     Stainless Steel (304 passive)

    •     Silicon bronze

    •     Stainless Steel (316 active)

    •     Monel 400

    •     Phosphor bronze

    •     Admiralty brass

    •     Cupronickel

    •     Molybdenum

    •     Red brass

    •     Brass plating

    •     Yellow brass

    •     Naval brass 464

    •     Uranium 8% Mo
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    •     Niobium 1% Zr

    •     Tungsten

    •     Stainless Steel (304 active)

    •     Tantalum

    •     Chromium plating

    •     Nickel (passive)

    •     Copper

    •     Nickel (active)

    •     Cast iron

    •     Steel

    •     Lead

    •     Tin

    •     Indium

    •     Aluminum

    •     Uranium (pure)

    •     Cadmium

    •     Beryllium

    •     Zinc plating (see galvanization)

    •     Magnesium




Pourbaix diagram for iron
Stability diagrams are able to condense a great amount of information into a compact representation, and are
widely employed in geochemistry and corrosion engineering. The Pourbaix diagram for iron is one of the more
commonly seen examples:

Three oxidation states of iron (0, +2 and +3) are represented on this diagram. The stability regions for the
oxidized iron states are shown only within the stability region of H2O. Equilibria between species separated by
vertical lines are dependent on pH only.

The +3 oxidation state is the only stable one in environments in which the oxidation level is controlled by
atmospheric O2. This is the reason the Earth’s crust contains iron oxides, which developed only after the
appearance of green plants which are the source of O2.
Iron is attacked by H+ to form H2 and Fe(II); the latter then reacts with O2 to form the various colored Fe(III)
oxides that constitute “rust”.
Numerous other species such as oxides and hydrous oxides are not shown. A really “complete” diagram for
iron would need to have at least two additional dimensions showing the partial pressures of O2 and CO2.
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More reading: Appendix           Pourbaix            |     Redox Reaction




A simple experiment
Procedure:

          Prepare 200 ml of agar-agar solution. Measure out a mass of 2.0 grams of powdered agar-agar. Heat 200 ml of
          water to boiling. Remove the water from the heat and add the agar-agar powder slowly while constantly stirring.
          Once the agar has dissolved, add 5 drops of phenolphthalein solution or 5 drops of bromothymol blue

          Take two nails (or strips of pure iron) and wrap them in the strips of metal. One nail should be wrapped with zinc
          metal and the other nail wrapped with copper metal. Place these two wrapped nails into a petri dish. Be sure the
          nails do not touch. (The zinc and copper metals should be rubbed down and cleaned with sandpaper before they
          are wrapped around the nails). Make sure the nails are not galvanized or have some other type of coating. The
          idea is to use iron.

          Slowly pour the agar-agar solution into the petri dishes to a depth of about 0.5 cm above the nails and metals.

          Allow the petri dishes to remain untouched for a day or two. From time to time make observations. At the end of
          the next day and then at the end of the second day make and record observations.

Note: Phenolphthalein is used as an acid or base indicator where in contact or presence of acid it will turn colorless and
with base,




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Observation: 观察实验




Figure 1. Using Phenolphthalein as indicator.
Iron wrapped in zinc is on the left and iron wrapped in copper is on the right.




Questions:
   1. What changes did you observe in the petri dish? Why did the color changes occur where they did?
   2. In which nail did the iron of the nail corrode?
   3. Why did the iron nail corrode in the one situation and not in the other?
   4. Explain "corrosion" or "rust" in an electrochemical point of view.
   5. What does the "pink" color (if phenolphthalein was used) indicate?
   6. What is a cathode and what is an anode?
   7. What is oxidation?


Explanations:

    1. As can be seen in Figure 1, the iron strip which is wrapped in copper corroded. Pink color is found around the
         copper strip and the iron can be seen to be turning orange-yellow. This is only after 5 hours. More corrosion would
         be visible days later. The second strip of iron is not corroded. Pink is found on the iron and nothing by the zinc
         strip. The color changes occurred where they did as a result of the corrosion.

    2. In the strip of iron wrapped with copper the iron corroded. Iron metal oxidizes faster or more easily than does the
         copper. It is said that the iron is oxidized and the copper is reduced. What is happening is that the iron is losing
         electrons and the copper is gaining electrons. The copper is considered the cathode in this case and the iron is
         considered to be the anode. The iron metal loses electrons and turns into an iron ion according to this equation:

                      Fe (s) → Fe + 2 e Equation 1.1
                                 +2    -



                      These two electrons travel through the iron metal to the copper. At the copper there is water and
                      oxygen which take the two electrons and use them to form hydroxide ions as in Equation 1.2:

                      ½ O2 (g) + H2O (l) + 2 e- → 2 OH- Equation 1.2

                      This excess of OH- produced causes the solution next to the copper to be pink. Hydroxide ions (OH-)
                      make a solution to be basic which turns pink in the presence of phenolphthalein.

                      What ultimately happens in the case of the iron metal wrapped with copper is that the iron metal
         loses two electrons which are used by water and oxygen to make hydroxide ions. It is evident that the hydroxide
         ions are formed at the copper surface because of the pink that exists around the copper. The iron ions that are
         formed react with oxygen and water to form "rust" as is seen in Equation 1.3:

         Fe+2 + ½ O2 (g) + H2O (l) → Fe (OH)2 (s) Equation 1.3

         This Fe (OH)2 (s) combines with a second molecule of Fe (OH)2 (s) in the presence of oxygen to form
         iron(III)oxide (the more common form of rust) and water.



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    2 Fe (OH)2 (s) + ½ O2 (g) → Fe2O3 (s) + 2H2O (l) Equation 1.4

    Thus iron "rusts" and the copper does not react with anything.

3. In the other situation in which iron is wrapped with zinc the opposite occurs. In this case zinc is oxidized faster or
    more easily than the iron and therefore it undergoes a very similar reaction as did the iron in the last example.
    Here zinc loses two electrons and forms a Zn +2 ion. On the surface of the iron the same reaction occurs as did on
    the copper. Water and oxygen combine with the two electrons to make hydroxide ions, which turn the solution
    next to the iron surface pink. In this case the zinc is considered to be the anode and the iron is considered to be
    the cathodeThis has very practical implications. The auto industry and boating industry have used this idea to
    prevent automobiles and the steel hulls of ships from rusting. Water is a crucial component to act as a medium to
    transfer electrons. Iron metal will not "rust" when it is in dry air. So these industries, knowing that zinc, aluminum,
    and magnesium oxidize or "rust" faster and more easily than iron, place these metals adjacent to the steel so that
    these metals will "rust" before the iron does.

4. See number 2.

5. The pink color indicates that hydroxide ions are produced. This indicates a chemical reaction has occurred. The
    location of the pink indicates that the metal nearest to it was producing the hydroxide ions, and therefore, was the
    metal "gaining" electrons. This metal which "gained" electrons is said to have been "reduced" while the metal
    which "lost" the electrons is said to have been "oxidized" or "rusted" or "corroded".

6. The cathode is the place in an electrochemical cell to where the electrons travel. The anode is the place in an
    electrochemical cell from where the electrons came.

7. Oxidation is the "loss of electrons". It is usually comparable to "rusting" or "corroding" because the metal loses
    electrons, turns into an ion, and therefore, there are less "metal" atoms around. Thus the metal is said to have
    corroded.




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1.3.2 Why corrosion cells form
Corrosion cells are created on metal surfaces in contact with an electrolyte because of energy differences
between the metal and the electrolyte. Different area on the metal surface could also have different potentials
with respect to the electrolyte. These variations could be due to:


         Metallurgical factors, due to fabrication, field installations etc.:
           Compositions.
           Microstructures.
           Inclusions.
           Precipitations.
           Heat treatment.
           Mechanical rolling and tempering.
           Welding.
           Work hardening.
           Fabrication, installation and external stress, strain factors.


         Environmental factors.
              Concentration Cells.
              Environmental induced SCC, SSC, HIC etc.
              Microbial MIC etc.
              Temperature induced corrosion.
              Mechanical environmental induced erosion, fretting, cavitation etc.
              Galvanic, CP and Impressed current anodic dissolution, stray current, cathodic embrittlement etc.

Above also include corrosion mechanisms of non-electrolytic nature.

Discussion:


1.3.2.1 Metallurgical Factors:
Carbon and low alloy steels are the most widely used material in the oilfield. Stainless steels (Fe-Cr-Ni), and
nickel-base corrosion resistant alloys (CRA), such as Incoloys (Ni-Fe-Cr), Inconels (Ni-Cr), Hastelloys (Ni-Cr-
Mo-Fe-Co) etc., are also used in highly corrosive environments.
Steel is an alloy of iron (Fe) and carbon (C). Carbon is fairly soluble in liquid iron at steel making temperatures,
however, it is practically insoluble in solid iron (0.02% at 723C), and trace at room temperature. Pure iron is
soft and malleable; small amounts carbon and manganese are added to give steel its strength and toughness.

Most of the carbon is oxidized during steelmaking. The residual carbon and post-fabrication heat treatment
determines the microstructure, therefore strength and hardness of steels. Carbon steels are then identified by
their carbon contents, i.e., low-carbon or mild steel, medium carbon (0.2- 0.4 % C), high-carbon (up to 1% C)
steels, and cast irons (>2 % C). American Iron and Steel Institute (AISI) designation 10xx series represent
plain carbon steels, last two digits indicating the carbon content. For instance, AISI 1036 steel, commonly used
in sucker rods, contain 0.36% carbon. Low alloy steels contain 1-3% alloying elements, such as chromium-
molybdenum steels, 4140 (1% Cr-0.2% Mo-0.4% C), for improved strength and corrosion resistance. American
Petroleum Institute (API) specifications also provide guidelines for strength and chemical composition of oilfield
steels.




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The microstructure of a low-carbon pipe steel is shown (magnified 100X) in (a) transverse and (b) in
longitudinal sections, where light grains are ferrite and the dark grains are pearlite. Other impurities in iron may
also migrate to grain boundaries forming micro-alloys that may have entirely different composition from the
grains, hence may have different corrosion properties. As in the case of intergranular corrosion, grain
boundary precipitation, notably chromium carbides in stainless steels, is a well recognized and accepted
mechanism of weld decay. In this case, the precipitation of chromium carbides is induced by the welding
operation when the heat affected zone (HAZ) experiences a particular temperature range (550oC~850oC). The
precipitation of chromium carbides consumed the alloying element - chromium from a narrow band along the
grain boundary and this makes the zone anodic to the unaffected grains. The chromium depleted zone
becomes the preferential path for corrosion attack or crack propagation if under tensile stress.




Low-carbon pipe steel is shown (magnified 100X) in transverse sections.




Same low-carbon pipe steel is shown (magnified 100X) in longitudinal sections,

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In a corrosive environment, either grains or the grain boundaries having different composition can become
anodic or cathodic, thus forming the corrosion cells. Hydrogen evolution reaction can take place on iron
carbide, and spheroidized carbon in steels, and graphite in cast irons, in acidic solutions with relative ease;
areas denuded in carbon become anodic and corrode preferentially. Therefore, post-weld heat treatment of
steels is critical in order to prevent corrosion of the heat affected zone (HAZ), sensitization and intergranular
corrosion in stainless steels.

Other metallurgical factors include improper heat treatment for stress relief after hot rolling, upsetting, or
excessive cold working; slag inclusions, mill scale, water deposited scale and corrosion product scales, nicks,
dents and gouges on the metal surface. Scars caused by pipe wrench, tongs, and other wellhead equipment
on sucker rods and tubing would become anodic and corrode downhole. Likewise, new threads cut into pipe
will be anodic and corrode in the absence of suitable corrosion protection.

Deformation caused by cold bending or forcing piping into alignment will create internal stresses in the metal.
The most highly stressed areas will become anodic with respect to the rest of the metal. Hammer marks, nicks
and gauges will also act as stress raisers and may cause fatigue failures.

Sections of the same steel may corrode differently due to variations in the concentration of aggressive ions in
the environment. For instance, a casing or a pipeline could pass through several formations or soils with
different water composition, hence, sections of the casing or the pipe could experience different rates of
corrosion. Similarly, a pipeline crossing a river will be exposed to higher concentration salts as compared to
dry land. It is difficult to predict the effect of higher salt concentrations but, generally, sections of steel exposed
to higher salt concentrations become anodic and corrode.

Differences in the oxygen concentration on the metal surface (differential aeration or differential oxygen
concentration cells) cause particularly insidious forms of corrosion. A common example is corrosion of pipes
under paved roads, parking lots, or pavements.

Lack of oxygen under the pavement render that section of the pipe anodic, hence pipe corrodes preferentially.
Similarly, loose backfill placed into ditch to cover a pipeline is more permeable to oxygen diffusion; the topside
of the pipe will become cathodic, and the bottom resting on undisturbed soil will become anodic and corrode.
Crevice and pitting corrosion mechanisms in aerated systems can also be explained by differential
concentration cells.




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            Intergranular Corrosion: Knife-Line Attack (KLA)

 Recognition: What is knife-line attack? Knife-Line Attack (KLA) is a form of intergranular corrosion of an
alloy, usually stabilized stainless steel, along a line adjoining or in contact with a weld after heating into the
sensitization temperature range.
The corrosive attack is restricted to extremely narrow line adjoining the fusion line. Attack appears razor-sharp
(and hence the name of "knife-line" attack). It is possible to visually recognize knife-line attack if the lines are
already formed in the along the weld.




 Mechanisms: What causes knife-line attack? For stabilized stainless steels and alloys, carbon is bonded
with stabilizers (Ti or Nb) and no weld decay occurs in the heat affected zone during welding. In the event
of a subsequent heat treatment or welding, however, precipitation of chromium carbide is possible and this
leaves the narrow band adjacent to the fusion line susceptible to intergranular corrosion.


Prevention: How to prevent knife-line attack? Knife-Line Attack can be prevented through:
    •     Heat treatment - heating the weld to 1065oC to re-stabilize the material.



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Pearlite.




Scanning micrograph of a silicate inclusion found in workpiece W1. (b) EDX analysis of the silicate inclusion showing its
chemical composition. (c) Scanning micrograph of sulfide inclusion found in workpieces W1 and W2. (d) EDX analysis of
the sulfide inclusion showing its chemical composition.




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1.3.2.2 Environmental Factors
Corrosion gas and microbes.

There are many unique environments in the oil field industry where corrosion commonly occurs. Oxygen (O2) , which is a
strong oxidizer, is one of the most corrosive gases when present. Other common corrosive gases in the oil field are carbon
dioxide (CO2) and hydrogen sulfide (H2S), which form weak acids in water. Microbial activity may cause corrosion alone,
create more corrosive gases, and/or act to induce blockage within pipes.

Corrosion rates of steel versus oxygen, carbon dioxide, and hydrogen sulfide. Note the different gas
concentrations on the x axis.




O2 Corrosion


O2 Information

Oxygen dissolved in water is one of the primary causes of corrosion in the oil field. When oxygen is present,
the most common types of corrosion include pitting corrosion and uniform corrosion.

Oxygen is a strong oxidant and reacts quickly with metal. The maximum amount of oxygen in water is only 8
ppm, so the mass transport of oxygen is the rate limiting step in oxygenated non-acidic environments.
Controlling the rate of oxygen transport (often by controlling flow velocity) is thus critical to corrosion control.

O2 corrosion products include iron oxides, including FeO(OH) - goethite, Fe2O3 - hematite, Fe3O3 - magnetite,
and FeO(OH) - ferrous hydroxide.


Differential Aeration

Corrosion may occur in oilfield applications due to the existence of differential aeration. In these cases, one
section of the metal is exposed to oxygen while the other is not. The section with no aeration becomes anodic,
and is subject to preferential corrosion. This can occur with pipelines and other metals near the surface. The
first figure shows an example of how a corrosion cell can form when a pipe is buried below the surface. The
soil above the pipe can become aerated due to the digging and backfilling process, so the top of the pipe is

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second figure, a section of pavement restricts oxygen from reaching the pipe in the part of the pipe under the
pavement. That part of the pipe becomes anodic and corrodes preferentially.




Where Found

Although it is not normally present at depths below around 330 ft (100 m), oxygen is often introduced in oil
production through leaking pump seals, casing and process vents, open hatches, and open handling. In
addition, oxygen removal processes such as gas stripping and chemical scavenging often fail, allowing oxygen
contamination in waterflood systems.

Oxygen corrosion occurs commonly in drilling fluid, primary production in rod pumped wells, outdoor rod
storage (rusting), oxygen entry into wellbore through annulus, lower part of well including casing, pump, tubing,
lower part of rod string


Prevention / Mitigation

Oxygen removal may be done by mechanical and chemical means. Mechanical means include gas stripping
and vacuum deaeration; chemical means include sodium sulfite, ammonium bisulfite and sulfur dioxide.
Mechanical means of oxygen removal are usually employed when large quantities of oxygen need to be
removed, while chemical means are used to remove small quantities of oxygen and may be used to remove
residual oxygen after mechanical means have been used.

It is often more economical to exclude oxygen from oilfield equipment than to remove it after it has entered the
system. The most common way of excluding oxygen is through the use of gas blankets, composed of oxygen
free gas such as natural gas (methane) or nitrogen. Gas blankets may be used on water supply wells and
water storage tanks, supply wells and producing wells, and pumps. Most tanks only require a few ounces of
pressure. The regulator should supply gas at a rate adequate to maintain pressure when the fluid level drops.
Maintenance of valve stems and pump packing is also important.

To reduce or prevent corrosion in an O2 environment:
Drilling - oxygen scavengers
Producing wells - corrosion inhibitors, oxygen scavengers, elimination of O2 sources
Flowlines - corrosion inhibitors, oxygen scavengers, elimination of O2 sources



More reading:


                          Corrosion Control in Pipelines Using Oxygen Stripping




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Signs of oxygen corrosion include wide shallow pits and reddish brown rust.




Oxygen corrosion also causes large areas of metal loss on sucker rods


It is virtually impossible to keep oxygen out of any tophole system. Downhole systems do not have oxygen,
unless oxygen is injected with treating chemicals or other secondary recovery methos are used, such as
firefloods. Oxygen from the air can react with iron sulfides to form iron oxides. The presence of iron oxides as
corrosion by-products is a strong indication that oxygen corrosion is occurring in the system. If X-Ray
Diffraction (XRD) finds magnetite (Fe3O4), hematite (Fe2O3), and / or akaganeite [Fe+3(O,OH,Cl)], which is an
iron oxy chloride, it is a strong indication that oxygen corrosion is occurring.

The topography of oxygen corrosion pits includes the following characteristics:
    •     round pits

    •     shallow pits

    •     sloping sidewalls

    •     tend to grow into one another

    •     bright red rust color
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Oxygen is not determined directly by XRF, however, subtracting the sum of all the elements from 100% gives
the oxygen level. Oxygen corrodes carbon steel forming iron oxides as the corrosion by-products.

Oxygen corrosion is usually controlled by the addition of oxygen scavengers to the system. Oxygen
scavengers help to reduce the oxygen level, and hence control Oxygen Corrosion. Note that the selection of a
particular oxygen scavenger should be based on compatibility, cost, and other pertinent factors.




CO2 Corrosion


CO2 Information

Carbon dioxide systems are one of the most common environments in the oil field industry where corrosion
occurs. Carbon dioxide forms a weak acid known as carbonic acid (H2CO3) in water, a relatively slow reaction.
However, CO2 corrosion rates are greater than the effect of carbonic acid alone. Cathodic depolarization may
occur, and other attack mechanisms may also be at work. The presence of salts is relatively unimportant.

Corrosion rates in a CO2 system can reach very high levels (thousands of mils per year), but it can be
effectively inhibited. Velocity effects are very important in the CO2 system; turbulence is often a critical factor in
pushing a sweet system into a corrosive regime. This is because it either prevents formation or removes a
protective iron carbonate (siderite) scale.

Conditions favoring the formation of the protective iron carbonate scale are elevated temperature, increased
pH (bicarbonate waters) and lack of turbulence. Magnetite scales are also formed in CO2 systems, and
corrosion product scales often consist of layers or mixtures of siderite and magnetite.

The maximum concentration of dissolved CO2 in water is 800 ppm. When CO2 is present, the most common
forms of corrosion include uniform corrosion, pitting corrosion, wormhole attack, galvanic ringworm corrosion,
heat affected corrosion, mesa attack, raindrop corrosion, erosion corrosion, and corrosion fatigue. The
presence of carbon dioxide usually means no H2 Embrittlement. CO2 corrosion products include iron carbonate
(siderite, FeCO3), Iron oxide, and magnetite. Corrosion product colors may be green, tan, or brown to black.


Where Found

As stated before, CO2 corrosion is one of the most common environments where corrosion occurs, and exists
almost everywhere.

Areas where CO2 corrosion is most common include flowing wells, gas condensate wells, areas where water
condenses, tanks filled with CO2, saturated produced water and flowlines, which are generally corroded at a
slower rate because of lower temperatures and pressures. For more information on specific equipment
corrosion issues,

CO2 corrosion is enhanced in the presence of both oxygen and organic acids, which can act to dissolve iron
carbonate scale and prevent further scaling.


Prevention / Mitigation

To reduce or prevent corrosion in an CO2 environment:
Drilling - pH control with caustic soda
Producing wells - corrosion inhibitors
Flowlines - continuous corrosion inhibitor injection

Prediction of corrosion

In sweet gas wells with a pH of 7 or less,

          CO2 partial pressure of 30 psi usually indicates corrosion.

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         CO2 partial pressure of 7 - 30 psi may indicate corrosion.

         CO2 partial pressure of 7 psi is usually considered non-corrosive.




Uniform Corrosion




Pitting Corrosion showing wormhole attack pattern, where pits are interconnected.




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Galvanic ringworm corrosion, often occurring four to six inches from the upset, where carbon particles have
been spheroidized




                                                                    Heat-affected zone (HAZ) corrosion is
                                                                    a type of galvanic corrosion which
                                                                    occurs along a weld seam.




                                                                    Raindrop attack occurs in gas
                                                                    condensate wells. In areas, water
                                                                    condenses on the metal surface,
                                                                    causing deep pits with tails.




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CO2 corrosion in flowing environments




                                                         Mesa attack is a form of CO2 corrosion
                                                         that occurs in flowing environments,
                                                         and occurs where a protective iron
                                                         carbonate coating is worn away in
                                                         areas.




                                                         Erosion Corrosion, or flow-enhanced
                                                         corrosion, usually occurs in areas
                                                         where the diameter of the pipe or
                                                         direction of flow is changing. Severe
                                                         metal loss can quickly occur.




                                                         Corrosion due to fatigue occurs in
                                                         areas of cyclic stresses. Here we see
                                                         fatigue corrosion in a drill pipe.




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Water with dissolved CO2 led to diffusion of atomic hydrogen (H) which combined as molecular hydrogen (H2)
in voids. The pressure buildup in these voids led to the cracking.




Carbon Dioxide Attack Connection irregularities caused turbulence in the wet CO2 natural gas. This turbulence
prevented formation of the normal protective film.

API literature states that steel equipment is susceptible to carbon dioxide corrosion when the partial
pressure of carbon dioxide is greater than 7 psi. This partial pressure of carbon dioxide is calculated by
multiplying the operating pressure by the mol % of carbon dioxide in the system and dividing by 100. For
instance, in a well with 1000 psi pressure and 0.5 mol % carbon dioxide, the carbon dioxide partial pressure
would be 1000 x 0.5 = 500 / 100 = 5 psi carbon dioxide.

The topography of carbon dioxide corrosion pits includes the following characteristics:
    •     sharp edges

    •     smooth sidewalls

    •     smooth bottoms

    •     pits tend to run into each other
The main corrosion by-product that indicates carbon dioxide corrosion is taking place is siderite (FeCO3).
Magnetite (Fe3O4) and hematite (Fe2O3), both iron oxides, could indicate that carbon dioxide corrosion is
occurring. The main mechanism occurring is indicated by the following equation:

                                             2Fe + 2CO2 + O2 → 2FeCO3



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Note that in the above equation, oxygen is required to form siderite. Another indication that carbon dioxide
corrosion is occurring is the amount of carbonates present in the deposits. If the deposits contain over 3%
carbonates, then most likely carbon dioxide is present in the system.

Carbon dioxide corrosion is usually controlled with the addition of a corrosion inhibitor to the system. A
corrosion inhibitor effective in a carbon dioxide environment should be specified. Note that the selection of a
particular corrosion inhibitor should be based on compatibility, cost, and other pertinent factors. Corrosion
resistant alloys (CRAs) can also be added to help prevent carbon dioxide corrosion.




H2S Corrosion


H2S, polysulfides, and sulfur Information

The maximum concentration of H2S in water is 400 ppm. Wells with large amounts of H2S are usually labeled
sour; however wells with only 10 ppm or above can be labeled sour. Partial pressures of only 0.05 H2S are
considered corrosive.

The primary problem in the presence of H2S is metal embrittlement, caused by penetration of H2 in metal. The
attack mechanism is complex, with many postulated routes. May involve SH- ion, since it is the only dissolved
sulfur ion.

Hydrogen sulfide is a weak acid when dissolved in water, and can act as a catalyst in the absorption of atomic
hydrogen in steel, promoting sulfide stress cracking (SSC) in high strength steels. Polysulfides and sulfanes
(free acid forms of polysulfides) may be formed when hydrogen sulfide reacts with elemental sulfur. These
sulfanes are produced along with other gaseous constituents. As pressure decreases up the production tubing,
the sulfanes dissociate and elemental sulfur precipitates, which can cause plugging.

Iron sulfides are often formed from corrosion reactions, and can be important in corrosion control, especially at
lower temperatures and low H2S partial pressures, where a protective film often forms. However, in order for
this protective film to form, the absence of oxygen and chloride salts is required.

In environments with hydrogen sulfide (H2S) corrosion, the most common types include uniform corrosion,
pitting corrosion, corrosion fatigue, sulfide stress cracking, hydrogen blistering, hydrogen embrittlement, and
stepwise cracking.

Corrosion products include black or blue-black iron sulfides, pyrite, greigite, mackinwaite, kansite, iron oxide
(Fe3O4), magnetite, sulfur (S), and sulfur dioxide (SO2).


Where Found

H2S corrosion can be found in production wells, flowlines, and during drilling. Areas where H2S corrosion is
common include sucker rods


Prevention / Mitigation

To reduce or prevent corrosion in an H2S environment:
Drilling - High pH, zinc treatments
Production - corrosion inhibitors
Flowlines - Corrosion inhibitors, H2S scavengers

Predicting corrosion

Sour gas wells may be corrosive if the pH is 6.5 or less, and H2S concentration is 250 ppm or more.




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                                                     Signs of hydrogen sulfide corrosion
                                                     include shallow round pits with etched
                                                     bottoms.




                                                     H2S Attack on sucker rods followed by
                                                     corrosion fatigue break, caused by
                                                     alternating stresses.




                                                     Sulfide stress cracking occurs when
                                                     H2S corrosion is accelerated by
                                                     stresses.




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                                                                          Hydrogen embrittlement fractures are
                                                                          caused by hydrogen entering the metal
                                                                          and concentrating internally in high-
                                                                          stress areas, making the metal very
                                                                          brittle. Hydrogen induced cracking can
                                                                          also occur if the metal is subjected to
                                                                          cyclic stresses or tensile stress.




Hydrogen sulfide corrosion, also known as sour corrosion, has plagued oilfield equipment. The level of sulfur
and sulfides in the deposits are an indication as to whether hydrogen sulfide corrosion is occurring or not.
Furthermore, when tested by X-Ray Diffraction (XRD), iron sulfides of all forms, for example, pyrite, pyrrhotite,
troilite, etc., are indications that hydrogen sulfide corrosion is occurring. Another indicator of hydrogen sulfide
corrosion is a positive spot test for iron sulfides in the form of a yellow precipitate and a rotten eggs odor, when
the steel is tested with Baroid's Iron Sulfide Detecting Solution (15% HCl + Sodium Arsenite).

The topography of hydrogen sulfide corrosion pits includes the following characteristics:
    •     conically-shaped

    •     sloping sidewalls

    •     etched bottoms
The main corrosion by-product that indicates hydrogen sulfide corrosion is taking place is pyrite (FeS2).
Pyrrhotite (Fe7S8) and troilite (FeS), which are iron sulfides, could indicate that hydrogen sulfide corrosion is
occurring. The main mechanism occurring is indicated by the following equation:

                                             Fe + H2S → FeS + H2


Note that in the above equation, hydrogen is evolved as a corrosion by-product. Further note that FeS is not
always the form of hydrogen sulfide present. As discussed above, pyrite (FeS2) and pyrrhotite (Fe7S8) could
be the form of iron sulfide resulting from the above equation. If there is hydrogen sulfide present in the system,
then there is a risk of hydrogen sulfide corrosion.

Hydrogen sulfide corrosion is usually controlled with the addition of a corrosion inhibitor to the system. A
corrosion inhibitor effective in a hydrogen sulfide environment should be specified. Note that the selection of a
particular corrosion inhibitor should be based on compatibility, cost, and other pertinent factors. Corrosion
resistant alloys (CRAs) are also used to control hydrogen sulfide attack.




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Sulfide Stress Cracking - NACE MR0175

The NACE Standard MR0175, "Sulfide Stress Corrosion Cracking Resistant Metallic Materials for Oil Field
Equipment" is widely used throughout the world. The standard specifies the proper materials, heat treat
conditions and strength levels required to provide good service life in sour gas and oil environments. NACE
(National Association of Corrosion Engineers) is a worldwide technical organization which studies various
aspects of corrosion and the damage that may result in refineries, chemical plants, water systems, and other
industrial systems.


History

MR0175 was first issued in 1975, but the origin of the document dates to 1959 when a group of engineers in
Western Canada pooled their experience in successful handling of sour gas. The group organized as NACE
committee T-1B and in 1963 issued specification 1B163, "Recommendations of Materials for Sour Service." In
1965, NACE organized the nationwide committee T-1F-1 which issued 1F166 in 1966 and MR0175 in 1975.
The specification is revised on an annual basis.

NACE committee T-1F-1 continues to have responsibility for MR0175. All revisions and additions must be
unanimously approved by the 500-plus member committee T-1, Corrosion Control in Petroleum Production.
MR0175 is intended to apply only to oil field equipment, flow line equipment, and oil field processing facilities
where H2S is present. Only sulfide stress cracking (SSC) is addressed. Users are advised that other forms of
failure mechanisms must be considered in all cases. Failure modes, such as severe general corrosion,
chloride stress corrosion cracking, hydrogen blistering or step-wise cracking are outside the scope of the
document. Users must carefully consider the process conditions when selecting materials.

While the standard is intended to be used only for oil field equipment, industry has taken MR0175 and applied
it to many other areas including refineries, LNG plants, pipelines, and natural gas systems. The judicious use
of the document in these applications is constructive and can help prevent SSC failures wherever H2S is
present.


Requirements
The various sections of MR0175 cover the commonly available forms of materials and alloy systems. The
requirements for heat treatment, hardness levels, conditions of mechanical work, and post-weld heat treatment
are addressed for each form of material. Fabrication techniques, bolting, platings, and coatings are also
addressed.




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                                                   Figure 1




                                                   Figure 2

Figures 1 and 2 taken from MR0175 define the sour systems where SSC may occur. Low concentrations of
H2S at low pressures are considered outside the scope of the document. The low stress levels at low
pressures or the inhibitive effects of oil may give satisfactory performance with standard commercial
equipment. Many users, however, have elected to take a conservative approach and specify NACE
compliance any time a measurable amount of H2S is present. The decision to follow MR0175 must be made by


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the user based on economic impact, the safety aspects should a failure occur, and past field experience.
Legislation can impact the decision as well. MR0175 must now be followed by law for sour applications under
several jurisdictions; Texas (Railroad Commission), off-shore (under U.S. Minerals Management Service), and
Alberta, Canada (Energy Conservation Board).


The Basics of Sulfide Stress Cracking




                                                    Figure 3

SSC develops in aqueous solutions as corrosion forms on a material. Hydrogen ions are a product of many
corrosion processes (Figure 3). These ions pick up electrons from the base material producing hydrogen
atoms. At that point, two hydrogen atoms may combine to form a hydrogen molecule. Most molecules will
eventually collect, form hydrogen bubbles, and float away harmlessly. Some percentage of the hydrogen
atoms will diffuse into the base metal and embrittle the crystalline structure. When the concentration of
hydrogen becomes critical and the tensile stress exceeds the threshold level, SSC occurs. H2S does not
actively participate in the SSC reaction; sulfides promote the entry of the hydrogen atoms into the base
material.

In many instances, particularly among carbon and low alloy steels, the cracking will initiate and propagate
along the grain boundaries. This is called intergranular stress cracking. In other alloy systems or under specific
conditions, the cracking will propagate through the grains. This is called transgranular stress corrosion
cracking. Sulfide stress cracking is most severe at ambient temperature, 20° to 120°F (-7° to 49°C). Below
20°F (-7°C) the diffusion rate of the hydrogen is so slow that the critical concentration is never reached. Above
120°F (49°C) the diffusion rate is so fast that the hydrogen passes through the material in such a rapid manner
that the critical concentration is not reached. The occurrence of stress corrosion cracking above 120°F (49°C)
is still likely and must be carefully considered when selecting material. In most cases, the stress corrosion
cracking will not be SSC but some other form. Chloride stress corrosion cracking is likely in deep sour wells as
most exceed 300°F (149°C) and contain significant chloride levels.

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                                                                                Figure 4

The susceptibility of a material to SSC is directly related to its strength or hardness level. This is true for
carbon steels, stainless steels, and nickel based alloys. When carbon or alloy steel is heat treated to
progressively higher hardness levels, the time to failure decreases rapidly for a given stress level (Figure 4).
Years of field experience have shown that good SSC resistance is obtained below 22 HRC for the carbon and
low alloy steels. SSC can still occur below 22 HRC, but the likelihood of failure is greatly reduced.


Carbon Steel

Carbon and low alloy steels have acceptable resistance to SSC provided their processing is carefully
monitored. The hardness must be less than 22 HRC. If welding or significant cold working is done, stress relief
is required. Even though the base metal hardness of a carbon or alloy steel is less than 22 HRC, areas of the
heat effected zone will be harder. Post-weld heat treatment will eliminate these excessively hard areas.

ASME SA216 grades WCB and WCC are the most commonly used body casting materials. It is Fishers™
policy to stress relieve all WCB and WCC castings to MR0175 whether they have been welded or not. This
eliminates the chance of a weld repair going undetected and not being stress-relieved.

ASME SA352 grades LCB and LCC are very similar to WCB and WCC. They are impact tested at -50°F (-
46°C) to ensure good toughness in low temperature service. LCB and LCC are used in the northern U.S.,
Alaska, and Canada where temperatures commonly drop below the -20°F (-32°C) permitted for WCB. All LCB
and LCC castings to MR0175 are also stress-relieved.


Cast Iron

Gray, austenitic, and white cast irons cannot be used for any pressure retaining parts, due to low ductility.
Ferritic ductile iron to ASTM A395 is acceptable when permitted by ANSI, API, or industry standards.


Stainless Steel

UNS S41000 stainless steel (410 stainless steel) and other martensitic grades must be double tempered to a
maximum allowable hardness level of 25 HRC. Post-weld heat treatment is also required. S41600 stainless
steel is similar to S41000 with the exception of a sulfur addition to produce free machining characteristics. Use
of free machining steels is not permitted by MR0175.

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CA6NM is a modified version of the cast S41000 stainless steel. MR0175 allows its use, but specifies the
exact heat treatment required. Generally, the carbon content must be restricted to 0.3 percent maximum to
meet the 23 HRC maximum hardness. Post-weld heat treatment is required for CA6NM.

The austenitic stainless steels have exceptional resistance to SSC in the annealed condition. The standard
specifies that these materials must be 22 HRC maximum and free of cold work to prevent SSC. The cast and
wrought equivalents of 302, 304, 304L, 305, 308, 309, 310, 316, 316L, 317, 321, and 347 are all acceptable
per MR0175.

Post-weld heat treatment of the 300 Series stainless steels is not required. The corrosion resistance may be
effected by welding. However, this can be controlled by using the low carbon grades, or low heat input levels
and low interpass temperatures.

Wrought S17400 (17-4PH) stainless steel is allowed, but must be carefully processed to prevent SSC. The
standard now gives two different acceptable heat treatments for S17400. One treatment is the double H1150
heat treatment which requires exposing the material at 1150°F (621°C) for four hours followed by air cooling
and then exposing for another four hours at 1150°F (621°C). A maximum hardness level of 33 HRC is
specified. The second heat treatment is the H1150M treatment. First, the material is exposed for two hours at
1400°F (760°C), then air cooled and exposed for four hours at 1150°F (621°C). The maximum hardness level
is the same for this condition.

CB7Cu-1 (Cast 17-4PH) is not approved per MR0175. However, many users have successfully applied it for
trim parts in past years in the same double heat treated conditions as the wrought form.

Two high strength stainless steel grades are acceptable for MR0175. The first is S66286 (grade 660 or A286)
which is a precipitation hardening alloy with excellent resistance to SSC and general corrosion. The maximum
hardness level permitted is 35 HRC.

The second material is S20910 (XM-19) which is commonly called Nitronic 50R. This high strength stainless
steel has excellent resistance to SSC and corrosion resistance superior to S31600 or S31700. The maximum
allowable hardness is 35 HRC. The "high strength" condition, which approaches 35 HRC, can only be
produced by hot working methods. Cold drawn S20910 is also acceptable for shafts, stems, and pins. It is our
experience that the SSC resistance of S20910 is far superior to S17400 or other austenitic stainless steels at
similar hardness levels. The only other materials with similar stress cracking resistance at these strength levels
are the nickel-based alloys which are, of course, much more expensive. A few duplex stainless steels are now
acceptable per MR0175. Wrought S31803 (2205) and S32550 (Ferralium 255) are acceptable to 28 HRC.
Wrought S32404 (Uranus 50) is acceptable to 20 HRC. Only one cast duplex stainless steel is acceptable,
alloy Z 6CNDU20.08M, NF A 320-55 French National Standard.


Nonferrous Alloys

The final category in MR0175 is the nonferrous materials section. In general, the nickel-based alloys are
acceptable to a maximum hardness level of 35 HRC. All have excellent resistance to SSC. Commonly used
acceptable materials include nickel-copper alloys N04400 (alloy 400) and N04405 (alloy 405) and the

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precipitation hardening alloy N05500 (K500). The nickel-iron-chromium alloys include alloys N06600 (alloy 600)
and N07750 (alloy X750). The acceptable nickel-chromium-molybdenum alloys include alloys N06625 (alloy
625), and N10276 (alloy C276). The precipitation hardening grade N07718 (alloy 718) is also acceptable to 40
HRC. Where high strength levels are required along with good machinability, The Emerson Process
Management Regulator Division uses N05500, N07718, N07750, or N09925 (alloy 925). They can be drilled or
turned, then age hardened. Several cobalt based materials are acceptable, including R30035 (alloy MP35N),
R30003 (Elgiloy), and R30605 (Haynes 25 or L605).

Aluminum based and copper alloys may be used for sour service, but the user is cautioned that severe
corrosion attack may occur on these materials. They are seldom used in direct contact with H2S.

Several wrought titanium grades are now included in MR0175. The only common industrial alloy is R50400
(grade 2).


Springs

Springs in compliance with NACE represent a difficult problem. To function properly, springs must have very
high strength (hardness) levels. Normal steel and stainless steel springs would be very susceptible to SSC and
fail to meet MR0175.

In general, very soft, low strength materials must be used. Of course, these materials produce poor springs.
The two exceptions allowed are the cobalt based alloys, such as R30003, which may be cold worked and
hardened to a maximum hardness of 60 HRC and alloy N07750 which is permitted to 50 HRC.


Coatings

Coatings, platings, and overlays may be used provided the base metal is in a condition which is acceptable per
MR0175. The coatings may not be used to protect a base material which is susceptible to SSC. Coatings
commonly used in sour service are chromium plating, electroless nickel (ENC) and ion nitriding. Overlays and
castings commonly used include CoCr-A (StelliteR or alloy 6), R30006 (alloy 6B), and NiCr-C (ColmonoyR 6)
nickel-chromium-boron alloys. Tungsten carbide alloys are acceptable in the cast, cemented, or thermally
sprayed conditions. Ceramic coatings such as plasma sprayed chromium oxide are also acceptable.

ENC is often used by the Emerson Process Management Regulator Division as a wear-resistant coating. As
required by MR0175, it is applied only to acceptable base metals. ENC has excellent corrosion resistance in
sour, salt containing environments.


Stress Relieving

Many people have the misunderstanding that stress relieving following machining is required by MR0175.
Provided good machining practices are followed using sharp tools and proper lubrication, the amount of cold
work produced is negligible. SSC resistance will not be affected. MR0175 actually permits the cold rolling of
threads, provided the component will meet the heat treat conditions and hardness requirements specified for
the given parent material. Cold deformation processes such as burnishing are also acceptable.

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Bolting

Bolting materials must meet the requirements of MR0175 when bolting is directly exposed to a sour
environment. Standard ASTM A193 grade B7 bolts or A194 grade 2H nuts can be used per MR0175 provided
they are outside of the sour environment. If the bolting will be deprived atmospheric contact by burial,
insulation, or flange protectors, then grades of bolting such as B7 and 2H are unacceptable. The most
commonly used fasteners for "exposed" applications are ASTM A193 grade B7M bolts and A194 grade 2M
nuts. They are tempered and hardness tested versions of the B7 and 2H grades. HRC 22 is the maximum
allowable hardness.

Many customers use only B7M bolting for bonnet, packing box, and flange joints. This reduces the likelihood of
SSC if a leak develops and goes undetected or unrepaired for an extended time. It must be remembered,
however, that use of lower strength bolting materials such as B7M often requires pressure vessel derating.


Composition Materials

MR0175 does not address elastomer and polymer materials. However, the importance of these materials in
critical sealing functions cannot be overlooked. User experience has been successful with elastomers such as
nitrile, neoprene, fluoroelastomer (FKM), and perfluoroelastomer (FFKM). In general, fluoropolymers such as
teflon (TFE) can be applied without reservation within their normal temperature range.


Codes and Standards

Applicable ASTM, ANSI, ASME, and API standards are used along with MR0175 as they would normally be
used for other applications. The MR0175 requires that all weld procedures be qualified to these same
standards. Welders must be familiar with the procedures and capable of making welds which comply.


The Commercial Application of NACE

Special documentation of materials to MR0175 is not required by the standard and NACE itself does not issue
any type of a certification. It is the producer's responsibility to properly monitor the materials and processes as
required by MR0175.

It is not uncommon for manufacturers to "upgrade" standard manufactured components to MR0175 by
hardness testing. This produces a product which complies with MR0175, but which may not provide the best
solution for the long-term. If the construction was not thoroughly recorded at the outset, it may be difficult to get
replacement parts in the proper materials. The testing necessary to establish that each part complies is quite
expensive. And, due to the "local" nature of a hardness test, there is also some risk that "upgraded" parts do
not fully comply.

With proper in-house systems, it is quite simple to confidently produce a construction which can be certified to
MR0175 without the necessity of after manufacture testing. This eliminates many costly extras and additionally
provides a complete record of the construction for future parts procurement. An order entry, procurement, and



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manufacturing system which is integrated and highly structured is required in order to confidently and
automatically provide equipment which complies.

Due to its hierarchical nature and its use by all company functions, the Emerson Process Management
Regulator Division system is ideal for items such as MR0175 which requires a moderate degree of control
without undue cost. In order to illustrate the system used by the Emerson Process Management Regulator
Division, an example will be used.

Most products produced by the Emerson Process Management Regulator Division (including products to
MR0175) will be specified by a Fisher Standard (FS) number. These numbers (e.g. FSED-542) completely
specify a standardized construction including size, materials, and other characteristics. The FS number is a
short notation which represents a series of part groups (modules) describing the construction. One module
may represent a 3-inch WCB valve body with ANSI Class 300 flanges, another may specify a certain valve
plug and seat ring. The part numbers which make up these modules are composed of a drawing number and a
material/finish identifier. The drawing describes the dimensions and methods used to make the part, while the
material/finish reference considers material chemistry, form, heat treatment, and a variety of other variables.
The part number definition also includes a very specific "material reference number" which is used to identify a
material specification for purchase of materials. The material specification includes the ASME designation as
well as additional qualifiers, as necessary, to ensure compliance with specifications such as NACE MR0175.

For NACE compliant products, an FS number and a NACE option are generally specified. The FS number
establishes the standard construction variation. The option modifies the construction and materials to comply
totally with MR0175 requirements. The option eliminates certain standard modules and replaces them with
NACE suitable modules. Each part in a NACE suitable module has been checked to assure that it complies to
the specification in form and manufacturing method and that it is produced from an appropriate material.

It is due to this top-to-bottom system integrity that the Emerson Process Management Regulator Division can
be confident of MR0175 compliance without the need for extensive test work. At each level of the system
documentation, there are specific references to and requirements for compliance to MR0175. Further, since
the construction is permanently documented at all levels of detail, it is possible to confidently and simply
procure replacement parts at any future date.

Test documentation is available in a variety of forms, including certificates of compliance, hardness test data,
chemical and physical test reports, and heat treat reports. Since these items will have some cost associated
with them, it is important to examine the need for documentation in light of the vendor's credibility and
manufacturing control systems. The Emerson Process Management Regulator Division's normal
manufacturing processes and procedures assure that all NACE specified products will comply without the need
for additional test expense.




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SRB (Microbial Influenced Corrosion)



Microbe Information

The mechanisms commonly thought to be involved in MIC include:

    •     Cathodic depolarization, whereby the cathodic rate limiting step is accelerated by micro-biological
          action.

    •     Formation of occluded surface cells, whereby microorganisms form "patchy" surface colonies. Sticky
          polymers attract and aggregate biological and non-biological species to produce crevices and
          concentration cells, the basis for accelerated attack.

    •     Fixing of anodic reaction sites, whereby microbiological surface colonies lead to the formation of
          corrosion pits, driven by microbial activity and associated with the location of these colonies.

    •     Under-deposit acid attack, whereby corrosive attack is accelerated by acidic final products of the MIC
          "community metabolism", principally short-chain fatty acids.

Microbes fall into two basic groups, aerobic and anaerobic. These two groups are based on the kind of
environment they prefer, either with or without oxygen. Slime formers form a diverse group of aerobic bacteria.
Common anaerobic bacteria include Sulfur/sulfate reducing bacteria (SRB's) and organic acid formers.

Microbes tend to form colonies, with different characteristics from the outside to inside. On the outside,
"slimers" may produce polymers (slime) that attract inorganic material, making the colony look like a pile of
mud and debris. These aerobic organisms can efficiently use up all available oxygen, giving anaerobic
microbes (SRB's) inside the colony a hospitable environment, allowing enhanced corrosion under the colony.

Microbially influenced corrosion (MIC) is a special danger when steels or alloys of aluminum and copper are in
constant contact with nearly neutral water, of pH 4 to 9, 50° to 122°F (10° to 50°C), especially when stagnant.
Microbially influenced corrosion mostly takes the form of pitting corrosion.

Corrosion products and effects include iron sulfates, slime, plugging, and bacteria growths. Sulfate-reducing
                                                                        2-
bacteria (SRB) are anaerobic bacteria which metabolize sulfates (SO4 ) and produce sulfuric acids or H2S,
thus introducing hydrogen sulfide into the system. SRB colonies can also form deposits that are conducive to
under-deposit corrosion (crevice corrosion.)


Where Found

Water storage tanks are a common site where MIC occurs. SRB's can contaminate tanks, which must then be
cleaned and sterilized because it is impossible for biocides to penetrate the large amounts of sludge and
debris in tank bottoms. Flow lines are another common MIC site, especially at the bottom of the line where
water accumulates. MIC has also been detected at the 3 o'clock and 9 o'clock positions, presumably at the oil
and water interface.


Prevention / Mitigation

To reduce or prevent microbial corrosion:
Drilling - biocides
Production - biocides, chlorine dioxide
Flowlines - biocides, chlorine dioxide
Cost considerations - Continuous vs. batch; EPA; biostat vs. biocide




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                                                     Bacterial attack is usually characterized
                                                     by rounded pits with etched sides,
                                                     edges, and bottoms.




                                                     MIC pits often have a terraced effect.




                                                     Although MIC normally occurs at the
                                                     bottom of the line where water
                                                     accumulates, it has also been detected
                                                     at the 3 o'clock and 9 o'clock positions,
                                                     presumably at the oil and water
                                                     interface.




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                                                                         One of the quick texts for SRB is the
                                                                         pipe cleaner test. Positive results are
                                                                         shown in these examples.




Bacterial growths tend to thrive in a downhole environment. Bacteria tend to propagate faster in the presence
of water or liquid. There are many tests that can be run to determine the presence or absence of sulfate
reducing bacteria (SRB), acid producing bacteria (APB), and general heterotopic bacteria (GHB). Also, the
presence of aerobes and anaerobes can be determined.

The topography of microorganism influenced corrosion pits includes the following characteristics:
    •     volcano-shaped craters

    •     bulls-eye patterns

    •     terraced sidewalls

    •     sloping edges

    •     etched edges

Bacterial counts are usually reported to the nearest power of 10. Hence, there could be 100 to 1000 colonies
per milliliter of SRB, 10 to 100 colonies per milliliter of APB, and 1000 to 10000 colonies per milliliter of GHB.
Additional counts can be given for aerobes and anaerobes. Note that some testing facilities will only report one
figure, for example, 1000 colonies per milliliter of SRB. This should be taken as the upper limit, and would
equate to 100 to 1000 colonies per milliliter of SRB.

Bacteria is usually controlled by the addition of biocide to the system. Biocides help to reduce the bacterial
counts, and hence control Microorganism Influenced Corrosion. Note that the selection of a particular biocide
should be based on compatibility, cost, and other pertinent factors.


  More on MIC




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http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/corrosion.html




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Chapter 2:
Forms of Corrosion
The forms of corrosion described here use the terminology in use at NASA-KSC. There are other equally
valid methods of classifying corrosion, and no universally-accepted terminology is in use. Keep in mind that a
given situation may lead to several forms of corrosion on the same piece of material.



     Illustration                                                            Form of Corrosion

                                                                            Uniform Corrosion

                                                                            This is also called general
                                                                            corrosion.   The    surface
                                                                            effect produced by most
                                                                            direct chemical attacks
                                                                            (e.g., as by an acid) is a
                                                                            uniform etching of the
                                                                            metal.




                                                                            Galvanic Corrosion
                                                                            Galvanic corrosion is an
                                                                            electrochemical action of
                                                                            two dissimilar metals in the
                                                                            presence of an electrolyte
                                                                            and an electron conductive
                                                                            path. It occurs when
                                                                            dissimilar metals are in
                                                                            contact.




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                                                     Concentration Cell
                                                     Corrosion

                                                     Concentration cell corrosion
                                                     occurs when two or more
                                                     areas of a metal surface are
                                                     in contact with different
                                                     concentrations of the same
                                                     solution.




                                                     Pitting Corrosion
                                                     Pitting corrosion is localized
                                                     corrosion that occurs at
                                                     microscopic defects on a
                                                     metal surface. The pits are
                                                     often found underneath
                                                     surface deposits caused by
                                                     corrosion product
                                                     accumulation.




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                                                     Crevice Corrosion
                                                     Crevice or contact corrosion
                                                     is the corrosion produced at
                                                     the region of contact of
                                                     metals with metals or
                                                     metals with nonmetals. It
                                                     may occur at washers,
                                                     under barnacles, at sand
                                                     grains, under applied
                                                     protective films, and at
                                                     pockets formed by threaded
                                                     joints.




                                                     Filiform Corrosion
                                                     This type of corrosion
                                                     occurs on painted or plated
                                                     surfaces when moisture
                                                     permeates the coating.
                                                     Long branching filaments of
                                                     corrosion product extend
                                                     out from the original
                                                     corrosion pit and cause
                                                     degradation of the
                                                     protective coating.




                                                     Intergranular Corrosion
                                                     Intergranular corrosion is an
                                                     attack on or adjacent to the
                                                     grain boundaries of a metal
                                                     or alloy.




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                                                     Stress Corrosion
                                                     Cracking
                                                     Stress corrosion cracking
                                                     (SCC) is caused by the
                                                     simultaneous effects of
                                                     tensile stress and a specific
                                                     corrosive environment.
                                                     Stresses may be due to
                                                     applied loads, residual
                                                     stresses from the
                                                     manufacturing process, or a
                                                     combination of both.




                                                     Corrosion Fatigue
                                                     Corrosion fatigue is a
                                                     special case of stress
                                                     corrosion caused by the
                                                     combined effects of cyclic
                                                     stress and corrosion. No
                                                     metal is immune from some
                                                     reduction of its resistance to
                                                     cyclic stressing if the metal
                                                     is in a corrosive
                                                     environment.




                                                     Fretting Corrosion
                                                     The rapid corrosion that
                                                     occurs at the interface
                                                     between contacting, highly
                                                     loaded metal surfaces when
                                                     subjected to slight vibratory
                                                     motions is known as fretting
                                                     corrosion.




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                                                     Erosion Corrosion

                                                     Erosion corrosion is the
                                                     result of a combination of
                                                     an aggressive chemical
                                                     environment and high fluid-
                                                     surface velocities.




                                                     Dealloying
                                                     Dealloying is a rare form of
                                                     corrosion found in copper
                                                     alloys, gray cast iron, and
                                                     some other alloys.
                                                     Dealloying occurs when the
                                                     alloy loses the active
                                                     component of the metal and
                                                     retains the more corrosion
                                                     resistant component in a
                                                     porous "sponge" on the
                                                     metal surface.




                                                     Hydrogen Damage
                                                     Hydrogen embrittlement is
                                                     a problem with high-
                                                     strength steels, titanium,
                                                     and some other metals.
                                                     Control is by eliminating
                                                     hydrogen from the
                                                     environment or by the use
                                                     of resistant alloys.




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                                                     Corrosion in Concrete
                                                     Concrete is a widely-used
                                                     structural material that is
                                                     frequently reinforced with
                                                     carbon steel reinforcing
                                                     rods, post-tensioning cable
                                                     or prestressing wires. The
                                                     steel is necessary to
                                                     maintain the strength of the
                                                     structure, but it is subject to
                                                     corrosion.




                                                     Microbial Corrosion

                                                     Microbial corrosion (also
                                                     called microbiologically -
                                                     influenced corrosion or
                                                     MIC) is corrosion that is
                                                     caused by the presence
                                                     and activities of microbes.
                                                     This corrosion can take
                                                     many forms and can be
                                                     controlled by biocides or by
                                                     conventional corrosion
                                                     control methods.




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                           Uniform / General Corrosion
This is also called general corrosion. The surface effect produced by most direct chemical attacks (e.g., as by
an acid) is a uniform etching of the metal. On a polished surface, this type of corrosion is first seen as a
general dulling of the surface and, if allowed to continue, the surface becomes rough and possibly frosted in
appearance. The discoloration or general dulling of metal created by its exposure to elevated temperatures is
not to be considered as uniform etch corrosion. The use of chemical-resistant protective coatings or more
resistant materials will control these problems.

While this is the most common form of corrosion, it is generally of little engineering significance, because
structures will normally become unsightly and attract maintenance long before they become structurally
affected. The facilities shown in the picture below show how this corrosion can progress if control measures
are not taken.

Uniform corrosion is the regular, uniform removal of metal from a surface. In uniform corrosion, microscopic
anodic areas (where metal dissolution occur), and cathodic areas (where hydrogen evolution or oxygen
reduction occur), frequently alternate. If, however, impurities are present on the metal surface, such as carbide
precipitates, then corrosion can be localized around the precipitate.

In the oilfield, uniform corrosion may be observed in tubing and sucker rods, possibly following an acidizing
treatment.

The rate of uniform corrosion can be calculated as shown in the example below. Uniform corrosion is usually
measured in mpy (mils per year, 1 mil = 1/1000 inch).


Example: A steel coupon of 4 x 2 x 1/8 inches is placed in an acid solution for one week, and loses 90 mg.
Calculate the rate of corrosion in mpy. Assume that steel is iron only.

Surface Area = 2(4 in x 2 in) + 2(4 in x 1/8 in) + 2(2 in x 1/8 in) = 17.5 in2

90 mg Fe           x 1 cm3          x (365 days) x 1 in x 1000 mil
        2
(17.5 in )(7 days) 7870 mg Fe         1 year    2.54 cm3 1 in

= 2 mpy


The following is an example of uniform corrosion caused by CO2.




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DESCRIPTION

General attack is typically caused by uniform general corrosion. Uniform corrosion can be described as follows:
Corrosion reaction that takes place uniformly over the surface of the material, thereby causing a general
thinning of the component and an eventual failure of the material.

Prevention or Remedial Action

    •     selection of a more corrosion resistant alloy (i.e. higher alloy content or more inert alloy)

    •     Utilize coatings to act as a barrier between metal and environment.

    •     Modify the environment or add chemical inhibitors to reduce corrosion rate.

    •     Apply cathodic protection.

    •     Replace with corrosion resistant non-metallic material.

Standard Test Methods

    •     ASTM G-31 - laboratory immersion corrosion testing of metals.

    •     ASTM G-4 - corrosion coupon tests in plant equipment.

    •     ASTM G-54 - practice for simple static oxidation testing.

    •     ASTM G-59 - practice for conducting potentiodynamic polarization resistance measurements.

    •     NACE TM0169 - laboratory corrosion testing of metals for the process industries.

    •     NACE TM0274 - dynamic corrosion testing of metals in high temperature water.

    •     ASTM B-117 - salt fog testing.

    •     ASTM G-85 - modified salt spray (fog) testing.

    •     ASTM D-2776 - test for corrosivity of water in the absence of heat transfer, by electrical methods.

    •     ASTM D-2688 - test for corrosivity of water in the absence of heat transfer, by weight loss methods.

    •     ASTM G-91 - test method of monitoring atmospheric SO2 using the sulfation plate technique.

Evaluation of General Corrosion

The predominant standard utilized for general corrosion assessment is ASTM G31. This standard gives
guidelines for conducted simple immersion corrosion tests. Important considerations when conducting such
tests in either the laboratory, field or plant setting are:

    •     Adequate solution volume for the surface area of corroding specimens in test.

    •     Electric isolation of the specimens from other specimens and any dissimilar metals in the system.

    •     Exposure of specimens to more than one phase, if applicable, since corrosion rates can change
          substantially in the different phases especially as water and impurity contents vary.

    •     Other test conditions such as flow rate, temperature, and aeration can produce variable results and
          locally high corrosion rates.

Methods of specimen surface preparation and post-test cleaning should be controlled as defined in the test
standards.




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                                     Galvanic Corrosion
Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and
an electron conductive path. It occurs when dissimilar metals are in contact.

It is recognizable by the presence of a buildup of corrosion at the joint between the dissimilar metals. For
example, when aluminum alloys or magnesium alloys are in contact with steel (carbon steel or stainless steel),
galvanic corrosion can occur and accelerate the corrosion of the aluminum or magnesium. This can be seen
on the photo above where the aluminum helicopter blade has corroded near where it was in contact with a
steel counterbalance.




Galvanic corrosion can be defined simply as being the effect resulting from contact between two different
metals or alloys in a conducting corrosive environment. Another term employed is galvanic coupling.



When a metal is immersed in any electrolytic solution, it is possible to measure its dissolution (natural
corrosion). For each solution, it is possible to establish a "galvanic series", that is, a ranking of different metals
and alloys as a function of this measured potential. When two different metals or alloys immersed in the same
solution are joined together electrically, an electric current will be set up between them, resulting from the short
circuit created. The coupling potential must of necessity lie between the two potentials for the uncoupled
metals and an increase in corrosion is generally observed in the less noble alloy and a decrease or
suppression of corrosion in the more noble material.

Due to modifications in the electrolyte, inversions may occur in the potential series. Thus, zinc covered with
corrosion products can become more "noble" than iron in certain hot waters (problem encountered in domestic



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hot water tanks); tin can become less "noble" than iron in organic acid solutions (problem encountered in food
cans).

For a given current between two different metals, the current density, and hence the rate of dissolution of the
less noble metal (anode,) will be greater the smaller the surface area SA of the anode. The use of unfavorable
surface area ratios has led to many expensive and often spectacular failures.
Some Means of preventing galvanic corrosion : choose metal combinations in which the constituents are as
close as possible in the corresponding galvanic series, avoid an unfavorable surface area ratio. Wherever
possible, use a seal, insulator, coating, etc. to avoid direct contact between two different metals, avoid
threaded junctions between materials widely separated in the galvanic series,




Galvanic Series in Sea Water

Noble
(least active)

Platinum
Gold
Graphite
Silver
18-8-3 Stainless steel, type 316 (passive)
18-8 Stainless steel, type 304 (passive)
Titanium
13 percent chromium stainless steel, type 410 (passive)
7NI-33Cu alloy
75NI-16Cr-7Fe alloy (passive)
Nickel (passive)
Silver solder
M-Bronze
G-Bronze
70-30 cupro-nickel
Silicon bronze
Copper
Red brass
Aluminum bronze
Admiralty brass
Yellow brass
76NI-16Cr-7Fe alloy (active)
Nickel (active)
Naval brass
Manganese bronze
Muntz metal
Tin
Lead
18-8-3 Stainless steel, type 316 (active)
18-8 Stainless steel, type 304 (active)
13 percent chromium stainless steel, type 410 (active)
Cast iron
Mild steel
Aluminum 2024
Cadmium
Alclad
Aluminum 6053
Galvanized steel
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Zinc
Magnesium alloys
Magnesium

Anodic
(most active)




                                                                            The natural differences in metal
                                                                            potentials produce galvanic
                                                                            differences, such as the galvanic
                                                                            series in sea water. If electrical
                                                                            contact is made between any two of
                                                                            these materials in the presence of
                                                                            an electrolyte, current must flow
                                                                            between them. The farther apart
                                                                            the metals are in the galvanic
                                                                            series, the greater the galvanic
                                                                            corrosion effect or rate will be.
                                                                            Metals or alloys at the upper end
                                                                            are noble while those at the lower
                                                                            end are active. The more active
                                                                            metal is the anode or the one that
                                                                            will corrode.

                                                                            Control of galvanic corrosion is
                                                                            achieved by using metals closer to
                                                                            each other in the galvanic series or
                                                                            by electrically isolating metals from
                                                                            each other. Cathodic protection can
                                                                            also be used to control galvanic
                                                                            corrosion effects.




Copper connected to steel resulted in this galvanic corrosion.

The scuba tank above suffered galvanic corrosion when the brass valve and the steel tank were wetted by
condensation. Electrical isolation flanges like those shown on the right are used to prevent galvanic corrosion.
Insulating gaskets, usually polymers, are inserted between the flanges, and insulating sleeves and washers
isolate the bolted connections.

The photo below shows the corrosion caused by a stainless steel screw causing galvanic corrosion of
aluminum. The picture shows the corrosion resulting from only six months exposure at the Atmospheric Test
Site.

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Galvanic corrosion occurs when two dissimilar metals are connected electrically and are in contact with an
electrolyte solution. One of the two metals is corroded preferentially; this metal is the anode and the un-
attacked metal is the cathode in the galvanic couple.




One example found in the oilfield is when a new section of pipe is added to an older section. The new pipe
becomes anodic and corrodes preferentially.

                                                              The Galvanic Series is a list sorted by corrosion
                                                              potentials for various alloys and pure metals in sea
                                                              water. It should not be confused with the emf
                                                              series. The emf series is a list of half-cell
                                                              potentials for standard state conditions measured
                                                              with respect to the standard hydrogen electrode,
                                                              while the Galvanic Series is based on corrosion
                                                              potentials in sea water.

                                                              Each metal or alloy has a unique corrosion
                                                              potential, Ecorr, when immersed in a corrosive
                                                              electrolyte. The most negative or active alloy is
                                                              always attacked preferentially by galvanic
                                                              corrosion, whereas the more noble metal becomes
                                                              cathodic (where reduction of hydrogen ions or
oxygen takes place) and is protected from corrosion.

Often the relative areas of each metal exposed are more important than their position in the galvanic series. If
the anode (more active metal) has a large area with respect to the cathode (more noble metal), the small area
of the cathode will not provide enough current to support uniform corrosion of the anode. However, if the
anode is small in comparison to the cathode, the rate of corrosion of the anode will be greatly accelerated and
corrosion will be localized adjacent to the more noble metal. When using coatings to prevent galvanic
corrosion, it is important to coat the more noble metal rather than the active metal, so that when defects are
introduced to the coat, the effects are not catastrophic.

There are some well-known examples of bimetallic (galvanic) corrosion. For example, N-80 couplings
connected to J-55 tubing always corrode preferentially to the J-55 grade at fairly rapid rates in wet CO2



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environment. Stainless steel valve in cast steel body also create a galvanic couple. Corrosion occurs
immediately adjacent to the more noble metal.

                                                                            Galvanic corrosion is also frequently
                                                                            observed in downhole pumps. Pump
                                                                            barrels, balls and cages are usually
                                                                            made of different alloys that may form
                                                                            galvanic couples. Pump barrels are
                                                                            also chromium plated for increased
                                                                            abrasion resistance. However,
                                                                            chromium plate may be scored by
                                                                            sand grains or crack, which leads to
                                                                            severe galvanic corrosion that is rapid
                                                                            and usually catastrophic. Electro less
                                                                            nickel plating also suffers from
                                                                            galvanic effects

                                                                            There are many subsets of galvanic
corrosion. A piece of metal is not uniform on the micro-scale, but contains grain boundaries and precipitates.
These precipitates are electrochemically different from the base metal, and may act as cathodes or anodes
with respect to the base metal.

Stainless steel, an alloy of chromium (Cr), nickel and iron, requires at least 12% Cr for passivity. If stainless
steel is heated to a high temperature (such as 425 C), chromium carbide precipitates will start to form along
grain boundaries, leaving a zone depleted of chromium. The precipitates will dissolve back into the grain
structure when heated above 850 C and fast cooled (quenched) back to room temperature.

Stainless steel may become sensitized during welding. The area surrounding the weld bead is known as a
heat affected zone (HAZ), a zone depleted of chromium, which will preferentially dissolve away. Therefore,
post-welding heat treatment or the use of low-carbon varieties is needed to prevent grain boundary corrosion.

The following picture shows a weld at the granular level:

                                                                        Another well-known example of HAZ
                                                                        corrosion in wet CO2 service is the failure
                                                                        of upset J-55 tubing that has not been full-
                                                                        length normalized (heat treated) after
                                                                        upsetting. This form is known as
                                                                        “ringworm” corrosion and it usually occurs
                                                                        4-6 inches below the upset in the heat-
                                                                        affected zone that has a different
                                                                        microstructure from the rest of the tubing.




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Aluminum Galvanic Corrosion: The aluminum coupled to carbon steel in this hot water system corroded badly
due to the galvanic couple.

Minimizing the Effect of Galvanic Attack


Galvanic attack can be minimized, as can other forms of corrosion, by correct design. The use of galvanically
compatible materials and the use of electrical insulation between dissimilar materials will help. Not coating
the anodic surface in case of pinhole damage to it is also useful as this could give rapid local attack.

The galvanic effect is the reason why different phases and segregated regions in alloy microstructures will
have varying resistance to corrosion. This effect is made good use of when polished specimens are selectively
attacked by etching in order to reveal and study microstructures features under the microscope. In stainless
steels Cr-depleted zones around Cr-rich second phases will be less noble and as such will be subject to highly
localized attack leading to inter-dendritic and/or intergranular forms of corrosion




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Stray Current Corrosion Electrical appliances were grounded to this gas pipeline. The stray currents led to
localized attack.

Testing Description

Accelerated corrosion which can occur when dissimilar metals are in electrical contact in the presence of an
electrolyte (i.e. conductive solution). An example of this corrosion phenomenon is increased rate of corrosion
of steel in seawater when in contact with copper alloys. Galvanic attack can be uniform in nature or localized at
the junction between the alloys depending on service conditions. Galvanic corrosion can be particularly severe
under conditions where protective corrosion films do not form or where they are removed by conditions of
erosion corrosion.

Prevention or Remedial Action

    •     selection of alloys which are similar in electrochemical behavior and/or alloy content.

    •     area ratio of more actively corroding material (anode) should be large relative to the more inert
          material (cathode).

    •     use coatings to limit cathode area.

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    •     insulate dissimilar metals.

    •     use of effective inhibitor.

Standard Test Methods

    •     ASTM G-71 - guide for conducting and evaluating galvanic corrosion tests in electrolytes.

    •     ASTM G-82 - guide for development and use of a galvanic series for predicting galvanic corrosion
          performance.

    •     ASTM G-104 - test method for assessing galvanic corrosion caused by the atmosphere.

Evaluation for Galvanic Corrosion

Many people utilized the standard galvanic series of materials in seawater to predict service performance
relative to galvanic corrosion. In fact, this galvanic series is specific to only seawater at near ambient
conditions. Other factors such as temperature and the presence of other chemical species can greatly affect
the rank ordering of materials. Such differences in environmental conditions can reverse galvanic couples
whereby the material expected to be the cathode may actually be the anode and experience severe corrosion.
In making galvanic corrosion measurements, it is good practice to try to separate the effects if crevices
between contacting materials and actual galvanic corrosion. This is the reason that in many tests, the
actual electrical coupling of the two materials is performed in a region protected from the environment or
externally from the environment. The external coupling is a good idea since it allows for measurement of
the mixed potential of the couple and the galvanic corrosion current. While the potential serves as a
measure of the thermodynamic driving force for galvanic corrosion, it is the galvanic corrosion current that
indicates the acceleration of corrosion by the influence of the galvanic couple.




More reading:




Galvanic Corrosion

   http://www.key-to-steel.com/Articles/Art160.htm
   http://www.corrosionclinic.com/types_of_corrosion/galvanic_corrosion.htm
   http://www.roymech.co.uk/Useful_Tables/Corrosion/Cor_bi_met.html




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                        Concentration Cell Corrosion
Concentration cell corrosion occurs when two or more areas of a metal surface are in contact with different
concentrations of the same solution. There are three general types of concentration cell corrosion:
       1. metal ion concentration cells
       2. oxygen concentration cells, and
       3. active-passive cells.

Metal Ion Concentration Cells

In the presence of water, a high concentration of metal ions will exist under faying surfaces and a low
concentration of metal ions will exist adjacent to the crevice created by the faying surfaces. An electrical
potential will exist between the two points. The area of the metal in contact with the low concentration of metal
ions will be cathodic and will be protected, and the area of metal in contact with the high metal ion
concentration will be anodic and corroded. This condition can be eliminated by sealing the faying surfaces in a
manner to exclude moisture. Proper protective coating application with inorganic zinc primers is also effective
in reducing faying surface corrosion.


Oxygen Concentration Cells


A water solution in contact with the metal surface will normally contain dissolved oxygen. An oxygen cell can
develop at any point where the oxygen in the air is not allowed to diffuse uniformly into the solution, thereby
creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration
cells are under either metallic or nonmetallic deposits (dirt) on the metal surface and under faying surfaces
such as riveted lap joints. Oxygen cells can also develop under gaskets, wood, rubber, plastic tape, and other
materials in contact with the metal surface. Corrosion will occur at the area of low-oxygen concentration
(anode). The severity of corrosion due to these conditions can be minimized by sealing, maintaining surfaces
clean, and avoiding the use of material that permits wicking of moisture between faying surfaces.


Active-Passive Cells


Metals that depend on a tightly adhering passive film (usually an oxide) for corrosion protection; e.g., austenitic
corrosion-resistant steel, can be corroded by active-passive cells. The corrosive action usually starts as an
oxygen concentration cell; e.g., salt deposits on the metal surface in the presence of water containing oxygen
can create the oxygen cell. If the passive film is broken beneath the salt deposit, the active metal beneath the
film will be exposed to corrosive attack. An electrical potential will develop between the large area of the
cathode (passive film) and the small area of the anode (active metal). Rapid pitting of the active metal will
result. This type of corrosion can be avoided by frequent cleaning and by application of protective coatings.




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                                        Pitting Corrosion
Passive metals, such as stainless steel, resist corrosive media and can perform well over long periods of time.
However, if corrosion does occur, it forms at random in pits. Pitting is most likely to occur in the presence of
chloride ions, combined with such depolarizers as oxygen or oxidizing salts. Methods that can be used to
control pitting include maintaining clean surfaces, application of a protective coating, and use of inhibitors or
cathodic protection for immersion service. Molybdenum additions to stainless steel (e.g. in 316 stainless) are
intended to reduce pitting corrosion.




The rust bubbles or tubercules on the cast iron above indicate that pitting is occurring. Researchers have
found that the environment inside the rust bubbles is almost always higher in chlorides and lower in pH (more
acidic) than the overall external environment. This leads to concentrated attack inside the pits.




Similar changes in environment occur inside crevices, stress corrosion cracks, and corrosion fatigue cracks. All
of these forms of corrosion are sometimes included in the term "occluded cell corrosion."




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Pitting corrosion can lead to unexpected catastrophic system failure. The split tubing above left was caused by
pitting corrosion of stainless steel. A typical pit on this tubing is shown above right.

Sometimes pitting corrosion can be quite small on the surface and very large below the surface. The figure
below left shows this effect, which is common on stainless steels and other film-protected metals. The pitting
shown below right (white arrow) led to the stress corrosion fracture shown by the black arrows.




Pitting Corrosion on Metal Surface


Pitting is one of the most destructive forms of corrosion as it will potential cause equipment failures due to
perforation / penetration. pitting generally occurs on metal surfaces protected by oxide film such as Stainless
steel, aluminum, etc. Typically for boiler and feed water system, pitting corrosion rate increase dramatically
with the increase of oxygen content in the fluid.




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Pitting can occur in any metal surfaces. Following are some pictures of pitting corrosion.




Pitting corrosion on external pipe surface




Pitting corrosion on external pipe surface




H2S Pitting corrosion on internal pipe surface




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Co2 Pitting corrosion on internal pipe surface

Mechanism
Lets look at figure below, oxygen rich fluid in contact with metal surface (at the top of the pit) will becomes
the cathode. At the bottom of the pit, low in oxygen level becomes the anode. this will form a complete
circuit where metal at the pit (FE) will be ionized to release electron (e) and form ion Ferum (FE2+), this
electron will travel to the top of pit to react with Oxygen (O2) (and water, H2O) to form ion hydroxides (OH-).
Ion Ferum (FE2+) will react with ion hydroxides (OH-) to form Ferum Oxide (Fe2O3) which typically a brown
rust. Deeper the pit leeser the oxygen content and higher the potential and pitting corrosion rate.




Severity of pitting corrosion
Knowing that pitting can cause failure due to perforation while the total corrosion, as measured by weight
lossm might be rather minimal, experience shown that rate of penetration may be 10 to 100 times that by
general corrosion, pitting corrosion has been considered to be more dangerous than the uniform
corrosion damage because it is very difficult to detect, predict and design against. General metal weight
loss method almost impossible to detect the internal pitting corrosion.


Pitting corrosion shape
Pits formed due to pitting corrosion can become wide and shallow or narrow and deep which can rapidly
perforate the wall thickness of a metal. Following picture demonstrate several types of pitting corrosion shape.
This has made it even more difficult to be detected especially undercutting, subsuface and horizontal type.

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Different Equation for Pitting Resistance Equivalent Number (PREN)




pitting corrosion is one of the most common localized corrosion attack and most destructive form of corrosion
in metal and alloy. Out of so many type of alloy, how to differential the pitting resistivity of particular metal and
alloy compare to the other? Pitting Resistance Equivalent Number is used.
Pitting Resistance Equivalent Number (PREN) is an index common used to measure and compare resistance
level of a particular metal and alloy to pitting corrosion.


PREN can be calculated, using the alloy chemical composition, to estimate relative pitting resistance of metal
and alloys.
Common equation for PREN calculation as followed:
PREN = %Cr + m.(%Mo) + n.(%N)
Per experiments, m range from 3.0 to 3.3 whilst n range from 12.8 to 30.
For ferritic grades Stainless Steel, the formula employed is:


PRE = % Cr + 3.3 (% Mo)


For austenitic grades Stainless Steel, the formula employed is:
PREN = %Cr + 3.3(%Mo) + 30(%N)
For duplex (austenitic-ferritic) grade Stainless Steel, the formula employed is:


PREN = %Cr + 3.3(%Mo) + 16(%N)
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For high Ni-Cr-Mo alloys e.g. Inconel 625, Hastelloy, etc, the formula employed is:
PREN = %Cr + 1.5(%Mo + %W + %Nb)
Where:
Cr - Chromium
Mo - Molybdenum
W - Tungsten
Nb - Niobium
Pitting is one of main problem for material expose to seawater. Minimum PREN required for material expose to
seawater is 40. Duplex Stainless steel, Super duplex stainless steel, etc are exhibiting PREN > 40.


Description

Pitting corrosion is highly localized corrosion occurring on a metal surface. Pitting is commonly observed on
surfaces with little or no general corrosion. Pitting typically occurs as a process of local anodic dissolution
where metal loss is exacerbated by the presence of a small anode and a large cathode.

Prevention or Remedial Action

There are several preventive approaches to avoid pitting. There are:

    •     Proper material selection e.g. SS316 with molybdenum having higher pitting resistance compare to
          SS304

    •     Use higher alloys (ASTM G48) for increased resistance to pitting corrosion

    •     Control oxygen level by injecting oxygen scavenger in boiler water system

    •     Control pH, chloride concentration and temperature

    •     Cathodic protection and/or Anodic Protection

    •     Proper monitoring of oxygen & chloride contents by routine sampling

    •     Agitation of stagnant fluid

    •     increase velocity of media and/or remove deposits of solids from exposed metal surface.

    •     selection of alloy with higher alloy content (e.g. in stainless alloys higher Cr, Mo and N content
          according to the following formula):

          PI = Cr + 3.3(Mo) + X(N) where PI is pitting index and
          x = 0 for ferritic stainless steels
          x = 16 for duplex (austenitic/ferritic) stainless steels
          x = 30 for austenitic stainless steels

          For more severe pitting service in some environments Ti - and Zr - alloys may also be appropriate.

    •     Use of effective chemical inhibitor to enhance resistance to localized attack.




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    •     Deaeration of aerated environments to reduce localized corrosion through elimination of oxygen
          concentration cell mechanism.

Standard Test Methods

    •     ASTM G-46 - practice for examination and evaluation of pitting corrosion.

    •     ASTM G-48 - test methods for pitting and crevice corrosion resistance of stainless steels and related
          alloys by the use of ferric chloride solution.

    •     ASTM G - standard reference test method for making poteniostatic and potentiodynamic anodic
          polarization measurements.

    •     ASTM G-61 - test method for conducting cyclic potentiodynamic polarization measurements for
          localized corrosion susceptibility of iron, nickel or cobalt based alloys.

    •     NACE TM0274 - dynamic corrosion testing of metals in high temperature water.

    •     ASTM G-85 - modified salt spray (fog) testing.

Evaluation of Pitting Corrosion

The extent of pitting corrosion can vary greatly depending on the exposure conditions and surface condition of
the material. Commonly used methods to determine the pitting corrosion resistance are

              •   Simple exposure of corrosion coupons to standardized environments of know severity (ASTM
                  G48).

              •   Evaluation of coupons and metal surfaces with standardized techniques to categorize the
                  nature of the pitting attack (ASTM G46).

              •   Use of electrochemical techniques (ASTM G61) to characterize the current-potential
                  polarization behavior of the material in specific service environments to identify materials
                  susceptible to pitting attack.

Most important in studies of pitting corrosion are the use of visual examination and/or metallographic
techniques to characterize the physical nature of the localized corrosive attack. Electrochemical
measurements should always be supplemented by such techniques to obtain the most accurate indications.
Typically, the most relevant information is the maximum attack depth and/or rate since these parameters will
most directly indicate the serviceability of actual components in service.




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                                     Crevice Corrosion

This form of attack is generally associated with the presence of small volumes of stagnant solution in occluded
interstices, beneath deposits and seals, or in crevices, e.g. at nuts and rivet heads. Deposits of sand, dust,
scale and corrosion products can all create zones where the liquid can only be renewed with great difficulty.
This is also the case for flexible, porous or fibrous seals (wood, plastic, rubber, cements, asbestos, cloth, etc.).
Crevice corrosion is encountered particularly in metals and alloys which owe their resistance to the stability of
                                                                                                -    +
a passive film, since these films are unstable in the presence of high concentrations of Cl and H ions.
The basic mechanism underlying crevice corrosion in passivatable alloys exposed to aerated chloride-rich
media is gradual acidification of the solution inside the crevice, leading to the appearance of highly aggressive
local conditions that destroy the passivity.
in an interstice, convection in the liquid is strongly impeded and the dissolved oxygen is locally rapidly
exhausted. A few seconds are sufficient to create a "differential aeration cell" between the small deaerated
interstice and the aerated remainder of the surface. However, "galvanic" corrosion between these two zones
remains inactive.
                                                                                            +
                               As dissolution of the metal M continues, an excess of Mn ions is created in the
                                                                                                             -
                               crevice, which can only be compensated by electromigration of the Cl ions (more
                                                                                                         -
                               numerous in a chloride-rich medium and more mobile than OH ions). Most
                               metallic chlorides hydrolyze, and this is particularly true for the elements in
                               stainless steels and aluminum alloys. The acidity in the crevice increases (pH 1-3)
                                                 -
                               as well as the Cl ion concentration (up to several times the mean value in the
                               solution). The dissolution reaction in the crevice is then promoted and the oxygen
                               reduction reaction becomes localized on the external surfaces close to the
                               crevice. This "autocatalytic" process accelerates rapidly, even if several days or
                               weeks were necessary to get it under way.
                               Means of preventing or limiting crevice corrosion : Use welds rather than bolted
                               or riveted joints, design installations to enable complete draining (no corners or
                               stagnant zones), hydrofuge any interstices that cannot be eliminated, and in
particular, grease all seals and seal planes, use only solid, non-porous seals, etc.




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  Crevice or contact corrosion is the corrosion produced at the region of contact of metals with metals or
metals with nonmetals. It may occur at washers, under barnacles, at sand grains, under applied protective
films, and at pockets formed by threaded joints. Whether or not stainless steels are free of pit nuclei, they are
always susceptible to this kind of corrosion because a nucleus is not necessary.
  Cleanliness, the proper use of sealants, and protective coatings are effective means of controlling this
problem. Molybdenum-containing grades of stainless steel (e.g. 316 and 316L) have increased crevice
corrosion resistance.




                                              The crevice corrosion shown above happened when an
                                              aerospace alloy (titanium - 6 aluminum - 4 vanadium) was used
                                              instead of a more corrosion-resistant grade of titanium. Special
                                              alloying additions are added to titanium to make alloys which are
                                              crevice corrosion resistant even at elevated temperatures.
                                                Screws and fasteners have are common sources of crevice
                                              corrosion problems. The stainless steel screws shown below
                                              corroded in the moist atmosphere of a pleasure boat hull.


Crevice corrosion and pitting corrosion are related because they both require stagnant water, chloride, and
oxygen or carbon dioxide. The mechanism of corrosion is very similar for both.



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Crevice corrosion tends to occur in crevices (stagnant, shielded areas) such as those formed under gaskets,
washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and
clamps.




TESTING DESCRIPTION

Crevice corrosion is localized corrosion which may occur in small areas of stagnant solution in crevices, joints
and under corrosion deposits (i.e. under deposit corrosion).

PREVENTION OR REMEDIAL ACTION

    •     redesign of equipment to eliminate crevices.

    •     close crevices with non-absorbent materials or incorporate a barrier to prevent of moisture penetration
          into crevice.

    •     prevent or remove builds-up of scale or other solids on surface of material.

    •     use of one-piece or welded construction versus bolting or riveting.

    •     select more corrosion resistant or inert alloy (note: see pitting corrosion for more information).

STANDARD TEST METHODS

    •     ASTM G-48 - test methods for pitting and crevice corrosion resistance of stainless steels and related
          alloys by the use of ferric chloride solution.

    •     ASTM G-78 - guide for crevice corrosion testing of iron-base and nickel-base stainless alloys in sea
          water and other chloride-containing aqueous media.


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Evaluation of Crevice Corrosion

The principal reference for the evaluation of crevice corrosion is ASTM G78. The extent of crevice corrosion
can be greatly influenced by the nature of the crevice and the technique utilized in the exposure test. Typically,
tighter crevices promote greater localized corrosive attack. The use of serrated TFE or ceramic washers is one
of the most common methods for obtaining reproducible simulation of crevice corrosion. These washers are
bolted to the specimen using a corrosion resistant bolt with constant applied torque for each crevice washer
assembly. In most cases, the rate of crevice attack in not constant. Initially, there is an incubation period where
the attack rate is essentially zero. However, as the corrosivity of the crevice environment increases with
exposure time, the local attack rate can actually increase with time in test. Therefore, multiple exposure
periods may be needed to accurately assess crevice corrosion attack rates.




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                                       Pitting Corrosion

Pitting Corrosion is "self nucleating" crevice corrosion, starting at occluded cells. Corrosion products often
cover the pits, and may form "chimneys". Pitting is considered to be more dangerous than uniform corrosion
damage because it is more difficult to detect, predict and prevent. A small, narrow pit with minimal overall
metal loss can lead to the failure of an entire engineering system.




Schematic of an actively growing pit in iron

Once initiated, both crevice and pitting corrosion can be explained by differential concentration cells, Cathodic
reactions, i. e. oxygen reduction or hydrogen evolution may start in the crevice or the pits. Large surface areas
will become cathodic and pits or crevices will become anodic and corrode. Metal dissolution will thus be
concentrated in small areas and will proceed at much higher rates than with uniform corrosion. Large crevices
are less likely to corrode because water movement causes mixing and replenishes oxygen, hydrogen ions,
bicarbonate or hydrogen sulfide.

The chloride ion acts as a catalyst in pitting and crevice corrosion. In other words, increases the corrosion rate
but is not used up in the reaction. It has the ability to absorb on the metal surface or the passive films and
polarize the metal, initializing localized corrosion. (e.g. pitting corrosion of austenitic stainless steels (304) in
salt water). This photo is an example of crevice corrosion on a tubing end.




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Pitting corrosion is frequently observed in CO2 and H2S environments in the oilfield. Pits will generally initiate
due to local breakdown of corrosion product films on the surface and corrosion will proceed at an accelerated
rate. In sweet (CO2) systems, the pits are generally small with sharp edges and smooth rounded bottoms. Pits
may become connected as the corrosion damage increases. Corrosion products are dark brown to grayish
black and loosely adhering. In sour (H2S) systems, the pits are usually shallow round depressions with etched
bottoms and sloping sides. Generally, the pits are not connected, and corrosion products are black and tightly
adhering to the metal surface.




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The first image is an example of CO2 pitting, and the second is an example of H2S pitting.



Pitting corrosion is particularly insidious. The attack is in the form of highly localized holes that can penetrate
inwards extremely rapidly, while the rest of the surface remains intact. A component can be perforated in a few
days with no appreciable loss in weight on the structure as a whole.
Pitting corrosion is most aggressive in solutions containing chloride, bromide or hypochlorite ions. Iodides and
fluorides are much less harmful. The presence of sulfides and H2S enhances pitting corrosion, and
systematically impairs the resistance criteria for this type of attack. The thiosulphate species plays a similar
role, since its electrochemical reduction causes "sulphidation" of the exposed metallic surfaces.
The presence of an oxydizing cation (Fe+3, Cu+2, Hg+2, etc.) enables the formation of pits even in the absence
of oxygen. However, in the presence of oxygen, all chlorides become dangerous, and this is also true in the
presence of hydrogen peroxide.

The stainless steels are particularly sensitive to pitting corrosion, but other metals, such as passive iron,
chromium, cobalt, aluminum, copper and their alloys are also prone to this form of damage.
Very often, in non-passivatable metals, a "tubercular" surface morphology is observed, beneath which pits
develop.
Contrary to crevice corrosion, the cause of pitting is not always completely local in nature. Thus, although
alterations or intrinsic defects at the metal-solution interface (e.g. inclusions emerging through the passive film
in stainless steels) often represent nuclei for local dissolution, all such potential nuclei are not attacked. The
stabilization and development of these nuclei always show a random nature. Galvanic coupling is then
established between the discontinuous zones, which form small anodes where metal dissolution occurs, and
the remainder of the surface where the cathodic reaction takes place.
Means of reducing or preventing pitting corrosion : Choose the material most appropriate for the service
conditions, avoid stagnant zones and deposits, Reduce the aggressivity of the medium, use cathodic
protection.




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Tubercle on the surface of a copper tube (corrosion by type I pits in sanitary cold water).




Pitting corrosion on the wall of an Cr18-Ni10 austenitic stainless steel tank.




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                                    Filiform Corrosion
                                                                                    This type of corrosion
                                                                                    occurs under painted or
                                                                                    plated     surfaces     when
                                                                                    moisture permeates the
                                                                                    coating.    Lacquers     and
                                                                                    "quick-dry" paints are most
                                                                                    susceptible to the problem.
                                                                                    Their    use     should    be
                                                                                    avoided unless absence of
                                                                                    an adverse effect has been
                                                                                    proven by field experience.
                                                                                    Where a coating is required,
                                                                                    it should exhibit low water
                                                                                    vapor            transmission
                                                                                    characteristics          and
                                                                                    excellent adhesion. Zinc-
                                                                                    rich coatings should also be
                                                                                    considered      for   coating
                                                                                    carbon steel because of
                                                                                    their cathodic protection
                                                                                    quality.

                                             Filiform corrosion normally starts at small, sometimes microscopic,
                                             defects in the coating. Lacquers and "quick-dry" paints are most
                                             susceptible to the problem. Their use should be avoided unless
                                             absence of an adverse effect has been proven by field experience.
                                             Where a coating is required, it should exhibit low water vapor
                                             transmission characteristics and excellent adhesion. Zinc-rich
                                             coatings should also be considered for coating carbon steel
                                             because of their cathodic protection quality.




The picture on the left shows filiform corrosion causing bleed-through on a welded tank. The picture on the
right shows "worm-like" filiform corrosion tunnels forming under a coating at the Atmospheric Test Site.

Filiform corrosion is minimized by careful surface preparation prior to coating, by the use of coatings that are
resistant to this form of corrosion (see above), and by careful inspection of coatings to insure that holidays, or
holes, in the coating are minimized.




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                              Intergranular Corrosion
Intergranular corrosion is an attack on or adjacent to the grain boundaries of a metal or alloy. A highly
magnified cross section of most commercial alloys will show its granular structure. This structure consists of
                                             quantities of individual grains, and each of these tiny grains has a
                                             clearly defined boundary that chemically differs from the metal
                                             within the grain center. Heat treatment of stainless steels and
                                             aluminum alloys accentuates this problem.
                                             The picture above shows a stainless steel which corroded in the
                                             heat affected zone a short distance from the weld. This is typical of
                                             intergranular corrosion in austenitic stainless steels. This corrosion
                                             can be eliminated by using stabilized stainless steels (321 or 347)
                                             or by using low-carbon stainless grades (304L or 3I6L).
  Heat-treatable aluminum alloys (2000, 6000, and 7000 series alloys) can also have this problem. See the
section on exfoliation corrosion below.
In most cases of corrosion, including uniform corrosion, the grain boundaries behave in essentially the same
way as the grains themselves. However, in certain conditions, the grain boundaries can undergo marked
localized attack while the rest of the material remains unaffected. The alloy disintegrates and loses its
mechanical properties.
This type of corrosion is due either to the presence of impurities in the boundaries, or to local enrichment or
depletion of one or more alloying elements. For example, small quantities of iron in aluminum or titanium
(metals in which iron has a low solubility), segregate to the grain boundaries where they can induce
intergranular corrosion. Certain precipitate phases (e.g. Mg5Al8, Mg2Si, MgZn2, MnAl6, etc.) are also known to
cause or enhance intergranular attack of high strength aluminum alloys, particularly in chloride-rich media.
The exfoliation corrosion phenomenon observed in rolled aluminum alloys is usually, but not always,
intergranular in nature. In this case, the corrosion products occupy a larger volume than the metal "consumed",
generating a high pressure on the slivers of uncorroded metal, leading to the formation of blisters.
Numerous alloy types can undergo intergranular attack, but the most important practical example is the
intergranular corrosion of austenitic stainless steels, related to chromium depletion in the vicinity of the
boundaries, due to the intergranular precipitation of chromium carbides (Cr23C6), during a "sensitizing" heat
treatment or thermal cycle.
Exfoliation Corrosion
                                             Exfoliation is a form of intergranular corrosion. It manifests itself by
                                             lifting up the surface grains of a metal by the force of expanding
                                             corrosion products occurring at the grain boundaries just below the
                                             surface. It is visible evidence of intergranular corrosion and most
                                             often seen on extruded sections where grain thickness is less than
                                             in rolled forms. This form of corrosion is common on aluminum,
                                             and it may occur on carbon steel.
                                             (See also other section on exfoliation corrosion)



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The picture on the left shows exfoliation of aluminum. Exfoliation of carbon steel is apparent in the channel on
the coating exposure panel on the right. The expansion of the metal caused by exfoliation corrosion can create
stresses that bend or break connections and lead to structural failure.




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Description




Metals and alloys are composed of grains similar to sand grains in a common sandstone. Intergranular
corrosion refers to the selective corrosion of the grain boundary regions. This attack is very common in some
stainless steels and nickel alloys. Some aluminum alloys can also exhibit intergranular and exfoliation (i.e.
corrosion at grain boundary sites parallel to the metal surface where corrosion products force apart the metal).

Prevention or Remedial Action

    •     Heat treatment of alloy to remove phases from grain boundary regions which reduce corrosion
          resistance (i.e. solution annealing).

    •     Use modified alloys which have eliminated such grain boundary phases through stabilizing elements
          or reduced levels of impurities:
          EXAMPLE: stainless steels such as AISI 304 or 316 can be "sensitized" by heating or welding in the
          range 900 to 1500 F. This forms carbide precipitates which reduce corrosion resistance of grain
          boundaries. The use of low carbon 304L or 316L will increase resistance to inter granular corrosion in
          welded components. for prolonged service at high temperature stabilized stainless steels (i.e. aisi 321
          and 347) will increase resistance to inter granular corrosion.

Standard Test Methods

    •     ASTM A 262 - practices for detecting susceptibility to intergranular attack in austenitic stainless steels.

    •     ASTM G-28 - test methods for detecting susceptibility to intergranular attack in wrought, nickel rich,
          chromium-bearing alloys.

    •     ASTM G-34 - test method for exfoliation corrosion susceptibility in 2xxx and 7xxx series aluminum
          alloys (EXCO test).

    •     ASTM G-66 - test method for visual assessment of exfoliation corrosion susceptibility of 5xxx series
          aluminum alloys (asset test).



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    •     ASTM G-67 - test method for determining the susceptibility to inter granular corrosion of 5xxx series
          aluminum alloys by mass loss after exposure to nitric acid (namlt test).

Evaluation of Intergranular Attack

The most common concern for stainless alloys is the influence of welding and/or heat treatment on
susceptibility to intergranular corrosion produced by carbide precipitation (i.e. senitization). Therefore, the
carbon content is an important metallurgical consideration with lower carbon (and nitrogen) materials or
materials that have been stabilized with additions of Ti or Nb showing lower tendencies to intergranular
corrosion. In evaluation, the tendencies for intergranular corrosion can vary greatly depending on the severity
of the test conditions and environment. Oftentimes, standardized environments are used such as those given
in ASTM A262.

Intergranular corrosion various alloys require the use of different environments:

    •     Aluminum alloys - acidified NaCl/HCl solution or HNO3solution.

    •     Magnesium alloys - NaCl/HF solution

    •     Copper alloys - NaCl solution with H2SO4 or HNO3.

    •     Lead alloys - Acetic acid or HF solutions




   More reading on corrosion on stainless steel




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                  Selective Leaching or Phase Attack

                                          The removal of one element from a solid solution alloy is often called
                                          leaching. The gradual loss of zinc from brass (dezincification) is
                                          perhaps the most well known example of this type of corrosion, but
                                          aluminum can also be leached from aluminum bronzes
                                          (dealuminification) and nickel from 70/30 Cupronickel alloys
                                          (denickelification).

                                          In each case initial corrosion dissolves both components of the alloy
                                          but the more noble metal, copper, is then precipitated from solution at
                                          the surface. This leads to increased solution of the parent alloy due to
                                          galvanic effects and hence further deposition of copper. The overall
                                          effect is to reduce the surface and underlying regions of a component
                                          to a spongy mass of material with much reduced mechanical strength,
                                          leading to possible collapse under normal working stresses.

                                          The tendency to this form of attack can be decreased by additional
alloying such as the addition of arsenic to brass and nickel to Al-bronzes. Leaching and other examples of the
selective attack are illustrated schematically in figure 1.




Figure 1. Leaching (top) and selective corrosion (bottom)




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Dealumnification of a C95800 nickel aluminum bronze pump impeller in service in a wastewater plant.


Graphitization of Cast Iron
A common form of leaching is the graphitization of cast irons. In slightly acidic waters both flake graphite (grey)
and nodular graphite (ductile) irons are corroded due to the anodic behavior of the matrix with respect to the
cathodic graphite. This results in the conversion of the structure to a weak porous mass of corrosion product
and graphite residue. However, there is often little sign of the extent of this damage from the outwards
appearance of the material, since the original shape and dimensions of components and pipes remain
unaffected. This highlights the importance of correct application of ultrasonic testing in the assessment of
condition of cast iron sections that may have suffered this form of attack.

In water pipes both internal and external graphitization may occur where soil chemistry is aggressive.
Corrosion mechanisms will also be subject to the influence of microbiological activity. In some cases, in
effluent lines and older water mains, pipe sections can be almost fully graphitized whilst still holding water.
They have been severely weakened, however, and are prone to sudden failure if water pressure changes, if
supporting soil moves or vibration from overhead traffic increases.

The graphitized surface can be easily penetrated by a screwdriver or knife and the extent of the damage
revealed by a examination under a microscope. Where it is cost effective graphitization is avoided by the use
of high nickel austenitic cast irons

Graphitization of cast iron pipe.




Copper-Nickel pipe selective attack on copper phase was initial suspected.




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On cleaning the surface dark corrosion products, green oxide indicate nickel selective attack.




                                                                                 Dealloying occurs when one or more
                                                                                 components      of   an     alloy     are   more
                                                                                 susceptible to corrosion than the rest, and
                                                                                 are preferentially dissolved. The most
                                                                                 important example of dealloying is the
                                                                                 removal of zinc from brass, known as
                                                                                 dezincification. Another common example
                                                                                 is graphitic corrosion, which occurs in gray
                                                                                 cast iron. In graphitic corrosion, the
                                                                                 graphite acts as a cathode, anodically
                                                                                 dissolving the iron and leaving a graphite
                                                                                 frame. This frame maintains its shape but
                                                                                 loses   mechanical        strength.     Graphitic
                                                                                 corrosion is observed in buried cast iron
                                                                                 pipe after many years exposure to soil; it
                                                                                 can also be seen in cast iron cannons in
                                                                                 ships that have been sunk at sea.




                                       Case Study #2: Graphitic Corrosion in Grey Cast Iron - Water Pipe
                                       Inspection

                                       These SEM images show cross sections of a grey cast iron water pipe. The
                                       cross section surfaces were ground and polished to reveal the continuous
                                       network of flake-like graphite peculiar to this form of iron.


                                       A grey cast iron pipe that has undergone graphitic corrosion often visually
   Graphitization of cast iron pipe.   appears to be fine other then some general surface corrosion. However,

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due possible subsurface attack a substantial portion of a pipes wall thickness can be converted to a weak and
brittle graphite network with dramatically reduced mechanical strength. Graphitic corrosion can lead to
catastrophic failure in grey cast iron pipes carrying water at relatively high pressures.

                                            The free surface at the left side of the first image was the outside
                                            surface of the previously buried pipe, which had been in contact
                                            with moist soil. A damaged area is plainly visible penetrating the
                                            pipe wall from the outer surface at the left. This form of attack,
                                            known as graphitic corrosion, is specific to grey cast iron. It occurs
                                            when the more noble graphite promotes the accelerated attack of
                                            the nearby iron metal through galvanic action in a corrosive
                                            environment such as a damp soil.




The free surface at the right side of the second image was the inner
surface of the same pipe, which had been in contact with potable
water. The inner surface clearly suffered corrosive attack resulting
in roughening and loss of wall thickness. Additional evidence of
graphitic corrosion is visible here. Metal loss due to galvanic attack
is obvious around several of the graphite flake clusters visible in
this cross section plane. This subsurface damage is possible
because of the continuous graphite network and would not have
been identified through a surface-based visual inspection.




An example of the use of electrochemical etching to reveal grain structure in a metallographic sample is shown
in Corrosion and Electrochemistry Case Study 1: Evaluating Chemical Plant Intergranular Corrosion with
Metallography and XPS Chemical Analysis.

See another example of metallographic microscopy images used to examine grain size and carbide precipitate
number and size in a sensitization investigation of 304 stainless steel using the ASTM G108 Test Method.

Description

Selective leaching/phase attack is the removal of one element from a metal or alloy by a corrosion process,
similarly, this process can also selectively remove one phase from an alloy. The most common example of this
form of attack is the removal of zinc (Zn) from brass alloys. In duplex stainless steels, some acidic
environments can selectively remove either the ferrite or austenite in the microstructure.

Prevention or Remedial Action

    •     reduce severity of environment through environmental control or addition of effective chemical
          inhibitors.
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   •     cathodic protection.

   •     use of coating to act as a barrier between the environment and the alloy.

Standard Test Methods

   •     ASTM G-31 - practice for laboratory immersion corrosion testing of metals.

   •     ASTM G-4 - method for conducting corrosion coupon tests in plant equipment.




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                            Stress Corrosion Cracking                                       Read More


Stress corrosion cracking (SCC) is the unexpected sudden failure of normally ductile metals or tough
thermoplastics subjected to a tensile stress in a corrosive environment, especially at elevated temperature (in
the case of metals). SCC is highly chemically specific in that certain alloys are likely to undergo SCC only
when exposed to a small number of chemical environments. The chemical environment that causes SCC for a
given alloy is often one which is only mildly corrosive to the metal otherwise. Hence, metal parts with severe
SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for
SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than
pure metals. The specific environment is of crucial importance, and only very small concentrations of certain
highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and
unexpected failure.

The stresses can be the result of the crevice loads due to stress concentration, or can be caused by the type
of assembly or residual stresses from fabrication (eg. cold working); the residual stresses can be relieved by
annealing.

Metals attacked
Certain austenitic stainless steels and aluminum alloys crack in the presence of chlorides, mild steel cracks in
the present of alkali (boiler cracking) and nitrates, copper alloys crack in ammoniacal solutions (season
cracking). This limits the usefulness of austenitic stainless steel for containing water with higher than few ppm
content of chlorides at temperatures above 50 °C. Worse still, high-tensile structural steels crack in an
unexpectedly brittle manner in a whole variety of aqueous environments, especially containing chlorides. With
the possible exception of the latter, which is a special example of hydrogen cracking, all the others display the
phenomenon of subcritical crack growth, i.e. small surface flaws propagate (usually smoothly) under conditions
where fracture mechanics predicts that failure should not occur. That is, in the presence of a corrodent, cracks
develop and propagate well below KIc. In fact, the subcritical value of the stress intensity, designated as KIscc,
may be less than 1% of KIc, as the following table shows:




                                                  KIc                          KIscc
                                   Alloy                SCC environment
                                             MN/m3/2                       MN/m3/2

                               13Cr steel    60         3% NaCl            12

                               18Cr-8Ni      200        42% MgCl2          10

                               Cu-30Zn       200        NH4OH, pH7         1

                               Al-3Mg-7Zn 25            Aqueous halides    5

                               Ti-6Al-1V     60         0.6M KCl           20




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                                                                           Stress     corrosion   cracking    (SCC)    is
                                                                           caused by the simultaneous effects of
                                                                           tensile stress and a specific corrosive
                                                                           environment. Stresses may be due to
                                                                           applied loads, residual stresses from the
                                                                           manufacturing process, or a combination
                                                                           of both.

                                                                           Cross sections of SCC frequently show
                                                                           branched cracks. This river branching
pattern is unique to SCC and is used in failure analysis to identify when this form of corrosion has occurred.

The photo below shows SCC of an insulated stainless-steel condensate line. Water wetted the insulation and
caused chlorides to leach from the insulation onto the hot metal surface. This is a common problem on steam
and condensate lines. Control is by maintaining the jackets around the lines so that moisture doesn't enter the
insulation or is quickly drained off.

                                        The next two photos show intergranular SCC of an aluminum aerospace
                                        part. The intergranular nature of the corrosion can be seen in the scanning
                                        electron microscope image on the left and in the microscopic cross section
                                        on the right. The arrows indicate the primary crack shown in both pictures.
                                        Note that secondary cracks are also apparent. These secondary cracks
                                        are common in stress corrosion cracking. The failure above occurred on
                                        an aluminum alloy subjected to residual stresses and salt water. Changes
                                        in alloy heat treatment recommended by KSC Materials Laboratory
                                        eliminated this problem.

                                        Several years ago, wide spread use of plastic tubing was started in new
                                        house construction and for repair of old systems. Flexible tubing was used
                                        to connect faucets to the fixed metal piping. The picture below shows
stress corrosion cracking after eight years in this service. The tubing was bent and stress cracks started at the
outside tensile side of the tube. Flexible plastic piping is now used less often in this service-especially for hot
water service.




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Stress corrosion cracking (SCC) is a process involving the initiation of cracks and their propagation, possibly
up to complete failure of a component, due to the combined action of tensile mechanical loading and a
corrosive medium. Indeed, it is the presence of tensile stresses that is dangerous, compressive stresses
exerting a protective influence.

SCC frequently occurs in media that are little or non-aggressive towards the metal or alloy concerned in the
absence of tensile loading (e.g. austenitic stainless steels in high temperature water and steam). The
associated weight losses are generally very small and even insignificant compared to the extent of the overall
damage incurred. This form of corrosion is of great practical importance and represents a permanent risk in
numerous industrial installations, in terms of both the economic consequences and the safety considerations
involved (personnel, equipment reliability, respect of the environment). There is no known category of
commercial metals and alloys that is fully immune to SCC. Even materials such as glasses, plastics and
rubbers can also be prone to this type of attack in certain conditions.
The time necessary for a part to fail by SCC can vary from a few minutes to several years.
Means of reducing or preventing stress corrosion cracking are : elimination of residual stresses by stress
relieving heat treatments, purification of the medium, choice of the most appropriate material, improvement of
the surface condition, avoid surface machining stresses, perform peening treatments on welds to induce
surface compressive stresses, apply external protection methods (cathodic protection, inhibitors and organic or
inorganic protective coatings).




Intergranular SCC in a copper alloy




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SCC Corrosion




Stress-corrosion cracking of stainless alloys

Stress corrosion cracking (SCC) is the formation of brittle cracks in a normally sound material through the
simultaneous action of a tensile stress and a corrosive environment. In most cases, SCC has been associated
with the process of active path corrosion (APC) whereby the corrosive attack or anodic dissolution initiates at
specific localized sites and is focused along specific paths within the material. In some cases, these are along
grain boundaries, in other cases, the path is along specific crystallographic within the grains. Quite often, SCC
is strongly affected by alloy composition, the concentration of specific corrodent species, and, to a lesser

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degree, the stress intensity. In some cases, this latter point may make the use of test methods based on
fracture mechanics concepts difficult to utilize effectively due to excessive crack branching and tendencies for
nonplanar propagation of cracks.

Furthermore, corrosion film characteristics (i.e., passivation) and local anodic attack (i.e., depassivation) serve
as controlling factors in SCC crack initiation and growth. Therefore, localized corrosion can promote SCC
making exposure geometry and specimen design important factors. In many cases, mechanical straining or
electrochemical inducements such as crevices or controlled potential are utilized to overcome the problems
and uncertainties of SCC initiation so that the inherent resistance of the material to SCC can be obtained at
reasonable test duration (see Table 1).

            Table 1 - Applied Potentials for SCC in Steel Exposed to Various Service Environments


                                  Environment        Potential rate (mV, SCE)

                                     Nitrate               -250 to +1200

                                Liquid ammonia            -400 to > +1500

                                   Carbonate                -650 to -550

                                   Hydroxide      -1100 to -850 and +350 to +500




Stress Corrosion Cracking Caustic leakage into a steam line embrittled this steam line causing cracking that
started near the welds.

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Nitrate Cracking Nitrate-contaminated rain water entering through insulation faults on an autoclave led to
stress corrosion cracking beginning at the weld.
The study of environmentally assisted cracking (EAC) in its most basic sense involves the consideration and
evaluation of the inherent compatibility between a material and the environment under conditions of either
applied or residual stress. This is a very broad topic encompassing many possible combinations of materials
and environments. However. it is also a critical consideration because equipment, components, and structures
are intended to be used under specific conditions of environment and stress. Furthermore, the materials used
in construction typically have a multitude of manufacturing and process variables that may affect materials



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performance. Testing for resistance to EAC is one of the most effective ways to determine the interrelation of
material, environmental, and mechanical variables on the cracking process.

The grand dimensions of this subject immediately limit attempts to make simplistic application of only a single
method of testing for all cases. Factors such as,

    1. material type,

    2. process history,

    3. product form,

    4. active cracking mechanism(s),

    5. loading configuration and geometry, and

    6. service environment conditions,

to name a few, can have major consequences in determining the type of specimen and test condition to be
utilized. The prudent approach to selection of testing methods is usually to assess these considerations along
with a survey of previous experiences provided from prior investigations for similar applications or from those
found in the published literature.

It can be said that there is no single perfect testing technique for the evaluation of EAC. However, the
evaluation of materials for EAC typically involves the use of the specimen and technique that takes into
account as many necessary factors as possible for the particular material and environment under
consideration. In some cases, this may mean the use of

    1. More than one type of test specimen

    2. Various alternative configurations of the same specimen

    3. Alternative test techniques with the same specimen (e.g. crevices applied potential, constant load, and
        slow strain rate)

Most of all, it is important to provide a link between the results of laboratory evaluations and real-world service
applications. This is often developed through studies involving:

    1. Integrated laboratory and field or in-plant tests

    2. Correlation of laboratory data with service experience

    3. Reviews of published literature on the service performance of similar materials

In any case, the evaluation of EAC susceptibility using laboratory testing methods can provide data resulting in
an increased confidence level. This often allows for an optimization of the materials of construction. By this it is
meant that the allowance for unpredictable service performance can be reduced resulting in a lower material
cost, reduced downtime, and a reduction in the number of costly failures.


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Evaluation of SCC and other forms of Environmentally Assisted Cracking (EAC)

The evaluation of SCC and EAC (e.g. , Stress Corrosion Cracking, Hydrogen Embrittlement, Liquid Metal
Embrittlement) requires understanding of various materials, mechanical and environmental factors that come
together to produce resistance or susceptibility to cracking. In many cases, SCC involves the combination of
tensile stress and local anodic attack which dictate the period of incubation prior to the initiation of SCC.
Therefore, to conduct tests for SCC, either mechanical or electrochemical means are often utilized to promote
localized corrosion so that the inherent susceptibility of the material can be determined. Such techniques
include the use of slow strain rate, cyclic slow strain rate, fracture mechanics and electrochemical potential
control.

In some cases, where constant load tests are used, environmental cracks can initiate but not propagate
through the entire cross-section of the specimen. Therefore, the specimen may not fail, but significant cracking
may take place.

The study of environmentally assisted cracking (EAC) in its most basic sense involves the consideration and
evaluation of the inherent compatibility between a material and the environment under conditions of either
applied or residual stress. This is a very broad topic encompassing many possible combinations of materials
and environments. However. it is also a critical consideration because equipment, components, and structures
are intended to be used under specific conditions of environment and stress. Furthermore, the materials used
in construction typically have a multitude of manufacturing and process variables that may affect materials
performance. Testing for resistance to EAC is one of the most effective ways to determine the interrelation of
material, environmental, and mechanical variables on the cracking process.

The grand dimensions of this subject immediately limit attempts to make simplistic application of only a single
method of testing for all cases. Factors such as,

    1. material type,

    2. process history,

    3. product form,

    4. active cracking mechanism(s),

    5. loading configuration and geometry, and

    6. service environment conditions,

to name a few, can have major consequences in determining the type of specimen and test condition to be
utilized. The prudent approach to selection of testing methods is usually to assess these considerations along
with a survey of previous experiences provided from prior investigations for similar applications or from those
found in the published literature.

It can be said that there is no single perfect testing technique for the evaluation of EAC. However, the
evaluation of materials for EAC typically involve the use of the specimen and technique that takes into account



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as many necessary factors as possible for the particular material and environment under consideration. In
some cases, this may mean the use of

    1. More than one type of test specimen

    2. Various alternative configurations of the same specimen

    3. Alternative test techniques with the same specimen (e.g. crevices applied potential, constant load, and
        slow strain rate)

Most of all, it is important to provide a link between the results of laboratory evaluations and real-world service
applications. This is often developed through studies involving:

    1. Integrated laboratory and field or in-plant tests

    2. Correlation of laboratory data with service experience

    3. Reviews of published literature on the service performance of similar materials

In any case, the evaluation of EAC susceptibility using laboratory testing methods can provide data resulting in
an increased confidence level. This often allows for an optimization of the materials of construction. By this it is
meant that the allowance for unpredictable service performance can be reduced resulting in a lower material
cost, reduced downtime, and a reduction in the number of costly failures.




Ammonia Attack :A few parts per million of ammonia in boiler feed water caused failure of this bronze valve.




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Chloride Attack: Waste water with a high chloride content caused rapid corrosion of this stainless steel mixing
valve.




Galvanized Bolt: The zinc galvanizing on this bolt failed rapidly in the industrial atmosphere containing SO2
and ammonium nitrate.




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Chloride Attack on Titanium This titanium heating coil was attacked by chlorides in an acidic environment.


Chloride stress - corrosion cracking (CSCC) is initiation and propagation of cracks in a metal or alloy under
tensile stresses and a corrosive environment contains Chloride compounds. Once the crack is initiated, it will
propagate rapidly and potentially lead to catastrophic failure.

Factors that influence the rate and severity of cracking include

    •     chloride content

    •     oxygen content

    •     temperature

    •     stress level

    •     pH value of an aqueous solution
Higher chloride content in process fluid will increase potential of CSCC.

The severity of cracking increases with temperature. Figure below shows several Stainless Steel materials
increases it susceptibility to CSCC as temperature is increased.




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Source : Sandvik Material Technology

SAF 2205 (UNS 31803) = Duplex Stainless Steel
SAF 2507 (UNS 32750) = Super Duplex Stainless Steel

Material under pressure without Post weld heat treatment will experience high stress level. Higher the stress
level, higher the potential of CSCC.

Acidic process(low pH) with chloride content in it tends to increase the CSCC potential.

CASE STUDIES

Hot gas (Shell) is cooled by seawater (Tube) from 220 degC to 180 degC in a Shell & Tube heat exchanger.
Seawater is being heated from 30 degC to 35 degC and return to sea. The Shell and Tube material of
construction are Carbon steel (CS) and Duplex Stainless Steel (DSS) respectively. After 2 months in operation,
cracks occurred at the tube (DSS) and leads to major platform shutdown. Investigation found crack was
caused by CSCC at tube.
Why a CSCC occurred at DSS tube although the seawater temperature only 35 degC maximum ?


Eventhough the inlet and outlet temperature are below 150 degC, thermal designer may design the heat
exchanger with high heat flux in order to reduce the heat exchanger area and this result tube skin temperature
exceeded 150 degC. Condition with Seawater which contains ~20,000 mg/l Chloride, high in dissolved oxygen,
slightly acidic and skin temperature exceeded 150 degC is perfect combination conditions for CSCC to occur
for DSS. Those heat exchanger designer shall always check skin temperature profile especially for low flow
condition or specify better material i.e. Super DSS for above service.




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The following are some images of metal experienced Chloride Stress Corrosion Cracking.




                              Inter granular SCC of an Inconel heat exchanger tube




                   Trans granular SCC of 316 stainless steel chemical processing piping system




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                                CSCC occured on insulated vessel




                                CSCC occured on insulated vessel




                                CSCC occured on Condenser tube




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                                                        CSCC on pipe




                                                 Inter granular SCC of a pipe




Description

SCC is the brittle cracking of a metal due to the result of combined effects from localized corrosion and tensile
stress. there are many examples in which specific metals and environments in combination cause such
problems. a few examples include:

    •     brass - SCC in solutions with ammonia

    •     steel - SCC in caustic (high ph), amine solutions

    •     stainless steels and aluminum alloys - SCC in solutions containing chlorides.

    •     ti-alloys - SCC in nitric acid or methanol.

Stress Corrosion Cracking of Stainless Steel
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The example shown indicates many intersecting, branched cracks with a transgranular propagation mode.
These are typical of stress corrosion cracking (SCC) in austenitic stainless steel. In this case, however, the
alloy was reported to be resistant to SCC in the NaCl brine service environment. The location of cracking was
limited to a region covered by an elastomeric sleeve. Under the sleeve, evidence of severe general and pitting
corrosion were found and evidence of sulfur-containing corrosion products. Analysis of the elastomer indicate
that it was not the correct grade and chemical degradation had occurred in service to produce organic acids
and sulfur compounds. This local environment resulted in enhanced localized susceptibility of the material to
pitting corrosion and SCC.


Prevention or Remedial Action

    •     lower either applied or residual tensile stresses.

    •     modification of the environment to eliminate specific scc agent(s).

    •     change alloy or increase alloy content (i.e. stainless steels and nickel base alloys).

    •     cathodic protection to change corrosion potential out of scc range.

    •     add chemical inhibitor.

Standard Test Methods

    •     ASTM G-30 - practice for making and using U-bend ssc test specimens.

    •     ASTM G-38 - practice for making and using C-ring scc test specimens.

    •     ASTM G-39 - practice for preparation and use of bent-beam scc test specimens.

    •     ASTM G-44 - practice for evaluation of scc resistance of metals and alloys in 3.5% NaCl solution.

    •     ASTM G-49 - practice for preparation and use of direct tension scc test specimens.

    •     ASTM G-58 - practice for preparation of scc test specimens for weldments.

    •     aluminum alloys: ASTM G-44 (seawater - alternate immersion), ASTM G-47(high

    •     stainless steels and nickel base alloys: ASTM G-35 (polythionic acid),

    •     ASTM G-36 stainless steels (boiling MgCl2 solution)

    •     ASTM G-37: copper-zinc alloys (ammonia solution).

    •     ASTM D-807 steels (caustic).

    •     ASTM F-945 titanium (aircraft engine cleaning materials).

    •     ASTM G129: Slow Strain Rate Testing of Materials for Environmentally Assisted Cracking


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    •     ASTM G142 - Tensile tests method in hydrogen environments

    •     NACE TM0274 - dynamic corrosion testing of metals in high temperature water.




Corrosion Engineering and Metal Corrosion Testing
Services - Example 3:

A SEM/BSE image of a corrosion pit associated
with stress corrosion cracking in a stainless steel
drum. The Back Scatter Electron (BSE) imaging
mode of the Scanning Electron Microscope is
sensitive to compositional variations. The corrosion
products appear dark with the stainless steel base
metal appearing white. SEM/EDS analysis indicated
a high concentration of chlorine in the corrosion
deposit.
(Scanning Electron Microscope (SEM) photo, Mag:
100X)




           Azom SCC: http://www.azom.com/Details.asp?ArticleID=102
           Stress Corrosion cracking of Stainless Steel.
           Stress Corrosion Cracking.
           Piping Failures Q&A
           PWHT to avoid IGSC of Supermartensitic Stainless Steel.
           SCC of UNS 20910 SS Steel. http://web.nace.org/content/publications/mp/2007/0701058.pdf
           Stress Corrosion Cracking




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                                     Corrosion Fatigue

Corrosion fatigue is a special case of stress corrosion caused by the combined effects of cyclic stress and
corrosion. No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a
corrosive environment. Damage from corrosion fatigue is greater than the sum of the damage from both cyclic
stresses and corrosion. Control of corrosion fatigue can be accomplished by either lowering the cyclic
stresses or by corrosion control. The "beach marks" on the propeller shown below mark the progression of
fatigue on this surface.

                                             Similar beach marks are shown on the aerospace part below left.
                                             The high magnification scanning electron microscope image on
                                             the right shows striations (individual crack progression marks).
                                             The part shown below is also discussed in the section on fretting
                                             corrosion. An infamous example of corrosion fatigue occurred in
                                             1988 on an airliner flying between the Hawaiian Islands. This
                                             disaster, which cost one life, prompted the airlines to look at their
                                             airplanes and inspect for corrosion fatigue.




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Corrosion-fatigue differs from SCC by the fact that the applied stresses are no longer static, but cyclic
(periodically fluctuating or alternating loads).

In the case of steels, the conventional fatigue limit determined from so-called Wöhler curves (applied stress as
a function of cycles to failure
δ = f(N)) does not exist for tests performed in a corrosive medium. Whatever the stress level, failure will
eventually occur after a finite number of cycles.
The cracks are generally transgranular in nature, with little tendency for branching. However, a few small
secondary cracks may appear in the vicinity of the main crack.
Although there is no direct relationship between the sensitivity to corrosion-fatigue and the mechanical
properties of the material, high strength alloys tend to be most highly prone.

Corrosion-fatigue damage can be prevented or reduced by decreasing the tensile stresses, either by the use of
stress-relief annealing, by modifying component design, or by applying mechanical surface treatments such as
peening, to introduce surface compressive stresses. Improvement of the surface condition by polishing is
generally beneficial. Corrosion inhibitors are highly effective.




Applied stress versus cycles to failure.

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Failure Modes

Fatigue fractures are caused by the simultaneous action of cyclic stress, tensile stress, and plastic strain. The
cyclic stress and strain starts the crack, and the tensile stress produces crack growth. Defects, pits,
imperfections, .etc are initiators of fatigue. Corrosion fatigue occurs in corrosive environments, such as wash-
out.




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                                       Fretting Corrosion
Fretting-corrosion is a combined damage mechanism involving corrosion at points where two moving metal
surfaces make rubbing contact. It occurs essentially when the interface is subjected to vibrations (repeated
relative movement of the two contacting surfaces) and to compressive loads. The amplitude of the relative
movement is very small, typically of the order of a few microns. When the frictional movement in a corrosive
medium is continuous, the resulting process is termed tribo-corrosion.

Means of preventing fretting corrosion :

    •     lubrication with oils or greases, to reduce friction and exclude oxygen from the interface.

    •     Increase in the hardness of one or both materials in contact. Certain material combinations show
          better friction behavior than others. Surface hardening treatments can be beneficial.

    •     Use of seals to absorb vibrations and exclude oxygen and/or moisture.

    •     Reduction of the friction loads in certain cases, or on the contrary, increase of the friction loads to
          attenuate vibrations.

    •     Modification of the amplitude of the relative movement between the two contacting surfaces




Friction-wear at an axle-cylinder contact point.

The rapid corrosion that occurs at the interface between contacting, highly loaded metal surfaces when
subjected to slight vibratory motions is known as fretting corrosion.




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                                             The photo above shows fretting corrosion of a fence post and
                                             wires which swing in the wind and wear against the post. Both the
                                             fence post and the connecting wires are experiencing fretting
                                             corrosion.

                                             This type of corrosion is most common in bearing surfaces in
                                             machinery, such as connecting rods, splined shafts, and bearing
                                             supports, and often causes a fatigue failure. It can occur in
                                             structural members such as trusses where highly loaded bolts are
used and some relative motion occurs between the bolted members.

                                             Fretting corrosion is greatly retarded when the contacting surfaces
                                             can be well lubricated as in machinery-bearing surfaces so as to
                                             exclude direct contact with air.

                                             The bearing race above is a classic example of fretting corrosion.
                                             This is greatly retarded when the contacting surfaces can be well
                                             lubricated as in machinery-bearing surfaces so as to exclude
                                             direct contact with air.




The fretting on a large aluminum part (above left) led to deposits of debris (shown in the cross sections on the
right). The vibratory motions rubbing back and forth also produced the fatigue cracks shown in the section on
fatigue corrosion.

Fretting corrosion is a limited but highly damaging type of corrosion. It is caused by a slight vibration,
friction, or slippage between two contacting surfaces that are under stress and heavily loaded. It is usually
associated with machined parts. Examples of these parts are the area of contact of bearing surfaces, two
mating surfaces, and bolted or riveted assemblies. At least one of the surfaces must be metal. In fretting
corrosion, the slipping movement on the contacting surface destroys the protective films that are
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present on the metallic surface. This action removes fine particles of the basic metal. The particles oxidize and
form abrasive materials, which further agitate within a confined area to produce deep pits. Such pits are
usually located in an area that increases the fatigue failure potential of the metal. Early signs of
fretting corrosion are surface discoloration and the presence of corrosion products in lubrication. Lubrication
and securing the parts so that they are rigid are effective measures to prevent this type of corrosion.




Description

Fretting corrosion is corrosion that can occur on the load bearing contact surface between mating material. It is
caused by the combination of corrosion and the abrasive effects of corrosion product debris often seen in
equipment with moving or vibrating parts. Other problems induced by fretting corrosion include: surface pitting.
seizing and galling of mating surfaces. reduced fatigue life as a result of stress concentrations produced on the
metal surface.

Prevention or Remedial Action

    •     use of lubricants and surface coatings designed to improve lubricity and limit metal-on-metal wear.

    •     increased surface hardness.

    •     use of barriers to limit ingress of corrosive environment to mating surfaces.

    •     reduce bearing loads on mating surfaces.";

Standard Test Methods

    •     ASTM G-77 - practice for ranking materials to sliding wear using block-on-ring wear test.

    •     ASTM G-98 - test for galling resistance of materials.

Evaluation of Fretting Corrosion

Fretting corrosion is produced by the combined effects of corrosion and wear. Therefore, factors influencing
either the severity of corrosion or the bearing load between the surfaces can affect fretting corrosion.
Parameters that need to be controlled in fretting corrosion evaluations include:

              •   corrosive environment

              •   contact load

              •   amplitude and frequency of load fluctuations

              •   cycles

              •   temperature

              •   availability of moisture

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Typically the more volumous the corrosion product and the high the bearing loads, the more intense will be the
fretting corrosion response in service.


More reading:

Take special note as you examine the asperity model: The asperity contact points are very small, of the order
of microns in diameter. These points are distributed across an apparent contact area determined by the
geometry of the contact springs at the interface and the contact force exerted by the springs, due to their
deflection on mating. The electrical current across the contact interface must flow through the asperity contact
points, resulting in a resistance called constriction resistance. The magnitude of the constriction resistance
depends on the number, size, and distribution of the asperity contacts at the interface, because all the asperity
contacts are in parallel, electrically. Constriction resistance exists even in the ideal case, when all the asperity
contact interfaces are metal-to-metal, e.g. gold-to-gold or tin-to-tin. If any of the asperity interfaces are
compromised by corrosion films or contaminants, the constriction resistance will increase. This is the reason
why corrosion is a degradation mechanism for connectors. Loss of asperity contact area, or of asperity
contacts, due to corrosion or contamination can result in contact interface resistance increases that are
sufficient to lead to connector failures.




              Figure 1: Schematic illustration of the structure of a contact interface resulting from
                   the intrinsic surface roughness on the micro-scale of the contact interface.

The kinetics of corrosion mechanisms in connectors can be very complex, but for the purposes of this
discussion, two such mechanisms will be highlighted: surface corrosion and motion-induced corrosion, or
fretting corrosion. Surface corrosion is a concern for all connector interfaces, even gold. It is important to note
that the gold is not the source of corrosion products; rather it is the base metal of the contact spring, usually a
copper alloy, that is the corrosion source.



In motion-induced, or fretting corrosion, the term “fretting” refers to the small scale of a few, or up to a few tens
of micron’s repetitive motions. Driving forces for fretting include vibration, mechanical and thermal shock, and
thermal expansion mismatch due to temperature cycling. Those driving forces probably sound familiar, as they
are the conditioning methods for a number of connector test specifications to assess the stability of connector

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contact resistance. Fretting corrosion is the predominant degradation mechanism for tin-plated connector
systems. A discussion of the details of tin-to-tin contact interfaces helps us better understand the process.




                          Figure 2: Schematic illustration of the structure of a tin surface.

Figure 2 schematically illustrates the important characteristics of tin surfaces as they relate to connector
contact interfaces. Tin is a soft and ductile metal that always has a very hard, brittle, and thin oxide, of the
order of a hundredth of a micron, on its surface. Tin oxide is a semiconductor, but the hard-over-soft structure
of tin makes it very easy to disrupt and displace the tin oxide, so that direct tin-to-tin contact can result in a
metal-to-metal and, thus, low-contact resistance. The mechanics of the displacement are simple. The tin oxide,
being brittle and thin, cannot support an applied load, so the oxide cracks and the load transfers to the
underlying soft and ductile tin. The tin flows under the applied load and the cracks in the oxide widen with the
flowing tin extruding through the cracks to make contact to the surface applying the load. Thus, it is easy to
establish a low resistance, metal-to-metal, contact interface between two tin-plated surfaces. The potential
problem is maintaining the integrity of that interface under fretting conditions.




                       Figure 3: Schematic illustration of the kinetics of fretting corrosion.

Figure 3 schematically illustrates the kinetics of fretting corrosion of tin contact interfaces. The top figure shows
the initial interface created as the tin oxide is displaced. At this point the electrical resistance of the interface

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will be of the order of a milliohm or so. If the contact interface moves, it experiences a fretting event as a result
of any of the driving forces mentioned previously, and a new contact interface will be created in essentially the
same manner as the original interface. This new contact will have a similar contact resistance. At the site of the
original interface, the disrupted tin interface area will be exposed to air—specifically, to oxygen—and a new
layer of tin oxide will form at all the original contact points. This is the corrosion part of fretting corrosion. If the
fretting motions are repeated, each repetition will result in the formation of additional tin oxide debris in the
general area of the contact interface. As this debris accumulates in and around the contact interface, it
interferes with an increasing number of asperity contact spots and, eventually, the contact resistance of the
interface will increase. The rate of resistance increase is dependent on many factors, the most important being
the length of the fretting motion and the contact force. The importance of the length of motion is in its impact on
the accumulation of oxide debris at the interface. Small motions produce a small amount of debris, but the
debris remains at the contact interface. Longer motions may produce larger amounts of oxide debris, but the
debris may be pushed towards the end of the fretting motion track, reducing the immediate impact of the debris
on contact resistance. The effect of contact force is similar. Low forces will produce less wear, and, therefore,
less oxide debris, but high forces will be more effective at displacing the oxide debris towards the ends of the
fretting track. Needless to say, the geometry of the contact springs at the contact interface also plays an
important role. The kinetics of fretting corrosion are complex indeed.




       Figure 4: Schematic illustration of the relationship between contact resistance and fretting cycles.



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Figure 4 schematically illustrates the general relationship between the average resistance increase due to
fretting corrosion and the number of fretting cycles. The green curve is for a dry, non-lubricated tin interface.
The rapid increase in resistance generally occurs at the order of a few thousand fretting cycles. The magnitude
of resistance change can vary from tens of milliohms to ohms, and even open circuit. Two features, not shown
explicitly in the graph, merit discussion. The first feature is the time dependence of fretting corrosion. That time
is, of course, dependent on the rate of fretting cycles and fretting degradation kinetics. Suffice it to say that
fretting corrosion can lead to resistance increases of the order of ohms, in tens of minutes under severe
fretting conditions. Second, Figure 4 shows the average resistance, but that is not the whole story. If the
contact resistance was continuously monitored at a high sampling rate, intermittent high resistance events
would be noted before significant changes in average resistance would be recorded. The frequency of
intermittent and the magnitude of the resistance change at each intermittent event would increase dramatically
in the same manner as the average resistance as fretting corrosion continued.


OK, fretting corrosion as a degradation mechanism leading to contact resistance degradation is a real and
significant performance issue for connectors. What can be done about it? There are two general approaches to
fretting corrosion prevention: one directed at preventing fretting, and one at preventing corrosion.


Fretting motions can be prevented if the mechanical stability of the contact interface is sufficient to withstand
the driving forces for fretting motion in the application environment of concern. The most effective means of
providing mechanical stability is through high contact forces. High contact forces mean high friction forces at
the contact interface to resist the driving forces for fretting motions. This is the reason that contact forces for tin
connector systems are in the range of hundreds of grams, as compared to the hundred grams or less typical of
gold connector systems. There are, however, limits to the magnitude of contact force that can be employed.
The benefit of the friction force that comes with contact force in providing mechanical stability has a downside
in that the same contact force also increases the mating force of the connector system. This effect may limit
the number of positions that can be realized in a tin connector system. High contact forces also mean
enhanced wear of the contact surface at the interface. As mentioned, tin is a soft material, and high contact
forces will reduce the number of mating cycles the connector system can support before the tin is worn away.
Recall also that high forces will enhance the rate of fretting debris formation, if fretting motions are not
prevented. Thus, if the contact force is not sufficient to prevent fretting motions, the fretting degradation rate
may be significantly increased.


Preventing the “corrosion” part of fretting corrosion is accomplished by using a contact lubricant. Contact
lubricant is a generic term and includes lubricants that are intended to reduce friction, as well as lubricants to
prevent fretting corrosion. It is important to specify to any lubricant supplier that an anti-fretting lubricant is
desired to prevent the improper selection and application of lubricants. There are many formulations of anti-
fretting contact lubricants available in various consistencies and with application processes designed to suit
different operating conditions and applications. Properly formulated anti-fretting lubricants can be effective at
reducing the potential for fretting corrosion. An example is the white curve, the “active lubricant,” in Figure 4.
With this lubricant, the fretting cycling was carried out to 50,000 cycles with no significant degradation in
contact resistance.


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One concern with the use of contact lubricants is ensuring proper application of the lubricant, as well as
confirming its presence on the product as received. If the lubricant is to be self-applied, the costs and possible
environmental effects of the selected lubricant must be considered. An additional potential issue may arise in
applications where the potential dust and/or contamination are high. Some contact lubricants may tend to be
“tacky” and to retain dust with the dust itself then contributing to fretting degradation.


The major connector plating systems that are susceptible to fretting corrosion are tin and nickel. Flash gold
systems may become susceptible to fretting corrosion if the flash gold is worn away due to fretting wear or the
mating cycles of the connector and the nickel under plate is exposed.




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                                     Erosion Corrosion
  Erosion corrosion is the result of a combination of an aggressive chemical environment and high fluid-
surface velocities. This can be the result of fast fluid flow past a stationary object, such as the case with the oil-
field check valve shown on the left below, or it can result from the quick motion of an object in a stationary fluid,
such as happens when a ship's propeller churns the ocean.


Surfaces which have undergone erosion corrosion are generally fairly clean, unlike the surfaces from many
other forms of corrosion.


Erosion corrosion can be controlled by the use of harder alloys (including flame-sprayed or welded hard
facings) or by using a more corrosion resistant alloy. Alterations in fluid velocity and changes in flow patterns
can also reduce the effects of erosion corrosion.
  Erosion corrosion is often the result of the wearing away of a protective scale or coating on the metal
surface. The oil field production tubing shown above on the right corroded when the pressure on the well
became low enough to cause multiphase fluid flow. The impact of collapsing gas bubbles caused the damage
at joints where the tubing was connected and turbulence was greater.
  Many people assume that erosion corrosion is associated with turbulent flow. This is true, because all
practical piping systems require turbulent flow-the fluid would not flow fast enough if lamellar (nonturbulent)
flow were maintained. Most, if not all, erosion corrosion can be attributed to multiphase fluid flow. The check
valve on the left above failed due to sand and other particles in an otherwise noncorrosive fluid. The tubing on
the right failed due to the pressure differences caused when gas bubbles collapsed against the pipe wall and
destroyed the protective mineral scale that was limiting corrosion.




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Erosion corrosion is acceleration in the rate of corrosion attack in metal due to the relative motion of a
corrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surfaces of a
tube can result in rapidly increasing erosion rates and eventually a leak. Erosion corrosion can also be
aggravated by faulty workmanship. For example, burrs left at cut tube ends can upset smooth water flow,
cause localized turbulence and high flow velocities, resulting in erosion corrosion. A combination of erosion
and corrosion can lead to extremely high pitting rates.



Erosion-corrosion is most prevalent in soft alloys (i.e. copper, aluminum and lead alloys). Alloys which form a
surface film in a corrosive environment commonly show a limiting velocity above which corrosion rapidly
accelerates. With the exception of cavitation, flow induced corrosion problems are generally termed erosion-
corrosion, encompassing flow enhanced dissolution and impingement attack. The fluid can be aqueous or
gaseous, single or multiphase. There are several mechanisms described by the conjoint action of flow and
corrosion that result in flow-influenced corrosion:

Mass transport-control: Mass transport-controlled corrosion implies that the rate of corrosion is dependent
on the convective mass transfer processes at the metal/fluid interface. When steel is exposed to oxygenated
water, the initial corrosion rate will be closely related to the convective flux of dissolved oxygen towards the
surface, and later by the oxygen diffusion through the iron oxide layer. Corrosion by mass transport will often
be streamlined and smooth.

Phase transport-control: Phase transport-controlled corrosion suggests that the wetting of the metal surface
by a corrosive phase is flow dependent. This may occur because one liquid phase separates from another or
because a second phase forms from a liquid. An example of the second mechanism is the formation of
discrete bubbles or a vapor phase from boiler water in horizontal or inclined tubes in high heat-flux areas under
low flow conditions. The corroded sites will frequently display rough, irregular surfaces and be coated with or
contain thick, porous corrosion deposits.

Erosion-corrosion: Erosion-corrosion is associated with a flow-induced mechanical removal of the protective
surface film that results in a subsequent corrosion rate increase via either electrochemical or chemical
processes. It is often accepted that a critical fluid velocity must be exceeded for a given material. The
mechanical damage by the impacting fluid imposes disruptive shear stresses or pressure variations on the
material surface and/or the protective surface film. Erosion-corrosion may be enhanced by particles (solids or


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gas bubbles) and impacted by multi-phase flows. The morphology of surfaces affected by erosion-corrosion
may be in the form of shallow pits or horseshoes or other local phenomena related to the flow direction.

Corrosion Erosion

Air was sucked into the intake and the turbulence caused failure from a combination of corrosion and erosion.




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DESCRIPTION

Erosion corrosion is the corrosion of a metal which is caused or accelerated by the relative motion of the
environment and the metal surface. It is characterized by surface features with a directional pattern which are
a direct result of the flowing media. Erosion corrosion is most prevalent in soft alloys (i.e. copper, aluminum
and lead alloys). Alloys which form a surface film in a corrosive environment commonly show a limiting velocity
above which corrosion rapidly accelerates. Other factors such as turbulence, cavitation, impingement or
galvanic effects can add to the severity of attack.

Prevention or Remedial Action

    •     selection of alloys with greater corrosion resistance and/or higher strength.

    •     re-design of the system to reduce the flow velocity, turbulence, cavitation or impingement of the
          environment.

    •     reduction in the corrosive severity of the environment.

    •     use of corrosion resistant and/or abrasion resistant coatings.

    •     cathodic protection.

Standard Test Methods

    •     ASTM G-32 - method of vibratory cavitation erosion testing.

    •     ASTM G-73 - practice for liquid impingement erosion testing

    •     ASTM G-75 - test method for slurry abrasivity by miller number.

    •     ASTM G-76 - practice for conducting erosion tests by solid particle impingement using gas jet.

    •     NACE TM0170 - method of conducting controlled velocity laboratory corrosion tests.

    •     NACE TM0286 - cooling water test units incorporating heat transfer surfaces.

Evaluation of Erosion Corrosion

Many specialized tests have been utilized to evaluate erosion corrosion. Typically, the nature of the attack from
erosion corrosion and/or velocity accelerated corrosion can be vary specific to the geometry and exposure
conditions. Therefore, the results of tests and the test/service conditions must always be careful examined.
The most commonly utilized methods are spinning cylinder and disk apparatus since they are relatively easy to
set-up and they produce conditions that are easily evaluated. However, they do not always give conditions that
represent those in actual service. Recently, great use of jet impingement and actual pipe flow cells have been
utilized which can more accurately simulate conditions of turbulent flow and multiphase environments. These
tests should be conducted to produce carefully quantified conditions of wall shear stress that match those in
the intended service. The wall shear stress is a measure of the mechanical action produced on the surface of
the material by the flowing media and most directly relates to the damage or removal of normally protective
corrosion products and inhibitor films.




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                                  Dealloying Corrosion
Dealloying is a rare form of corrosion found in copper alloys, gray cast iron, and some other alloys. Dealloying
occurs when the alloy loses the active component of the metal and retains the more corrosion resistant
component in a porous "sponge" on the metal surface. It can also occur by re-deposition of the noble
component of the alloy on the metal surface.
Control is by the use of more resistant alloys-inhibited brasses and malleable or nodular cast iron.




The brass on the left dezincified leaving a porous copper plug on the surface. The gray cast iron water pipe
shown on the right photo has graphitized and left graphitic surface plugs which can be seen on the cut surface.
The rust tubercules or bubbles are also an indication of pitting corrosion.

The bottom photo shows a layer of copper on the surface of a de-alloyed 70% copper-30% nickel cupronickel
heat exchanger tube removed from a ship. Stagnant seawater is so corrosive that even this normally
corrosion-resistant alloy has corroded. Virtually all copper alloys are subject to de-alloying in some
environments.

                                                   a




This process, also called "dealloying" or "selective leaching", involves the selective dissolution of one of the
elements in a single phase alloy or one of the phases in a multiphase alloy
The most well known example is the dezincification of brass (e.g. 70Cu - 30Zn). In this case, the brass takes
on a red coppery tinge as the zinc is removed. It also becomes porous and very brittle, without modification to
the overall dimensions of the part

This problem can be overcome by choosing an alloy that is less prone, such as a copper-rich cupro-nickel.
Brasses with lower zinc contents or containing elements such as tin (1%) and/or small quantities of arsenic,
antimony, or phosphorus have much greater resistance.
Numerous other alloys are susceptible to selective corrosion in certain conditions. For example, denickelization


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can occur in Cu-Ni alloys, and dealuminization in aluminum bronzes, while the graphitization phenomenon in
grey cast irons is due to slow dissolution of the ferrite matrix.




Micrographic appearance of a dezincification of brass.




The brass on the left dezincified leaving a porous copper plug on the surface. The gray cast iron water pipe shown on the
right photo has graphitized and left graphitic surface plugs which can be seen on the cut surface. The rust tubercules or
bubbles are also an indication of pitting corrosion




Dezincification corrosion of an Admiralty brass exchanger tube in cooling water service.

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                                       Hydrogen Damages
Hydrogen damages can be broadly classified into 3 categories:

          Ductile hydrogen blistering.

          Brittle hydrogen embrittlement.

          High temperature H2 surface attack.

Hydrogen blistering can occur when hydrogen enters steel as a result of the reduction reaction on a metal
                                   +
cathode. Single-atom nascent H hydrogen atoms then diffuse through the metal until they meet with another
atom, usually at inclusions or defects in the metal. The resultant diatomic hydrogen molecules are then too big
to migrate and become trapped. Eventually a gas blister builds up and may split the metal.

Hydrogen Induced Cracking (HIC) or hydrogen embrittlement is a brittle mechanical fracture caused by
penetration and diffusion of atomic hydrogen into the crystal structure of an alloy. It occurs in corrosive
environment under tensile stress, similar to stress corrosion cracking (SCC); however, cathodic protection
initiates or enhances HIC but suppresses or stops SCC. The cracks are usually non-branching and fast
growing, and can be transgranular (through the grains) or intergranular (through the grain boundaries).
Hydrogen embrittlement is a problem with high-strength steels, titanium, and some other metals. Control is by
eliminating hydrogen from the environment or by the use of resistant alloys.

High temperature H2 attack occurs when an alloy is exposed to high temperature in H2 environment, It is most
surface phenomenon involve decarburizing, hydride H- formation and deterioration of mechanical properties
and post heat susceptibility to cracking.




Hydrogen Induce Cracking.



HIC occurs in high strength steels when atomic hydrogen dissolves in the crystal lattice of the metal rather
than forming H2 gas. In the oilfield, the presence of H2S gas often leads to sulfide stress cracking (SSC), which
is a special case of hydrogen induced stress cracking. A process resulting in a decrease of the toughness or ductility
of a metal due to the presence of atomic hydrogen.

The presence of hydrogen atoms in a metal crystal lattice can be extremely detrimental, leading to a
catastrophic loss of mechanical strength and ductility. It is generally accepted that the hydrogen is first of all
adsorbed on the metal surface before penetrating the lattice, where it diffuses in ionic form (i.e. as protons).
The hydrogen atoms can have various origins the surrounding atmosphere containing hydrogen or
hydrogenated compounds (H2S, NH3, H2O, etc.), electroplating processes during which the proton reduction
reaction occurs, electrochemical corrosion during which the cathodic reaction is proton reduction.
Once they have penetrated the crystal lattice, hydrogen atoms can cause several types of damage.



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•   Precipitation of brittle hydrides: this occurs in titanium and other metals with a high affinity for hydrogen (Ta,
    Zr, V, Pd ).

•   Recombination to molecular hydrogen: when the metal contains macroscopic discontinuities or
    microscopic defects, these can represent sites for the recombination of hydrogen atoms. The hydrogen
    molecules are unable to diffuse away into the lattice and it is possible to build up high local pressures,
    leading to the formation of flakes and blisters, and "ladder-type" cracking.

•   Hydrogen embrittlement: by interacting with lattice dislocations, hydrogen atoms cause a marked loss in
    the plastic strain capacity of the metal, which becomes brittle.

Hydrogen embitterment (or hydrogen grooving) is the process by which various metals, most importantly high-
strength steel, become brittle and crack following exposure to hydrogen. Hydrogen cracking can pose an
engineering problem especially in the context of a hydrogen economy. However, commercially workable and
safe technology exists globally in the hydrogen industry, which produces some 50 million metric tons per year.

Hydrogen embrittlement is also used to describe the formation of zircaloy hydride. This use of the term in this
context is common in the nuclear industry.




The broken spring above on the left was brought to the Materials Laboratory for failure analysis. Examination
at high magnification in the scanning electron microscope (above right) revealed intergranular cleavage
characteristic of hydrogen assisted cracking (hydrogen embrittlement). The part was zinc plated during
refurbishment, and the hydrogen which entered the metal during the plating process had not been baked out.
A post-plating bakeout procedure should be standard for high strength steels.

Process
The mechanism begins with hydrogen atoms diffusing through the metal. When these hydrogen atoms re-
combine in minuscule voids of the metal matrix to hydrogen molecules, they create pressure from inside the
cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile
strength, up to where it can crack open, in which case it would be called Hydrogen Induced Cracking (HIC).
High-strength and low-alloy steels, aluminum, and titanium alloys are most susceptible.




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Hydrogen embrittlement can happen during various manufacturing operations or operational use, anywhere
where the metal comes in contact with atomic or molecular hydrogen. Processes which can lead to this include
cathodic protection, phosphating, pickling, and electroplating. A special case is arc welding, in which the
hydrogen is released from moisture (for example in the coating of the welding electrodes; to minimize this,
special low-hydrogen electrodes are used for welding high-strength steels). Other mechanisms of introduction
of hydrogen into metal are galvanic corrosion, chemical reactions of metal with acids, or with other chemicals
(notably hydrogen sulfide in sulphide stress cracking, or SSC, a process of importance for the oil and gas
industries).

Counteractions-HIC

Means of preventing hydrogen embrittlement are;

     Control hardness.

     Control of stress level.

     Avoid hydrogen source

     Careful selection of materials of construction and plating systems.

     Heat treatment to remove absorbed hydrogen.

For prevention of hydrogen embrittlement: reduce the corrosion rate, modify the electroplating conditions,
change the alloy, take appropriate precautions during welding and so on.

If the metal has not yet started to crack, the condition can be reversed by removing the hydrogen source and
causing the hydrogen within the metal to diffuse out - possibly at elevated temperatures. Susceptible alloys,
after chemical or electrochemical treatments where hydrogen is produced, are often subjected to heat
treatment in order to remove absorbed hydrogen.

In the case of welding, often pre- and post-heating the metal is applied to allow the hydrogen to diffuse out
before it can cause any damage. This is specifically done with high-strength steels and low alloy steels such as
the chrome/molybdenum/vanadium alloys. Due to the time needed to re-combine hydrogen atoms to the
harmful hydrogen molecules, hydrogen cracking due to welding can occur over 24 hours after the welding
operation is completed.

Hydrogen may enter a metal surface by the cathodic reduction of hydrogen or water:

        2H+ + 2e- → 2HAdsorbed (acidic waters)
        2H2O + 2e- → 2HAdsorbed + 2OH- (neutral waters)

Normally, the adsorbed hydrogen at the surface recombines to form hydrogen gas:

        2HAdsorbed → H2

However, recombination poisons such as sulfide (S2-), prevent hydrogen gas from forming and the adsorbed
hydrogen moves through the metal, thereby weakening it. Hydrogen sulfide (H2S) is especially aggressive in
promoting hydrogen damage because it provides not only the sulfide poison, but hydrogen ions (H+) as well.


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Sulfide stress cracking (SSC) occurs in high-strength drill pipes, casing, tubing, and sucker rods. Like stress
corrosion cracking (SCC), cracking may not occur below a threshold stress, however, increasing strength and
applied stress, increasing H2S concentrations and increasing acidity (decreasing pH) increase SSC
susceptibility.

As opposed to SCC, decreasing temperature also increases SSC susceptibility. Time to failure is minimum at
room temperature. The ramification is that, steels become most susceptible to SSC near the surface where the
highest strength is required to carry the weight of the string. Increasing the wall thickness of the tubular can
reduce the applied stress thus allowing the use of lower strength steels, but strength must be balanced against
the applied load at the top of the joint due to increasing weight. High strength casing may be used deeper in
the well where temperatures are higher.

In SCC, failure initiates at the crevices on the metal surface, usually in the pits. Thus, SCC susceptibility of
steels is related to its susceptibility to pitting. Whereas SSC generally initiates at impurity inclusions in the
metal, hence it is dependent on the hydrogen absorption characteristics of the metal.

Microstructure of steel also influences the SSC susceptibility. Quenched and tempered steels have better SSC
resistance than normalized and normalized and tempered steels. Acceptable hardness limits for many alloys in
sour service are described in the National Association of Corrosion Engineers (NACE) Specification MR-01-75.
For SSC resistance, the hardness of carbon and low alloy steels must be maintained below 22 Rockwell
Hardness C (HRC). Tubular based on AISI 4100 series low-alloy steels are acceptable up to HRC 26. Higher
alloyed steels may have higher hardness levels.


Hydrogen Induced Cracking-Resistant Steel Plates
Sumitomo started research earlier, and has continued it in earnest, on mechanism of and counter-measures
against hydrogen induced cracking under humid hydrogen sulfide environment. Such research was started in
the course of the development of materials for line-pipes used for sour gas and/or sour oil, and achieved
results are ranked in the top level of world research in this area.
As a result, Sumitomo's hydrogen induced cracking resistant plate, "CR5" was developed and commercialized,
aimed at application for oil refining facilities.
CR50 is produced by treating 40 and 50 kg/mm2 strength class plates as countermeasures to prevent
hydrogen induced cracking, such as reducing quantity of inclusions, shape control of inclusions and addition of
infinitesimal amount of elements to inhibit hydrogen penetration into a plate.




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Schematic illustration of various
                                    1. Cracking in environment of humid hydrogen sulfide
cracks
                                    It was known for a long time since the old days that cracks occur under
                                    humid environment containing hydrogen sulfide.
                                    Mechanisms for such cracks are classified into the following two
                                    categories.


                                    (1) Sulfide stress corrosion cracking (SSC)
a. Blister
                                    It occurs when external stress (working stress, residual stress) is
b. HIC
                                    working on steel, and propagates to the vertical direction to axial stress.
c. SSC (low strength steel)
                                    It is also called Sulfide Stress Cracking (SC).
d. (high strength steel)

                                    (2) Hydrogen induced cracking (HIC)
                                    It occurs under a condition without external stress. The cracking is
Example of HIC cracking
                                    parallel to the plate surface and propagates stepwise to the thickness
                                    direction with time.
                                    Surface swelling due to occurrence of cracks on the surface or
                                    immediately beneath the surface is called blistering.




Cracking is stepwise and almost
goes through the thickness.




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Hydrogen Blistering.



A special case of hydrogen damage is known as hydrogen blistering. Hydrogen blistering occurs when
hydrogen atoms diffuse into the steel, and hydrogen gas nucleates at internal defects and inclusions, forming
voids which eventually generate enough pressure to locally rupture the metal.




Hydrogen blistering is occasionally observed in the oilfield in sour systems.




Hydrogen blistering is controlled by minimizing corrosion in acidic environments. It is not a problem in neutral
or caustic environments or with high-quality steels that have low impurity and inclusion levels.




Blistering related to excessive cathodic protection of an oil pipe collector




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High Temperature Hydrogen Attack. HTHA.




Hydrogen attack on steels is manifest as decarburization, intergranular fissuring, or blistering. These
conditions result in lowered tensile strength, ductility, and impact strength. The reaction of hydrogen with iron
carbide to form methane is probably the most important chemical reaction involved in the attack on steel by
hydrogen. Attack of steel at elevated temperatures and pressures is limited or prevented by the following
measures: (1) use of steel alloyed with strong carbide-forming elements, (2) use of liners of resistant alloy
steels, (3) substitution of resistant nonferrous alloys and (4) introduction of diffusion barrier.




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Hydrogen attack corrosion and cracking on the ID of an 1800 psig carbon steel boiler tube.

If steel is exposed to hydrogen at high temperatures, hydrogen will diffuse into the alloy and combine with
carbon to form tiny pockets of methane at internal surfaces like grain boundaries and voids. This methane
does not diffuse out of the metal, and collects in the voids at high pressure and initiates cracks in the steel.
This process is known as hydrogen attack and leads to decarburization of the steel and loss of strength.




High Temperature Hydrogen Attack (HTHA) is a form of degradation caused by hydrogen reacting with carbon to form
methane in a high temperature environment.

C + 4H --> CH4

The methane forms and stays in grain boundaries and voids however it does not diffuse out of the metal. Once it
accumulated in the grains and voids, it expands and forms blister , weaken the metal strength and initiate cracks in the
steel.

High-strength low-alloy steels are particularly susceptible to this mechanism, which leads to embrittlement of the bulk
parent metal (typical C-0.5 Mo steels). The embrittlement in the material can result in a catastrophic brittle fracture of the
asset.


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Description

Hydrogen induced damage describes any of a number of forms of degradation of metals caused by exposure
to environments (liquid or gas) which cause absorption of hydrogen into the material to cause degradation in
mechanical performance. Examples of hydrogen induced damage are:

1. Formation of internal cracks, blisters or voids in steels.

2. Embrittlement (i.e. loss of ductility).

3. High temperature hydrogen attack (i.e. surface decarburizing and chemical reaction with hydrogen).

Prevention or Remedial Action

1. internal cracking or blistering

              Use of steel with low levels of impurities (i.e. sulfur and phosphorus).

              Modifying environment to reduce hydrogen charging.

              Use of surface coatings and effective inhibitors.

2. hydrogen embrittlement

              Use of lower strength (hardness) or high resistance alloys.

              Careful selection of materials of construction and plating systems.

              Heat treatment to remove absorbed hydrogen.

3. high temperature hydrogen attack

              Selection of material (for steels, use of low and high alloy Cr-Mo steels; selected Cu alloys; non-
              ferrous alloys).

              Limit temperature and partial pressure H2.



Standard Test Methods



NACE TM0177 - laboratory testing of metals for resistance to sulfide stress cracking in H2S environments.

    •     NACE TM0284 - evaluation of pipeline and plate steels for resistance to stepwise cracking.

    •     ASTM G129 - slow strain rate test for determination of environmentally assisted cracking.

    •     ASTM G142 - tension tests in hydrogen environments.

    •     ASTM G146 - hydrogen induced disbonding of stainless clad steel plate in refinery hydrogen service.

    •     ASTM F-326 - method for electronic hydrogen embrittlement test for cadmium electroplating processes.

    •     ASTM F-519 - method for mechanical hydrogen embrittlement testing of plating processes and aircraft
          maintenance chemicals.

    •     ASTM A-143 - practice of safeguarding against embrittlement of hot dip galvanized structural steel
          products and detecting embrittlement.

    •     ASTM B-577 - hydrogen embrittlement of deoxidized and oxygen free copper.


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    •     NACE TM0177 - laboratory testing of metals for resistance to sulfide stress cracking in H2S
          environments

    •     F1459-06 Standard Test Method for Determination of the Susceptibility of Metallic Materials to
          Hydrogen Gas Embrittlement (HGE)



Evaluation for Hydrogen Induced Damage

Since hydrogen can induce many types of damage in engineering materials, it is impossible to look to only one
test method for all problems.

    •     Slow strain rate test methods are good to obtain general information on the inherent susceptibility to
          hydrogen embrittlement is a short period of time. However, the results will generally be very
          conservative.

    •     For higher strength materials, the use of constant load tests for determination of an apparent threshold
          stress for cracking is a generally accepted technique.

    •     Hydrogen induced cracking and blistering of low strength steels can be tested using non-stressed
          coupons exposed to the test environment. However, in some cases, the addition of an externally
          applied or residual tensile stress can cause materials to crack that do not show cracking in the non-
          stressed condition. Also, constant load specimens may not fail under tensile stress even though they
          may have extensive internal cracking or blistering.

High temperature hydrogen damage and disbonding must be evaluated for the specific conditions of time and
temperature for the intended use. However, it can in many cases, be accelerated with the combination of
higher temperature and/or hydrogen pressure.




          Clicks for more information on the subjects:

           Hydrogen Induced cracking along the fusion boundary of welding of dissimilar metals.
           Hydrogen Effects in Metals.
           Ferritic and austenitic sintered stainless steel fatigue cracking resistance propagation: Hydrogen
           embrittlement influences.
           Influences of thermo-hydrogen of micro structural evolution and hardness of Ti600 alloy.
           Hydrogen permeability and integrity of hydrogen transfers pipeline.
           Hydrogen delay cracking of high strength weldable steels.




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                                  Concrete Corrosion




The picture on the left shows cracking and staining of a seawall near the Kennedy Space Center. The pitting
corrosion in the right photo occurred on an aluminum railing on a concrete causeway over an inlet to the
Atlantic Ocean.

Concrete is a widely-used structural material that is frequently reinforced with carbon steel reinforcing rods,
post-tensioning cable or pre-stressing wires. The steel is necessary to maintain the strength of the structure,
but it is subject to corrosion. The cracking associated with corrosion in concrete is a major concern in areas
with marine environments (like KSC) and in areas which use deicing salts.

There are two theories on how corrosion in concrete occurs:
         Salts and other chemicals enter the concrete and cause corrosion. Corrosion of the metal leads to
         expansive forces that cause cracking of the concrete structure.

         Cracks in the concrete allow moisture and salts to reach the metal surface and cause corrosion.
Both possibilities have their advocates, and it is also possible that corrosion in concrete can occur either way.
The mechanism isn't truly important, the corrosion leads to damage, and the damage must be controlled.

In new construction, corrosion in concrete is usually controlled by embedding the steel deep enough so that
chemicals from the surface don't reach the steel (adequate depth of cover). Other controls include keeping the
water/cement ratio below 0.4, having a high cement factor, proper detailing to prevent cracking and ponding,
and the use of chemical admixtures. These methods are very effective, and most concrete structures, even in
marine environments, do not corrode.

Unfortunately, some concrete structures do corrode. When this happens, remedial action can include repairing
the cracked and spalled concrete, coating the surface to prevent further entry of corrosive chemicals into the
structure, and cathodic protection, an electrical means of corrosion control. KSC has experience with all of
these methods of controlling corrosion on existing concrete structures.




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                                    Microbial Corrosion                             Read More


Microbial corrosion (also called microbiologically-influenced corrosion or MIC) is corrosion that is caused by
the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides
or by conventional corrosion control methods.

There are a number of mechanisms associated with this form of corrosion, and detailed explanations are listed
at the bottom of this section. Most MIC takes the form of pits that form underneath colonies of living organic
matter and mineral and biodeposits. This biofilm creates a protective environment where conditions can
become quite corrosive and corrosion is accelerated.

The physical presence of microbial cells on a metal surface, as well as their metabolic activities, can cause
Microbiologically Influenced Corrosion (MIC) or biocorrosion. The forms of corrosion caused by bacteria are
not unique. Biocorrosion results in pitting, crevice corrosion, selective dealloying, stress corrosion cracking,
and under-deposit corrosion. The following mechanisms are some of the causes of biocorrosion.

Oxygen depletion or differential aeration cells




Nonuniform (patchy) colonization by bacteria results in differential aeration cells. This schematic shows pit
initiation due to oxygen depletion under a biofilm. (Borenstein 1994)



Nonuniform (patchy) colonies of biofilm result in the formation of differential aeration cells where areas under
respiring colonies are depleted of oxygen relative to surrounding noncolonized areas. Having different oxygen
concentrations at two locations on a metal causes a difference in electrical potential and consequently
corrosion currents. Under aerobic conditions, the areas under the respiring colonies become anodic and the
surrounding areas become cathodic.

Stainless steels’ protective film
Oxygen depletion at the surface of stainless steel can destroy the protective passive film. Remember that
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stainless steels rely on a stable oxide film to provide corrosion resistance. Corrosion occurs when the oxide
film is damaged or oxygen is kept from the metal surface by microorganisms in a biofilm.


Sulfate-reducing bacteria
Oxygen depletion at the surface also provides a condition for anaerobic organisms like sulfate-reducing
bacteria (SRB) to grow. This group of bacteria are one of the most frequent causes for biocorrosion. They
reduce sulfate to hydrogen sulfide which reacts with metals to produce metal sulfides as corrosion products.
Aerobic bacteria near the outer surface of the biofilm consume oxygen and create a suitable habitat for the
sulfate reducing bacteria at the metal surface. SRBs can grow in water trapped in stagnant areas, such as
dead legs of piping. Symptoms of SRB-influenced corrosion are hydrogen sulfide (rotten egg) odor, blackening
of waters, and black deposits. The black deposit is primarily iron sulfide. (Borenstein 1994 and Geesey 1994)



"One way to limit SRB activity is to reduce the concentration of their essential nutrients: phosphorus, nitrogen,
and sulfate. Thus, purified (RO or DI) waters would have less problem with SRBs. Also, any practices which
minimize biofilm thickness (flushing, sanitizing, eliminating dead-end crevices) will minimize the anaerobic
areas in biofilm which SRBs need" (Geesey 1994).




Byproducts of bacterial metabolism
Another corrosion mechanism is based on the by-products of bacterial metabolism.

Acid-producing bacteria
Bacteria can produce aggressive metabolites, such as organic or inorganic acids. For example, Thiobacillus
thiooxidans produces sulfuric acid and Clostridium aceticum produces acetic acid. Acids produced by bacteria
accelerate corrosion by dissolving oxides (the passive film) from the metal surface and accelerating the
cathodic reaction rate (Borenstein 1994).

Hydrogen-producing bacteria
Many microorganisms produce hydrogen gas as a product of carbohydrate fermentation. Hydrogen gas can
diffuse into metals and cause hydrogen embrittlement.




Iron bacteria
Iron-oxidizing bacteria, such as Gallionella, Sphaerotilus, Leptothrix, and Crenothrix, are aerobic and
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filamentous bacteria which oxidize iron from a soluble ferrous (Fe2+) form to an insoluble ferric (Fe3+) form.
The dissolved ferrous iron could be from either the incoming water supply or the metal surface. The ferric iron
these bacteria produce can attract chloride ions and produce ferric chloride deposits which can attack
austenitic stainless steel. For iron bacteria on austenitic stainless steel, the deposits are typically brown or red-
brown mounds.

Biofilm can be removed and/or destroyed by chemical and physical treatments. Chemical biocides can be
divided into two major groups: oxidizing and nonoxidizing. Physical treatments include mechanical scrubbing
and hot water.

The picture below shows a biofilm on a metallic condenser surface. These biofilms can allow corrosive
chemicals to collect within and under the films. Thus the corrosive conditions under a biofilm can be very
aggressive, even in locations where the bulk environment is noncorrosive.

MIC can be a serious problem in stagnant water systems such as the fire-protection system that produced the
pits shown above. The use of biocides and mechanical cleaning methods can reduce MIC, but anywhere
where stagnant water is likely to collect is a location where MIC can occur.

Corrosion (oxidation of metal) can only occur if some other chemical is present to be reduced. In most
environments, the chemical that is reduced is either dissolved oxygen or hydrogen ions in acids. In anaerobic
conditions (no oxygen or air present), some bacteria (anaerobic bacteria) can thrive. These bacteria can
provide the reducible chemicals that allow corrosion to occur. That's how the limited corrosion that was found
on the hull of the Titanic occurred. The picture below shows a "rusticle" removed from the hull of Titanic. This
combination of rust and organic debris clearly shows the location of rivet holes and where two steel plates
overlapped.

Much microbial corrosion involves anaerobic or stagnant conditions, but it can also be found on structures
exposed to air. The pictures below show a spillway gate from a hydroelectric dam on the Columbia River. The
stress corrosion cracks were caused by pigeon droppings which produced ammonia-a chemical that causes
stress corrosion cracking on copper alloys like the washers used on this structure. Since it's impossible to potty
train pigeons, a new alloy resistant to ammonia was necessary.

In addition to the use of corrosion resistant alloys, control of MIC involves the use of biocides and cleaning
methods that remove deposits from metal surfaces. Bacteria are very small, and it is often very difficult to get a
metal system smooth enough and clean enough to prevent MIC.

Typical corrosion morphology of line pipe steel induced by SRB-related MIC buried at anaerobic soil

Overview MIC is the one of major risk factor for underground pipelines. This interdisciplinary subject require
knowledge for corrosion science, surface chemistry, microbiology, soil science etc. Our continuous field and
laboratory experience for 6 years in this area makes it possible to detection, monitoring, mitigation of MIC
successfully. The expertise provide a better understanding of corrosion mechanisms, permitting the use of
cost-effective solutions to MIC problems .




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SEM photo of sulfate reducing bacteria (SRB) mixed with biogenic, porous iron sulfides,
attached to carbon steel surface exposed to anaerobic soil for 140 day




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Bacteria Stress Corrosion Cracking : Bacterial activity led to stress corrosion cracking in this 304 SS bolt. As the cracking
progressed, the bacteria colonized the cracks, causing more cracking.




Bacteria Nodule: Bacteria growth on a weld in a 304 SS tank




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Pipe Deposits Bacteria in untreated river water caused these deposits in a low-flow cooling water line




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Treated & Untreated River Water Bacteria deposits on the untreated water coupon contrast with the clean coupon in the
water treated with a biocide.




scanning electron micrograph image shows a metal surface from which the sulfate-reducing biofilm was scraped away, as
well as a portion of the metal surface still encrusted by biofilm and corrosion products. Pitting due to microbial corrosion is
evident in the exposed metal.

   More Reading on MIC of Piping

   MIC Predictive Maintenance for Fire Sprinkler Systems

   Microbial Lecture University of Florida: http://www.abe.ufl.edu/~chyn/age4660/lect.htm

   Microbial Diversity: http://www.learner.org/courses/biology/units/microb/index.html




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                                  Cavitation Corrosion
Cavitation:
Cavitation sometimes is considered a special case of erosion-corrosion and is caused by the formation and
collapse of vapor bubbles in a liquid near a metal surface. Cavitation removes protective surface scales by the
implosion of gas bubbles in a fluid. Calculations have shown that the implosions produce shock waves with
pressures approaching 60 ksi. The subsequent corrosion attack is the result of hydro-mechanical effects from
liquids in regions of low pressure where flow velocity changes, disruptions, or alterations in flow direction have
occurred. Cavitation damage often appears as a collection of closely spaced, sharp-edged pits or craters on
the surface.



In offshore well systems, the process industry in which components come into contact with sand-bearing
liquids, this is an important problem. Materials selection plays an important role in minimizing erosion corrosion
damage. Caution is in order when predicting erosion corrosion behavior on the basis of hardness. High
hardness in a material does not necessarily guarantee a high degree of resistance to erosion corrosion.
Design features are also particularly important.

It is generally desirable to reduce the fluid velocity and promote laminar flow; increased pipe diameters are
useful in this context. Rough surfaces are generally undesirable. Designs creating turbulence, flow restrictions
and obstructions are undesirable. Abrupt changes in flow direction should be avoided. Tank inlet pipes should
be directed away from the tank walls, towards the center. Welded and flanged pipe sections should always be
carefully aligned. Impingement plates of baffles designed to bear the brunt of the damage should be easily
replaceable.

The thickness of vulnerable areas should be increased. Replaceable ferrules, with a tapered end, can be
inserted into the inlet side of heat exchanger tubes, to prevent damage to the actual tubes. Several
environmental modifications can be implemented to minimize the risk of erosion corrosion. Abrasive particles
in fluids can be removed by filtration or settling, while water traps can be used in steam and compressed air
systems to decrease the risk of impingement by droplets. De-aeration and corrosion inhibitors are additional
measures that can be taken. Cathodic protection and the application of protective coatings may also reduce
the rate of attack.




Cavitation occurs in liquid when bubbles form and implode in pump systems or around propellers. Pumps put
liquid under pressure, but if the pressure of the substance drops or its temperature increases, it begins to


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vaporize, just like boiling water. Yet in such a small, sensitive system, the bubbles can't escape so they
implode, causing physical damage to parts of the pump or propeller.

A combination of temperature and pressure constraints will result in cavitation in any system. No manufacturer
or industrial technician wants to run pumps that keep getting affected by cavitation, as it will permanently
damage the chambers of the device. The vaporization actually causes a loud, rocky noise because the
bubbles are imploding and making the liquid move faster than the speed of sound!




Inside every pump, there is a propeller that draws liquid from one side of the chamber to the other. The liquid
normally continues out through a valve so it can do another job in a different part of the machine. Sometimes
this device is called an impeller. Even though the total chamber stays under the same pressure, and the
materials are temperature regulated, cavitation manages to occur right next to the surface of the propeller.

A propeller rotates through a liquid and actually creates localized differences in pressure along the propeller
blades. This can even occur underwater on a submarine or ship's propeller. The bubbles of cavitation appear
in low-pressure areas but then immediately want to implode with such force that they make dings and pits in
metal. A propeller exposed to cavitation resembles the surface of the moon, with tiny, scattered craters.

There are two types of cavitation that can occur in the different stages of pumping, but both are results of the
same phenomenon. Suction or classical cavitation occurs around the impeller as it is drawing liquid through
the chamber. The propeller's motion creates the changes in pressure necessary for vaporization.

Discharge or recirculation cavitation is the result of changing pressure at the point of exit, the discharge valve.
The valve is not able to let all the liquid through as fast as it should, so the currents' different velocities create
miniature changes in the uniform pressure. Even such small variations are enough to create the ideal
circumstances for cavitation. Cavitation mostly affected pump, propeller and fan-like rotating equipments.



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Cavitation damage




Cavitation damage to a Francis turbine.
Cavitation is, in many cases, an undesirable occurrence. In devices such as propellers and pumps, cavitation
causes a great deal of noise, damage to components, vibrations, and a loss of efficiency.

When the cavitation bubbles collapse, they force liquid energy into very small volumes, thereby creating spots
of high temperature and emitting shock waves, the latter of which are a source of noise. The noise created by
cavitation is a particular problem for military submarines, as it increases the chances of being detected by
passive sonar.

Although the collapse of a cavity is a relatively low-energy event, highly localized collapses can erode metals,
such as steel, over time. The pitting caused by the collapse of cavities produces great wear on components
and can dramatically shorten a propeller's or pump's lifetime.

After a surface is initially affected by cavitation, it tends to erode at an accelerating pace. The cavitation pits
increase the turbulence of the fluid flow and create crevasses that act as nucleation sites for additional
cavitation bubbles. The pits also increase the component's surface area and leave behind residual stresses.
This makes the surface more prone to stress corrosion.
Pumps and propellers
Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.

As an impeller's (in a pump), or propeller's (as in the case of a ship or submarine) blades move through a fluid,
low pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the
blades move, the lower the pressure around it can become. As it reaches vapor pressure, the fluid vaporizes
and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very
strong local shockwaves in the fluid, which may be audible and may even damage the blades.

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Cavitation in pumps may occur in two different forms:
Suction cavitation
Suction cavitation occurs when the pump suction is under a low-pressure/high-vacuum condition where the
liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the
pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This
imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating
under a suction cavitation condition can have large chunks of material removed from its face or very small bits
of material removed causing the impeller to look sponge-like. Both cases will cause premature failure of the
pump often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the
pump casing.
Discharge cavitation
Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a
pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the
majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid
flows around the impeller it must pass through the small clearance between the impeller and the pump
cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater (similar to what
occurs in a venturi) which turns the liquid into a vapor. A pump that has been operating under these conditions
shows premature wear of the impeller vane tips and the pump cutwater. In addition, due to the high pressure
conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme
conditions, this can break the impeller shaft.

Discharge cavitation is believed to be the cause of the cracking of joints.
Cavitation in engines
Some bigger diesel engines suffer from cavitation due to high compression and undersized cylinder walls.
Vibrations of the cylinder wall induce alternating low and high pressure in the coolant against the cylinder wall.
The result is pitting of the cylinder wall that will eventually let cooling fluid leak into the cylinder and combustion
gases to leak into the coolant.

It is possible to prevent this from happening with chemical additives in the cooling fluid that form a protecting
layer on the cylinder wall. This layer will be exposed to the same cavitation, but rebuilds itself.




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Cavitation : Low suction pressure led to suction bubbles forming that destroyed the protective film.




Stainless Steel Cavitation :Steam bubble formation due to inadequate suction pressure caused this damage to the 316
stainless impeller.


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Stainless Steel Erosion Corrosion This stainless impeller pumping a nitric acid / fertilizer slurry failed from a
combination of erosion and corrosion.




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DESCRIPTION

Cavitaion and erosion corrosion is the corrosion of a metal which is caused or accelerated by the relative
motion of the environment and the metal surface. It is characterized by surface features with a directional
pattern which are a direct result of the flowing media. Erosion corrosion is most prevalent in soft alloys (i.e.
copper, aluminum and lead alloys). Alloys which form a surface film in a corrosive environment commonly
show a limiting velocity above which corrosion rapidly accelerates. Other factors such as turbulence, cavitation,
impingement or galvanic effects can add to the severity of attack.

Prevention or Remedial Action

    •     selection of alloys with greater corrosion resistance and/or higher strength.

    •     re-design of the system to reduce the flow velocity, turbulence, cavitation or impingement of the
          environment.

    •     reduction in the corrosive severity of the environment.

    •     use of corrosion resistant and/or abrasion resistant coatings.

    •     cathodic protection.

Standard Test Methods

    •     ASTM G-32 - method of vibratory cavitation erosion testing.

    •     ASTM G-73 - practice for liquid impingement erosion testing

    •     ASTM G-75 - test method for slurry abrasivity by miller number.

    •     ASTM G-76 - practice for conducting erosion tests by solid particle impingement using gas jet.

    •     NACE TM0170 - method of conducting controlled velocity laboratory corrosion tests.

    •     NACE TM0286 - cooling water test units incorporating heat transfer surfaces.

Evaluation of Cavitation and Erosion Corrosion

Many specialized tests have been utilized to evaluate erosion corrosion. Typically, the nature of the attack from
erosion corrosion and/or velocity accelerated corrosion can be vary specific to the geometry and exposure
conditions. Therefore, the results of tests and the test/service conditions must always be careful examined.
The most commonly utilized methods are spinning cylinder and disk apparatus since they are relatively easy to
set-up and they produce conditions that are easily evaluated. However, they do not always give conditions that
represent those in actual service. Recently, great use of jet impingement and actual pipe flow cells have been
utilized which can more accurately simulate conditions of turbulent flow and multiphase environments. These
tests should be conducted to produce carefully quantified conditions of wall shear stress that match those in
the intended service. The wall shear stress is a measure of the mechanical action produced on the surface of
the material by the flowing media and most directly relates to the damage or removal of normally protective
corrosion products and inhibitor films.




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                             Liquid Metal Embrittlement

Certain materials exhibit general and/or localized corrosion and embrittlement when in contact with certain
liquid metals. Liquid metal embrittlement (LME) shows many of the characteristics of both SCC and HEC. For
example. LME is often preceded by an incubation period required for the liquid metal to penetrate oxide or
passive layers on the substrate material which is analogous to local depassivation prior to SCC. However. in
many cases, LME shows a very strong effect of stress intensity and a rapid transition from slow to rapid crack
growth similar to HEC .
Therefore. it is common in LME tests to utilize surface-active agents or dynamic strain to promote surface
attack and thereby reducing the incubation time required to initiate cracking. Second. tension, precracked. or
notched specimens and fracture mechanics methods as also utilized extensively in LME testing




Schematic differentiation of anodic stress corrosion cracking and cathodically sensitive hydrogen embrittlement.



Description

Corrosive degradation of metals in the presence of certain liquid metals such as mercury, zinc, lead, cadmium.
examples of liquid metal attack include: chemical dissolution. metal-to-metal alloying (i.e. amalgamation).
embrittlement and cracking.

Prevention or Remedial Action

    •     selection of compatible materials.

    •     removal of liquid metal from environment.

    •     application of resistant surface coating or treatment to act as a barrier between metal and environment.

    •     chemical dissolution and amalgamation - see test methods for general corrosion and pitting.

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    •     liquid metal embrittlement - see test methods for scc.

Standard Test Methods

    •     ASTM G129 - slow strain rate test for determination of environmentally assisted cracking.

    •     ASTM G-30 - practice for making and using U-bend SCC test specimens.

    •     ASTM G-38 - practice for making and using C-ring SCC test specimens.

    •     ASTM G-39 - practice for preparation and use of bent-beam SCC test specimens.

Evaluation for Liquid Metal Embrittlement (LME)

The evaluation of LME usually requires chemical or mechanical techniques to overcome the incubation period
for cracking. In much the same way that a localized corrosion event is needed to initiate SCC, local chemical
attack is usually a precursor for LME. Dynamically applied loads as in the slow strain rate test can be used to
break normally protective surface films to allow intimate contact of the material surface and the liquid metal.
Chemical agents can also be used to remove or breach this surface films and initiate localized attack so that
the inherent susceptibility of the material can be determined. In some cases, surface treatments may be
utilized to enhance resistance to LME. However, this should be conducted with extreme caution since damage
to this surface layer may induce cracking.



Certain materials exhibit general and/or localized corrosion and embrittlement when in contact with certain
liquid metals. Liquid metal embrittlement (LME) shows many of the characteristics of both SCC and HEC. For
example. LME is often preceded by an incubation period required for the liquid metal to penetrate oxide or
passive layers on the substrate material which is analogous to local depassivation prior to SCC. However. in
many cases, LME shows a very strong effect of stress intensity and a rapid transition from slow to rapid crack
growth similar to HEC .

Therefore. it is common in LME tests to utilize surface-active agents or dynamic strain to promote surface
attack and thereby reducing the incubation time required to initiate cracking. Second. tension, precracked. or
notched specimens and fracture mechanics methods as also utilized extensively in LME testing.




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                                  Exfoliation Corrosion

Intergranular Corrosion: Exfoliation Corrosion




Exfoliation corrosion is a more severe form of intergranular corrosion that can occur along aluminum grain
boundaries in the fuselage empennage and wing skins of aircraft. These grain boundaries in both aluminum
sheet and plate are oriented in layers parallel to the surface of the material, due to the rolling process. The
delamination of these thin layers of the aluminum, with white corrosion products between the layers,
characterizes exfoliation corrosion.

Exfoliation corrosion is often found next to fasteners where an electrically insulating sealant or a sacrificial
cadmium plating has broken down, permitting a galvanic action between the dissimilar metals. Where
fasteners are involved, exfoliation corrosion extends outward from the fastener hole, either from the entire
circumference of the hole, or in one direction from a segment of the hole. In severe cases, the surface bulges
outward, but in less severe cases, there may be no telltale blisters, and you can only detect the exfoliation
corrosion by nondestructive inspection methods that are not always very effective.

Controlled shot peening can be very effective in the process of both identifying and repairing exfoliation
corrosion damage. Service manuals normally call for the removal of the fasteners and then for the use of rotary
discs to sand away the corroded material, followed by blending the area and polishing out the tool marks.
Aircraft structural engineers have used Metal Improvement Company's controlled shot peening after removal
of visible exfoliation corrosion to compensate for the lower fatigue strength of the newly reduced cross-section.
The action of peening, however, will cause the surface to blister again, where deeper exfoliation corrosion is
present. The surface can then be redressed and repeened until no further blistering occurs. Metal
Improvement Company calls this process Search Peeningsm. The process provides both a reliable
nondestructive testing of the exfoliated material and a fatigue strength compensation for any reduced cross
section.



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Metal Improvement Company can perform its Search Peening process on-site at aircraft repair hangers to
address exfoliation corrosion.

Recognition

What is exfoliation? Exfoliation is yet another special form of intergranular corrosion that proceeds laterally
from the sites of initiation along planes parallel to the surface, generally at grain boundaries, forming corrosion
products that force metal away from the body of the material, giving rise to a layered appearance.
Exfoliation is sometimes described as lamellar, layer, or stratified corrosion. In this type of corrosion, attack
proceeds along selective subsurface paths parallel to the surface. It is possible to visually recognize this type
of corrosion if the grain boundary attack is severe otherwise microstructure examination under a microscope is
needed.




Exfoliation corrosion in an aluminum alloy exposed to tropical marine environment. Also note the paint failures
caused by corrosion of aluminum at the coating/aluminum interface.
Mechanisms What causes exfoliation? Exfoliation is a special type of intergranular corrosion that occurs on the
elongated grain boundaries. The corrosion product that forms has a greater volume than the volume of the
parent metal. The increased volume forces the layers apart, and causes the metal to exfoliate or delaminate.
Aluminum alloys are particularly susceptible to this type of corrosion.


Prevention


How to prevent exfoliation corrosion? Exfoliation corrosion can be prevented through:
the use of coatings
selecting a more exfoliation resistant aluminum alloy
using heat treatment to control precipitate distribution.




Exfoliation Corrosion: Exfoliation is a form of intergranular corrosion. It manifests itself by lifting up the surface
grains of a metal by the force of expanding corrosion products occurring at the grain boundaries just below the

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surface. It is visible evidence of intergranular corrosion and most often seen on extruded sections where grain
thickness is less than in rolled forms.
It is generally considered that exfoliation corrosion is due to the build-up of corrosion products that create a
wedging stress that lifts up the surface grains. However, the exfoliation mechanism is still under discussion:
possible operating mechanisms include intergranular corrosion of in plane grain boundaries accelerated by the
wedging effect, or crack propagation by a "purely" stress corrosion mechanism.
Exfoliation




 Exfoliation corrosion is a particular form of intergranular corrosion associated with high strength aluminum
alloys. Alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated,
flattened grains, are particularly prone to this damage.


Corrosion products building up along these grain boundaries exert pressure between the grains and the end
result is a lifting or leafing effect. The damage often initiates at end grains encountered in machined edges,
holes or grooves and can subsequently progress through an entire section.
Anisotropic grain structure of wrought aluminum alloys




SL = Short longitudinal LT = Longitudinal transverse ST = Short transverse




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Notice how the corrosion separates into distinct layers which have expanded to occupy a much larger area
than the original, un-corroded part. Obviously, the structural integrity of this part disappeared long ago.
Micrograph of a failed aircraft component




Exfoliation of a failed aircraft component made of 7075-T6 aluminum (picture width = 400 mm)




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Exfoliation Corrosion, Evaluation of Exfoliation Corrosion

EXFOLIATION is a structure-dependent form of localized (usually) intergranular corrosion that is most familiar
in certain alloys and tempers of aluminum.

The occurrence of exfoliation in susceptible materials is influenced to a marked degree by environmental
conditions. Figure 1 illustrates the broad range of behavior in different types of atmospheres. For example,
forged truck wheels made of an aluminum-copper alloy (2024-T4) give corrosion-free service for many years in
the warm climates of the southern and western United States, but they exfoliate severely in only 1 or 2 years in
the northern states, where deicing salts are used on the highways during the winter months.

Accelerated laboratory tests do not precisely predict long-term corrosion behavior; however, answers are
needed quickly in the development of new materials. For this reason, accelerated tests are used to screen
candidate alloys before conducting atmospheric exposures or other field tests. They are also sometimes used
for quality control tests. Several new laboratory tests for exfoliation corrosion have been standardized in recent
years under the jurisdiction of American Society for Testing and Materials (ASTM) Committee G-1 on the
Corrosion of Metals.

Test Method used:

ASTM G85 Standard Practice for Modified Salt Spray (Fog) Testing

The ASTM G85 standard consists of a set of 5 modifications to the ASTM B117 Salt Spray Test. These
modifications are applicable to ferrous and nonferrous metals, and also to organic and inorganic coatings.
These variations are useful when a different or more corrosive environment than the salt fog described in
Practice B 117 is desired.

This test standard comprises of five climate modifications to the basic ASTM B117 salt spray test. These five
modifications are known by the following annexes and descriptions:
ASTM G85 annex A1 – acetic acid salt spray test, continuous
This test is also referred to as an ASS test.
ASTM G85 annex A2 – cyclic acidified salt spray test
This test is also referred to as a MASTMAASIS test.
ASTM G85 annex A3 – seawater acidified test, cyclic
This test is also referred to as a SWAAT test.
ASTM G85 annex A4 – Sulphur dioxide (SO2 ) salt spray test, cyclic
This test is also referred to as an SO 2 test.
ASTM G85 annex A5 – dilute electrolyte cyclic fog /dry test
This test is also referred to as a PROHESION test.




The standard in salt spray testing ASTM B117
The American Society of Testing and Materials (ASTM) test B117 is one of the most widely adopted continuous salt
spray test specifications. Its use is internationally widespread and its provisions have been frequently re-written into
the national standards of other countries, and also appear in other industry specific corrosion test standards.

ASTM B117 has always been and excellent reference document for the salt spray practitioner, with many helpful
hints and tips contained in its useful appendixes. But since it is also regularly updated, by an active and broad based
ASTM sub-committee, it is a standard that is always evolving and becoming ever more ‘user friendly’. The suffix to
the main standard number indicates the year of publication. For example, ASTM B117 – 03 indicates a 2003



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publication date (which was the latest version available at the time of writing). Please check you are using the most
up to date edition available for your application.



Other Exfoliation Corrosion Tests:

Exfoliation Corrosion is a severe form of intergranular corrosion that can occur along aluminum grain
boundaries, parallel to the surface. Exfoliation Corrosion represents a special type of localized corrosion,
which develops under the surface of aluminum high-alloyed alloys.

If intergranular corrosion is allowed to propagate, delamination of the thin layers of aluminum, known as
exfoliation corrosion will occur. The resulting corrosion forces the metal upward, giving rise to a layered or
leaf-like appearance to the surface.

Exfoliation Corrosion Testing applies to all wrought products from industry, especially aeronautics, and can
include sheet, plate, extrusion and forging.

          ASTM G34-Describes a procedure for constant immersion exfoliation corrosion (EXCO)

          ASTM G66-Method covers a procedure for continuous immersion exfoliation corrosion testing of
          aluminum alloys (ASSET Test)
          ASTM G112-Covers the aspects of specimen preparation, exposure, inspection and evaluation for
          conducting exfoliation corrosion tests



ASTM G34 - 01(2007) Standard Test Method for Exfoliation Corrosion Susceptibility in 2XXX and 7XXX
Series Aluminum Alloys (EXCO Test)

Significance and Use

This test method was originally developed for research and development purposes; however, it is referenced,
in specific material specifications, as applicable for evaluating production material
Use of this test method provides a useful prediction of the exfoliation corrosion behavior of these alloys in
various types of outdoor service, especially in marine and industrial environments.4 The test solution is very
corrosive and represents the more severe types of environmental service, excluding, of course, unusual
chemicals not likely to be encountered in natural environments.
The exfoliation ratings were arbitrarily chosen to illustrate a wide range in resistance to exfoliation in this test.
However, it remains to be determined whether correlations can be established between EXCO test ratings and
realistic service conditions for a given alloy. It is an ongoing activity of the Task Group on Exfoliation Corrosion
of Aluminum Alloys (G01.05.02.08) to maintain outdoor exposure tests for this purpose. For example, it has
been reported that samples of Al-Zn-Mg-Cu alloys rated EA or P in a 48-h EXCO test did not develop more
than a slight amount of incipient exfoliation (EA) during six- to nine-year exposures to seacoast atmospheres,
whereas, ED rated materials in most cases developed severe exfoliation within a year in the seacoast
atmosphere. It is anticipated that additional comparisons will become available as the outdoor tests are
extended.

1. Scope
1.1 This test method covers a procedure for constant immersion exfoliation corrosion (EXCO) testing of high-
strength 2XXX and 7XXX series aluminum alloys.
Note 1—This test method was originally developed for research and development purposes; however, it is
referenced, in specific material specifications, as applicable for evaluating production material (refer to Section
14 on Precision and Bias).
1.2 This test method applies to all wrought products such as sheet, plate, extrusions, and forgings produced
from conventional ingot metallurgy process.
1.3 This test method can be used with any form of specimen or part that can be immersed in the test solution.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appropriate safety and health practices and determine
the applicability of regulatory limitations prior to use.



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ASTM G112 - 92(2003)
Standard Guide for Conducting Exfoliation Corrosion Tests in Aluminum Alloys


Significance and Use
Although there are ASTM test methods for exfoliation testing, they concentrate on specific procedures for
test methodology itself. Existent test methods do not discuss material variables that can affect
performance. Likewise they do not address the need to establish the suitability of an accelerated test for
alloys never previously tested nor the need to correlate results of accelerated tests with tests in outdoor
atmospheres and with end use performance.

This guide is a compilation of the experience of investigators skilled in the art of conducting exfoliation
tests and assessing the degree and significance of the damage encountered. The focus is on two general
aspects: guides to techniques that will enhance the likelihood of obtaining reliable information, and tips and
procedures to avoid pitfalls that could lead to erroneous results and conclusions.

The following three areas of testing are considered: the test materials starting with the “as-received”
sample up through final specimen preparation, the corrosion test procedures including choice of test,
inspection periods, termination point, and rating procedures, and analyses of results and methods for
reporting them.

This guide is not intended as a specific corrosion test procedure by which to evaluate the resistance to
exfoliation of an aluminum alloy product.

This guide is not intended as a basis for specifications, nor as a guide for material lot acceptance.

1. Scope

1.1 This guide differs from the usual ASTM standard in that it does not address a specific test. Rather, it is
an introductory guide for new users of other standard exfoliation test methods, (see Terminology G 15 for
definition of exfoliation).

1.2 This guide covers aspects of specimen preparation, exposure, inspection, and evaluation for conducting
exfoliation tests on aluminum alloys in both laboratory accelerated environments and in natural, outdoor
atmospheres. The intent is to clarify any gaps in existent test methods.

1.3 The values stated in SI units are to be regarded as the standard. The inch-pound units given in
parentheses are for information only.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It
is the responsibility of the user of this standard to establish appropriate safety and health practices and
determine the applicability of regulatory limitations prior to use.


2. Referenced Documents

G1 Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
G15 Terminology Relating to Corrosion and Corrosion Testing
G34 Test Method for Exfoliation Corrosion Susceptibility in 2XXX and 7XXX Series Aluminum Alloys (EXCO
Test)
G50 Practice for Conducting Atmospheric Corrosion Tests on Metals
G66 Method for Visual Assessment of Exfoliation Corrosion Susceptibility of 5XXX Series Aluminum Alloys
(ASSET Test)
G85 Practice For Modified Salt Spray (Fog) Testing
G92 Practice for Characterization of Atmospheric Test Sites



ISO 11881:1999
Corrosion of metals and alloys -- Exfoliation corrosion testing of aluminum alloys




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Chapter 3:
Corrosion Controls
There are a number of means of controlling corrosion. The choice of a means of corrosion control depends on
economics, safety requirements, and a number of technical considerations.



                                                                             Design.

                                                                             Materials Selection.

                                                                             Protective Coatings.

                                                                             Inhibitors and Other Means of
                                                                             Environmental Alteration.
                                                                             (Chemical Treatment)

                                                                             Corrosion Allowances.

                                                                             Cathodic Protection.

                                                                             Anodic Protection.




  Corrosion Protections of Metals - Overview
  Corrosion Control: http://www.vulcanhammer.net/marine/Mo307.pdf



Design.




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Engineering design is a complicated process that includes design for purpose, manufacturability, inspection,
and maintenance. One of the considerations often overlooked in designing manufactured products is
drainage. The corrosion of the automobile side panel above could have been minimized by providing
drainage to allow any water and debris to fall off of the car instead of collecting and causing corrosion from the
far side of the panel.


All of the other methods of corrosion control should be considered in the design process.




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Material Selections.



Carbon Steel

Stainless Steel

Aluminum

Copper Alloys

Titanium

Carbon Steel

Most large metal structures are made from carbon steel-the world's most useful structural material. Carbon
steel is inexpensive, readily available in a variety of forms, and can be machined, welded, and formed into
many shapes.

This large statue by Pablo Picasso in front of the Chicago city hall is made from a special form of carbon steel
known as weathering steel. Weathering steel does not need painting in many boldly exposed environments.
Unfortunately, weathering steel has been misused in many circumstances where it could not drain and form a
protective rust film. This has given the alloy a mixed reputation in the construction industry.

Where other means of corrosion control are not practical, other alloys can be substituted for carbon steel. This
normally doubles or more the material cost for a structure, and other corrosion control methods must be
considered before deciding on the use of more expensive alternates to carbon steel.

Some forms of carbon steel are subject to special types of corrosion such as hydrogen embrittlement, etc. It is
common practice to limit the allowable strength levels of carbon steel to avoid brittle behavior in environments
where environmental cracking may occur. High strength bolts cannot be galvanized for the same reason-a
concern that they may hydrogen embrittle due to corrosion on the surface.

Protective coatings, cathodic protection, and corrosion inhibitors are all extensively used to prolong the life of
carbon steel structures and to allow their use in environments such as the Kennedy Space Center where the
environment would otherwise be too corrosive for their use.

Stainless Steels

                               The stainless steel body on this sports car is one example of how stainless
                               steels can be used. The stainless steel is virtually immune to corrosion in this
                               application-at least in comparison to the corrosion that would be experienced by
                               conventional carbon steel or aluminum auto bodies.

                              Stainless steels are a common alternative to carbon steels. There are many
                              kinds of stainless steels, but the most common austenitic stainless steels (300-
                              series stainless steels) are based on the general formula of iron with
                              approximately 18% chromium and 8% nickel. These austenitic stainless steels
are frequently immune to general corrosion, but they may experience pitting and crevice corrosion and
undergo stress corrosion cracking in some environments.

Aluminum

Aluminum alloys are widely used in aerospace applications where their favorable strength-to-weight ratios
make them the structural metal of choice. They can have excellent atmospheric corrosion capabilities.


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Unfortunately, the protective properties of the aluminum oxide films that form on these alloys can break down
locally and allow extensive corrosion. This is discussed further in the section on intergranular corrosion.

The highway guardrail shown on the right is located near the ocean in Florida. The aluminum alloy maintains a
silvery shine except in locations where the passive film has suffered mechanical damage. The wear caused by
the rail touching the wooden post at this location destroyed the passive film on the edges of the rail and
allowed intergranular corrosion to proceed and cause the exfoliation corrosion shown above. While the
corrosion above is very interesting and makes for an interesting web site, it is important to note that the railing
is decades old and would have never lasted as long in this location if it were made of carbon steel.

Intergranular corrosion is a major problem on airplanes and other structures made from aluminum alloys. It
frequently occurs at bolt and rivet holes or at cutouts where the small grain boundaries perpendicular to the
metal surface are exposed.

Copper Alloys

Brasses and bronzes are commonly used piping materials, and they are also used for valves and fittings. They
are subject to stress corrosion cracking in the presence of ammonia compounds. They also suffer from
dealloying and can cause galvanic corrosion when coupled with steel and other structural metals. Most copper
alloys are relatively soft and subject to erosion corrosion.

The dezincification shown above could have been controlled by using inhibited brasses which have been
commercially available since the 1930's.

Titanium

Titanium is one of the more common metals in nature, but its limited use means that small-scale production
operations result in a relatively expensive metal. In the United States it finds extensive use in the aerospace
industry. The Japanese make extensive use of titanium in the chemical process industries.

There are two general types of titanium alloys-aerospace alloys and corrosion resistant alloys. The crevice
corrosion of an aerospace alloy flange in a saltwater application is a classic example of how titanium gets
misused.


Selection of materials:
http://www.hse.gov.uk/comah/sragtech/techmeasmaterial.htm
Ebooks on materials:
http://iran-eng.com/showthread.php?t=43015&page=14
Corrosion and material selection in desalination plants:
http://www.scribd.com/doc/7457739/Corrosion-and-Material-Selection-in-Desalination-Plants
Corrosion resistance alloys:
http://www.hpalloy.com/alloys/corrosionResistant.html
Chemical and material performance:
http://www.engineeringtoolbox.com/metal-corrosion-resistance-d_491.html
Materials selection at high temperature:
http://www2.mtec.or.th/th/research/famd/corro/mshtemp.htm
Material selection guides:
http://www.documentation.emersonprocess.com/groups/public_public_mmisami/documents/articles_articlesre
prints/mc-00992.pdf
Materials selection guides for valves:
http://d.scribd.com/docs/bkl25fpw3pcakaotfui.pdf
DOE fundamental handbooks on material Science-Vol1 & 2
http://hss.energy.gov/NuclearSafety/techstds/standard/hdbk1017/h1017v1.pdf
http://hss.energy.gov/NuclearSafety/techstds/standard/hdbk1017/h1017v2.pdf
Material handbooks collection:
http://community.h2vn.com/index.php?topic=96.0
http://iran-eng.com/showthread.php?t=43015&page=14




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Protective coating.


Protective coatings are the most commonly used method of corrosion control. They are the subject of several
sections of this web site.

Protective coatings can be metallic, such as the galvanized steel shown below, or they can be applied as a
liquid "paint." Most of the research and testing of protective coatings at the Kennedy Space Center is related to
paint-like protective coatings.




Filiform corrosion occurs underneath protective coatings. The air conditioner on the left is starting to show rust
stains due to problems with protective coating. The same types of problems are starting to appear on the
aluminum airplane wing shown on the right.




   Protective Coatings and Paints
http://www.vulcanhammer.net/marine/3_190_06.pdf
   Coating failures and solutions
http://www.sikkens.com/en/PaintSolutions/Blistering.htm
   Failure analysis of paints and coatings
http://www.matcoinc.com/files/PublicationPDFs/CoatingFailureAnalysis.pdf
    Norsok Standards on coatings
http://www.standard.no/imaker.exe?id=5438
   Jotun’s coating failures.
http://www.jotun.com/www/com/20020113.nsf?OpenDatabase&db=/www/com/20020115.nsf&v=1102&e=uk&
m=922&c=52CB8C0DAD610F78C1256C40006C2D04
   Early coatings failure of offshore platforms.
http://www.cathodicprotectionpapers.com/3coatingfailures
   Coating and lining failure analysis and standard test methods-CorrosionSource.
http://www.corrosionsource.com/handbook/CPS/cps_a_clf.htm




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Inhibitions and environmental alteration.                             [Inhibitor]



   Corrosion inhibitors are chemicals that are added to controlled environments to reduce the corrosivity of
these environments. Examples of corrosion inhibitors include the chemicals added to automobile antifreezes to
make them less corrosive. Most of the Kennedy Space Center's corrosion inhibitor research involves the
effectiveness of inhibitors added to protective coatings.

           [Inhibitor types]




Corrosion allowances.



                                                    Engineering designers must consider how much metal is
                                                    necessary to withstand the anticipated load for a given
                                                    application. Since they can make mistakes, the use of the
                                                    structure can change, or the structure can be misused,
                                                    they usually are required to over design the structure by a
                                                    safety factor that can vary from 20% to over 300%. Once
                                                    the necessary mechanical load safety factor has been
                                                    considered, it becomes necessary to consider whether or
                                                    not a corrosion allowance is necessary to keep the
                                                    structure safe if it does corrode.

                                                     The picture above shows extra steel added to the
                                                   bottom of an offshore oil production platform. The one
inch of extra steel was added as a corrosion allowance.




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                                          CP Tutorials-1
Cathodic protection.
                                          CP Tutorials-2

Cathodic protection is an electrical means of corrosion control. Cathodic protection can be applied using
sacrificial (galvanic) anodes or by means of more complicated impressed current systems.




  This Louisiana fishing boat has sacrificial zinc anodes welded to the hull to slow down corrosion.


Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the
cathode of an electrochemical cell.

It is a method used to protect metal structures from corrosion. Cathodic protection systems are most
commonly used to protect steel, water/fuel pipelines and storage tanks; steel pier piles, ships, offshore oil
platforms and onshore oil well casings.

A side effect of improperly performed cathodic protection may be production of molecular hydrogen, leading to
its absorption in the protected metal and subsequent hydrogen embrittlement.

Cathodic protection is an effective method of preventing stress corrosion cracking.


Galvanic CP


Today, galvanic or sacrificial anodes are made in various shapes using alloys of zinc, magnesium and
aluminum. The electrochemical potential, current capacity, and consumption rate of these alloys are superior
for CP than iron.

*Also Ag/AgCl in 20 ohm-cm seawater

Corrosion Potentials in Flowing Seawater (8-13 ft/s), Temperature Range 50-80 F (10-27 C)
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Galvanic anodes are designed and selected to have a more "active" voltage (technically a more negative
electrochemical potential) than the metal of the structure (typically steel). For effective CP, the potential of the
steel surface is polarized (pushed) more negative until the surface has a uniform potential. At that stage, the
driving force for the corrosion reaction is halted. The galvanic anode continues to corrode, consuming the
anode material until eventually it must be replaced. The polarization is caused by the current flow from the
anode to the cathode. The driving force for the CP current flow is the difference in electrochemical potential
between the anode and the cathode.



Impressed Current CP

For larger structures, galvanic anodes cannot economically deliver enough current to provide complete
protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power
source (a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes or
continuous ribbons of various specialized materials. These include high silicon cast iron, graphite, mixed metal
oxide, platinum and niobium coated wire and others.

                                  A cathodic protection rectifier connected to a pipeline

                                  A typical ICCP system for a pipeline would include an AC powered rectifier
                                  with a maximum rated DC output of between 10 and 50 amperes and 50 volts.
                                  The positive DC output terminal is connected via cables to the array of anodes
                                  buried in the ground (the anode ground bed). For many applications the
                                  anodes are installed in a 60 m (200 foot) deep, 25 cm (10-inch) diameter
vertical hole and backfilled with conductive coke (a material that improves the performance and life of the
anodes). A cable rated for the expected current output connects the negative terminal of the rectifier to the
pipeline. The operating output of the rectifier is adjusted to the optimum level by a CP expert after conducting
various tests including measurements of electrochemical potential.

Telephone wiring uses a form of cathodic protection. A circuit consists of a pair of wires, with forty-eight volts
across them when the line is idle. The more positive wire is grounded, so that the wires are at 0 V and -48 V
with respect to earth ground. The 0 V wire is at the same potential as the surrounding earth, so it corrodes no
faster or slower than if it were not connected electrically. The -48 V wire is cathodically protected. This means
that in the event of minor damage to the insulation on a buried cable, both copper conductors will be
unaffected, and unless the two wires short together, service will not be interrupted.

If instead the polarity were switched, so that the wires were at 0 V and +48 V with respect to the surrounding
earth, then the 0 V wire would be unaffected as before, but the +48 V wire would quickly be destroyed if it
came into contact with wet earth. The electrochemical action would plate metal off the +48 V wire, reducing its
thickness to the point that it would eventually break, interrupting telephone service. This choice of polarity was
not accidental; corrosion problems in some of the earliest telegraphy systems pointed the way.




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                                                  Testing

                                                  Electrochemical potential is measured with reference electrodes.
                                                  Copper-copper(II) sulfate electrodes are used for structures in
                                                  contact with soil or fresh water. Silver chloride electrodes are
                                                  used for seawater applications.

                                                  Silver/silver-chloride electrode is by far the most common
                                                  reference type used today because it is simple, inexpensive, very
                                                  stable and non-toxic. It is mainly used with saturated potassium
                                                  chloride electrolyte, but can be used with lower concentrations
                                                  such as 3.5 mol dm-3 or 1 mol dm-3 potassium chloride.
                                                  Silver/silver-chloride electrode is a referent electrode based on
                                                  the following halfreaction

                                           AgCl(s) + e-        Ag(s) + Cl-

Dependence of potential of silver/silver chloride electrode upon temperature and concentration of KCl
according to standard hydrogen electrode:

                                                    Potential vs. SHE / V
                                                  3.5 mol dm-
                                         t / °C         3
                                                                sat. solution

                                          15          0.212        0.209
                                          20          0.208        0.204
                                          25          0.205        0.199
                                          30          0.201        0.194
                                          35          0.197        0.189



Galvanized Steel




Galvanizing (or galvanising, outside of the USA) generally refers to hot-dip galvanizing which is a way of
coating steel with a layer of metallic zinc. Galvanized coatings are quite durable in most environments because
they combine the barrier properties of a coating with some of the benefits of cathodic protection. If the zinc
coating is scratched or otherwise locally damaged and steel is exposed, the surrounding areas of zinc coating

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form a galvanic cell with the exposed steel and protect it from corrosion. This is a form of localised cathodic
protection - the zinc acts as a sacrificial anode.


IMPACT AND ABRASION RESISTANCE


Hardness, ductility and adherence combine to provide the galvanized coating with unmatched protection
against damage caused by rough handling during transportation to and/or at the job site as well during its
service life. The toughness of the galvanized coating is extremely important since barrier protection is
dependent upon coating integrity.


Other coatings damage easily during shipment or through rough handling on the job site. Experts will argue
that all organic forms of barrier protection (such as paint) by their nature are permeable to some degree.
Correctly applied galvanized coatings are impermeable.




If the galvanized coating is physically damaged, it will continue to provide cathodic protection to the exposed
steel. If individual areas of underlying steel or iron become exposed by up to 1/4" diameter spot, the
surrounding zinc will provide these areas with cathodic protection for as long as the coating lasts.




Below the name of each layer in the figure appears its respective hardness, expressed by a Diamond Pyramid
Number (DPN). The DPN is a progressive measure of hardness. The higher the number the greater the
hardness. Typically, the Gamma, Delta, and Zeta layers are harder than the underlying steel. The hardness of
these inner layers provides exceptional protection against coating damage through abrasion. The Eta layer of
the galvanized coating is quite ductile, providing the coating with some impact resistance.PERFORMANCE AT
ELEVATED TEMPERATURES



    Galvanized coatings perform well under continuous exposure to temperatures up to 392o F (200o C).
Exposure to temperatures above this can cause the outer free zinc layer to peel from the underlying zinc-iron
alloy layer. However, the remaining zinc-iron alloy layer will provide good corrosion resistance and will
continue to protect the steel for a long time, depending upon its thickness.



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                                                            CORNER AND EDGE PROTECTION

                                                            The galvanizing process naturally produces coatings
                                                            that are at least as thick at the corners and edges as
                                                            the coating on the rest of the article. As coating damage
                                                            is most likely to occur at edges, this is where added
                                                            protection is needed most. Brush-applied or spray-
                                                            applied coatings have a natural tendency to thin at
                                                            corners and edges



A photomicrograph of a cross-section of an edge of a piece of galvanized steel.




This arrangement is called a galvanic cell. A typical cell might consist of two pieces of metal, one zinc and the
other copper, each immersed each in a solution containing a dissolved salt of the corresponding metal. The
two solutions are separated by a porous barrier that prevents them from rapidly mixing but allows ions to
diffuse through


If we connect the zinc and copper by means of a metallic conductor, the excess electrons that remain when
     2+
Zn        ions emerge from the zinc in the left cell would be able to flow through the external circuit and into the
                                                              2+
right electrode, where they could be delivered to the Cu           ions which become "discharged", that is, converted
into Cu atoms at the surface of the copper electrode. The net reaction is the oxidation of zinc by copper(II) ions:

               2+          2+
Zn(s) + Cu          → Zn        + Cu(s)


But this time, the oxidation and reduction steps (half reactions) take place in separate locations




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   Cathodic Protection Systems for Civil Works Structures
http://www.vulcanhammer.net/marine/EM-1110-2-2704.pdf
   Operation and Maintenance: Cathodic Protection Systems
http://www.vulcanhammer.net/marine/ufc_3_570_06.pdf
   Electrical Engineering Cathodic Protection
http://www.vulcanhammer.net/marine/3_570_02.pdf
   Cathodic and anodic protection:
http://cheserver.ent.ohiou.edu/ChE430(530)/cathodic_anodic_protection.pdf
   Corrosion and oxidation:
http://www.ecm.auckland.ac.nz/course/cm322/322PPT_06.pdf
   Metallic corrosion:
http://cheserver.ent.ohiou.edu/ChE430(530)/



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http://www.chem1.com/acad/webtext/elchem/ec2.html




Anodic Protection.


Anodic protection or anodizing, is an electrolytic passivation process used to increase the thickness of the
natural oxide layer on the surface of metal parts. Anodizing increases corrosion resistance and wear
resistance, and provides better adhesion for paint primers and glues than bare metal. Anodic films can also be
used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin
transparent coatings that add interference effects to reflected light. Anodizing is also used to prevent galling of
threaded components and to make dielectric films for electrolytic capacitors. Anodic films are most commonly
applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, and niobium.
This process is not a useful treatment for iron or carbon steel because these metals exfoliate when oxidized;
i.e. the iron oxide (also known as rust) flakes off, constantly exposing the underlying metal to corrosion. "Stay-
Brite" is sometimes used as market name for products made from anodised aluminium such as brass replica.

Read more……        Read More




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                                 Appendix




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Appendix A - Pourbaix Diagram                      Back          Read More

The effects of pH on the form in which an element in a given oxidation state exists in natural waters can be
summarized with predominance diagrams such as that for phosphorous (V) shown below.




However, if suitable reducing agents are present, the phosphorous may not remain in the +5 oxidation state.

Knowledge of the pH condition of the environment is not sufficient for predicting the form in which an element
will exist in natural waters. You must also take into consideration whether the aqueous environment is well
aerated (oxidizing) or polluted with organic wastes (reducing). In order to add this variable, we must expand
the predominance diagram to include the reduction potential of the environment as well as the pH. This type of
predominance diagram is known as a Pourbaix diagram.Eo-pH diagram, or pE-pH diagram.

Simplified Pourbaix diagram for 1 M iron solutions.




Low E (or pE) values represent a reducing environment. High E values represent an oxidizing environment.
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The pE scale is intended to represent the concentration of the standard reducing agent (the e-) analogously to
the pH scale representing the concentration of standard acid (H+). PE values are obtained from reduction
potentials by dividing Eoby 0.059.

Key to features on the diagram:
    •     Solid lines separate species related by acid-base equilibria (line a)

              o   line a shows the pH at which half of the 1 M iron is Fe3+ and half is precipitated as Fe(OH)2

              o   Pourbaix diagrams incorporate Z1/r calculations and acid-base equilibria

              o   the position of an acid-base equilibrium is dependent on the total concentration of iron

                           reducing the total concentration of Fe3+ will reduce the driving force of the precipitation

                           reducing the total iron concentration from 1 M to 10-6 M (more realistic concentrations
                           for geochemists and corrosion engineers) shifts the boundary from pH 1.7 to pH 4.2

                           In general, in more dilute solutions, the soluble species have larger predominance
                           areas.

    •     Solid double lines separate species related by redox equilibria (lines c & d)

              o   redox equilibria of species not involving hydrogen or hydroxide ions appear as horizontal
                  boundaries (line b)

              o   redox species of species involving hydrogen or hydroxide appear as diagonal boundaries
                  becuase they are in part acid-base equilibria (line c)

                           diagonal boundaries slope from upper left to lower right because basic solutions tend
                           to favor the more oxidized species

    •     Longer dashed lines enclose the theoretical region of stability of the water to oxidation or reduction
          ((lines d & f) while shorter dashed lines enclose the practical region of stability of the water (e & g)

              o   Dashed line d represents the potential of water saturated with dissolved O2at 1 atm (very well
                  aerated water).

              o   above this potential water is oxidized to oxygen:

                                              2 H2O + 4 H+ (aq) O2 + 4 e- Eo = +1.229 V

                           theoretically water should be oxidized by any dissolved oxidizing agent Eo > 1.229

                           in practice, about 0.5 V of additional potential is required to overcome the overvoltage
                           of oxygen formation (dashed line e)

    •     Dashed line f represents the potential of water saturated with dissolved H2 at 1 atm pressure (high
          level or reducing agents in solution).

    •     Below this potential water is reduced to hydrogen:

                                                  2 H+ + 2 e- Eo = +1.229 V

              o   in practice, an overvoltage effect prevents significant release of hydrogen until the lower
                  dashed line g is reached




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Uses of Pourbaix Diagrams:
   •     Any point on the diagram will give the termodynamically most stable (theoretically the most abundant)
         form of the element for that E and pH.

             o   E=+0.8 V and pH = 14
                 predominant form is FeO42-.


   •     The diagram gives a visual representation of the oxidizing and reducing abilities of the major stable
         compounds of an element

             o   Strong oxidizing agents and oxidizing conditions are found ONLY at the top of the diagram.
                 The lower boundaries of strong oxidizing agents are high on the diagram.

             o   Reducing agents and reducing conditions are found at the bottom of a diagram and nowhere
                 else.
                 Strong reducing agents have boundaries that are low on the diagram.

             o   A species that prevails from top to bottom at the pH in question has no oxidizing or reducing
                 properties at all within that range.




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                                                   EXAMPLE

          On the Pourbaix diagram for iron find:
              1. the chemical form of iron that is the strongest oxidizing agent.


              2. the form of iron that is the strongest reducing agent


              3. the form of iron that would predominate in a neutral solution at a
                 potential of 0.00V



              4. the standard reduction potential for the reduction of Fe2+ to Fe
                  metal




For some elements, the predominance area for a given oxidation state may disappear completely above or
below a given pH.




If the element is in an intermediate oxidation state, the element will undergo disproportionation at appropriate
pH's.




Notice that predominance areas are missing for hypochlorite, chlorite and chlorate ions. This is due to either
lack of electrochemical data for a species or (in this case) the fact that the ions are thermodynamically
unstable to disproportionation. In the case of chlorine the rates of disproportionation reactions are slow enough
that these chlorine species can be observed and used.




In predicting when cations and anions would react to form precipitates, we only considered the most stable
oxidation states of the elements so that interference of redox reactions between the anion and cation could be
avoided.                                                                                              Back
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                  Cations and anions will undergo redox reactions if the predominance areas of
                  their oxidation states do not coincide.



Ferrate ion is expected to be a feebly basic anion which should precipitate with feebly acidic cations. Ferrate
gives the expected precipitate with Ba2+ but not with Eu2+.

Eu2+ is a good reducing agent having no part of its predominance area above a potential of -0.429 V. There is
no overlap of this region with that of ferrate ion. A redox reaction will occur between the two species to yield
species that do have overlapping predominance areas -- Eu3+ and Fe3+




Pourbaix diagrams allow for more accurate predictions of the forms in which the different elements will exist in
natural waters.

    •     For a clean lake, the surface waters are well aerated and the dissolved oxygen concentrations are
          high enough to make the potential reasonably close to the Eo for oxygen.

    •     Conditions may approach anaerobic (actively reducing)approaching the lower boundary of the
          reduction of water to hydrogen for

              o    a lake highly polluted with organic reducing agents

              o    the bottom layer of a thermally stratified lake

              o    for a swamp




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More reading:




http://www.wou.edu/las/physci/chemhome/courses.htm




http://www.doitpoms.ac.uk/tlplib/pourbaix/index.php




http://engnet.anu.edu.au/DEcourses/engn4520/




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Answer 1: FeO42- is the strongest oxidizing agent




Answer 2: Elemental Fe




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Answer 3: Fe(OH)3




Answer 4: -0.5 V3




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APPENDIX B: Hydrogen damages:

                                     Factors Affecting In-Service Cracking
                                       of Weld Zone in Corrosive Service


January 1986

Category: Incidents

Summary: The following article is a part of National Board Classic Series and it was published in the National
Board BULLETIN . The article was reprinted in the January 1986 National Board BULLETIN . Permission to
reprint was granted by the Illinois Division of Boiler and Pressure Vessel Safety, D. R. Gallup, Superintendent.
(6 printed pages)

< This article describes the cause of failure of a monoethandamine (MEA) absorber vessel that ruptured in the
state of Illinois in 1984, resulting in 17 fatalities and property damage in excess of $100 million.




VESSEL DESCRIPTION

The ruptured vessel was designed in accordance with The American Society of Mechanical Engineers (ASME)
Boiler and Pressure Vessel Code, Section VIII rules. The vessel was constructed of 1 inch thick SA516 Gr 70
steel plates rolled and welded with full penetration submerged arc joints, without postweld heat treatment. The
cylindrical vessel measures 81/2 feet in diameter with hemispherical ends comprising an overall height of 55
feet. Operating conditions were 200 psig internal pressure containing largely propane and hydrogen sulfide at
100¡F. An internal system distributed monoethanolamine (MEA) through the vessel for the purpose of
removing hydrogen sulfide from the gas.




VESSEL OPERATING HISTORY

The vessel went into operation in 1969. Soon after start-up, hydrogen blisters were observed to be forming in
the bottom two courses of the cylindrical vessel wall. Metallurgical analysis showed laminations to be present
in the steel.

In 1974, due to the large blister area found in the second course, a full circumferential ring 8 feet high was
replaced in field by inserting a preformed ring in three equal circumferential segments. The welding was
accomplished by the shielded metal arc process ("stick welding") without preheating or postweld heat treating.

The ASME Code does not require preheating or postweld heat treatment for SA516 Gr 70 steel 1 inch thick or
less. However, this steel is slightly air hardenable during welding, depending on the welding process, position
and procedure employed. This material is classified as a P1, Group 2 material according to ASME Code
Section IX.


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The vessel was operated under the owner/user option of the Illinois Boiler and Pressure Vessel Safety Act and
received a certification inspection approximately every two years. Continuing corrosion problems in the lower
end of the vessel resulted in the installation of an internal Monel liner in 1976 covering the bottom head and
most of the first ring, stopping short of the replaced ring. Periodic internal inspections were mainly visual with
wall thickness determinations made by an ultrasonic thickness gauge.

Just prior to the rupture, an operator noted a horizontal crack about 6 inches long spewing a plume of gas.
While attempting to close off the main inlet valve, the operator noted the crack had increased in length to about
2 feet. As the operator was evacuating the area and as the firemen were arriving, the vessel ruptured releasing
a large quantity of flammable gas which ignited shortly thereafter creating a large fireball and the ensuing of
deaths and damage. The separation occurred along the lower girth weld joint made during the 1974 repair.
The upper portion of the vessel was propelled 3500 feet by the thrust of the escaping gas.




METALLURGICAL EXAMINATION

The fracture surfaces exhibited the presence of four major prerupture cracks in the heat affected zone (HAZ) of
the lower girth field repair weld. The cracks originated on the inside surface and had progressed nearly through
the wall over a period of time. The largest precrack was located in the same area as the prerupture leak
reported by the operator. In total, the four cracks encompassed a circumferential length of about 9 feet (33.7%
of circumference). The remainder of the fracture exhibited a fast running brittle separation.




Microscopic examination of various cross sections through the failed weld joint area showed the cracking
originated in a hard microstructure in the HAZ and progressed in a manner characteristic of hydrogen related
damage in hard steels (see figures above). The HAZ exhibited hardness of up to 45 HRC (Hardness Rockwell
"C") (450 Brinell), equivalent to a tensile strength of over 200,000 psi in the region of weld cracking. By
comparison, the base metal had a hardness value of less than 20 HRC (229 BHN [Brinell Hardness Number],
110,000 psi tensile strength). The following sections discuss technical factors contributing to in-service
cracking of weld joints under such conditions.




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WELDING FACTORS

Welding procedures adopted must take into account not only the minimum requirements of ASME Code
Section IX and the appropriate design section, but must also be suitable for the specific service conditions
likely to be encountered. Stress corrosion cracking, hydrogen embrittlement and corrosion fatigue are typical of
material/environment interactions that are not fully accounted for in the ASME Code design rules. Appreciation
of such potential problems is left to the process designer, vessel designer, owner, contractor or inspector.
Reliance on only the ASME Code rules is not enough to assure safety of vessels operating in many corrosive
environments.

The weld HAZ contains potentially crack susceptible metallurgical structure, hardness variations and residual
stresses that can promote various types of unexpected service induced cracking depending on the chemical
environment and operating temperature. Industry experience has shown that steel having a hardness of 22
HRC maximum is resistant to cracking even under severe exposure conditions where hydrogen can be
absorbed by the steel. At hardness levels above 22 HRC, steel becomes less resistant to hydrogen induced
cracking and other environmental effects. At high hardness (above about 40 HRC), steel becomes quite
susceptible to cracking in the presence of hydrogen.

In potentially critical environments, the weld joint properties must be carefully controlled. Weld HAZ hardness
is a function of the cooling rate after welding. Preheating to at least several hundred degrees and maintaining
an interpass temperature during welding can warm the joint area sufficiently to prevent rapid cooling after
welding. Carbon content and alloy composition will dictate the appropriate temperature. Rapid cooling of even
mild steel can result in unacceptably high HAZ hardness for service in aggressive chemical environments.

Postweld heat treating (PWHT) is often necessary in critical weld joints to temper (soften) or stress relieve
weld joints in rugged duty or aggressive chemical environments. Higher carbon steels and more alloyed steels
are nearly always given PWHT. Even when not specifically called for in ASME Code Section IX, preheating or
PWHT may be necessary. In hydrogen environments, avoiding formation of a hard HAZ is crucial. Other
corrosive environments present similar concerns.

The specific weld procedure employed must be developed by individuals with pertinent knowledge of the
ASME Code (which should be viewed as the minimum guideline) as well as material behavior expertise in
aggressive environments.

CORROSION FACTORS

There are many specific ways that corrosion may contribute to unexpected failures. Often, corrosion problems
are handled simply by making the component thicker (a corrosion allowance). This is appropriate so long as
the corrosive conditions are known, the vessel is periodically inspected and if the corrosion is not highly
localized. Corrosion fatigue, pitting, stress corrosion and hydrogen attack are examples of metal/environment
problems that cannot be adequately handled by a corrosion allowance and superficial inspection methods
alone.

Hydrogen-assisted cracking and stress corrosion cracking will not always be readily apparent. Carefully
preparing the surface for visual examination, along with other techniques such as dye penetrant, magnetic

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particle, or shear wave ultrasonic inspection methods, may be required to detect such defects. Corrosion-
enhanced damage is often associated with welds, nozzles, or areas of unstable environmental conditions;
places where either the environment, stress, or metallurgical condition may abruptly change.

                                                           High pressure hydrogen or acidic environments can
                                                           introduce damaging levels of hydrogen into steel,
                                                           particularly   hard   steels   or   hard   HAZs.    The
                                                           mechanism of hydrogen evolution and penetration
                                                           is illustrated above. The absorbed hydrogen atoms
                                                           are attracted to high stress regions in the structure,
                                                           such as crack-like defects. The combination of hard
                                                           steel   and    absorbed    hydrogen    leads   to   the
                                                           development of cracks. Once inside the steel, these
                                                           hydrogen atoms also migrate to inclusions or
                                                           laminations and create hydrogen fissures and
                                                           blisters.

                                                           Hydrogen sulfide, cyanide and arsenic, even in
                                                           trace deposits, are examples of materials that
                                                           greatly increase the amount of hydrogen that
                                                           becomes absorbed by steel. Therefore, under
                                                           acidic corrosive conditions, particularly those
                                                           environments that also contain hydrogen sulfide,
                                                           cyanide or arsenic, hydrogen damage can be
                                                           severe. Weld HAZ hardness must be carefully
                                                           controlled under these circumstances, regardless of
                                                           whether or not the ASME Code or the National
                                                           Board Inspection Code specifically address the
                                                           subject.

                                                           Welding procedures, repair methods, and
                                                           inspection procedures must include careful
consideration of potential failure modes in corrosive environments. If pressure vessels or allied components
are operating in an aggressive environment, special steps should be taken to assure that individuals with
pertinent expertise are involved in the planning and review stages of design, inspections and repairs. When
distress signals are present, take the time to evaluate the cause and determine what special precautions are
necessary.




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SUMMARY

The problems of in-service cracking of weld zones can be minimized by attention to the important factors
summarized below.

    •     Preheat or postweld heat treat weld joints that may develop a hard HAZ when corrosive conditions are
          met.

    •     Inspect weld HAZs for cracks by a suitable NDE method if hard HAZs are suspected.

    •     Field repair welds are likely to have hard HAZs unless proper preheat or PWHT is applied.

    •     Small welds on thick members and arc strikes are examples of conditions resulting in rapid heating
          and cooling and are likely areas for trouble.

    •     Shop welds made according to the ASME Code may also crack in service under severely corrosive
          conditions.

    •     Preheating field weld joints will help drive off the dissolved hydrogen that has been picked up by the
          steel in service.

    •     Be particularly cautious when inspecting critical components in unfamiliar corrosive service, especially
          when prior history reveals problems and when field repairs have been made.




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APPENDIX C: Degradation Mechanisms for the Oil and Gas Industry



API RP571 "Damage Mechanisms Affecting Fixed Equipment in the Refining Industry." This recommended
practice describes degradation mechanisms found in refineries, affected materials, critical factors used to
identify   the   mechanism,     affected    units   or   equipment,   appearance   or   morphology   of   damage,
prevention/mitigation measures, inspection and monitoring recommendations, and related mechanisms.
References are also provided where the reader may be looking for additional information regarding the
degradation mechanism.




Figure 1- Sand erosion of wellhead piping




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Figure 2- Erosion/Corrosion at a pipe elbow




Figure 3- Shackle pin from FPSO mooring chain




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Figure 4 - Galvanic corrosion of seawater cooler brass tube sheet connected to titanium distribution grid (bars shown
looking through nozzle) and copper nickel cover/nozzle.




Figure 5 - Steam manifold valve, located on ship deck, wet mineral wool insulation.


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Figure 6 - Corrosion under insulation (CUI) on steam condensate return line at main deck penetration.




Figure 7 - This design facilitates water entrapment, coating breakdown and accelerated corrosion




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Case Study: 1

Factors Affecting In-Service Cracking of Weld Zone in Corrosive
Service
Harold L. Schmeilski
Illinois Division of Boiler and Pressure Vessel Safety, D. R. Gallup, Superintendent.

January 1986

Category: Incidents

Summary: The following article is a part of National Board Classic Series and it was published in the National
Board BULLETIN . The article was reprinted in the January 1986 National Board BULLETIN . Permission to
reprint was granted by the Illinois Division of Boiler and Pressure Vessel Safety, D. R. Gallup, Superintendent.
(6 printed pages)

< This article describes the cause of failure of a monoethandamine (MEA) absorber vessel that ruptured in the
state of Illinois in 1984, resulting in 17 fatalities and property damage in excess of $100 million.

VESSEL DESCRIPTION

The ruptured vessel was designed in accordance with The American Society of Mechanical Engineers (ASME)
Boiler and Pressure Vessel Code, Section VIII rules. The vessel was constructed of 1 inch thick SA516 Gr 70
steel plates rolled and welded with full penetration submerged arc joints, without postweld heat treatment. The
cylindrical vessel measures 81/2 feet in diameter with hemispherical ends comprising an overall height of 55
feet. Operating conditions were 200 psig internal pressure containing largely propane and hydrogen sulfide at
100¡F. An internal system distributed monoethanolamine (MEA) through the vessel for the purpose of
removing hydrogen sulfide from the gas.

VESSEL OPERATING HISTORY

The vessel went into operation in 1969. Soon after start-up, hydrogen blisters were observed to be forming in
the bottom two courses of the cylindrical vessel wall. Metallurgical analysis showed laminations to be present
in the steel.

In 1974, due to the large blister area found in the second course, a full circumferential ring 8 feet high was
replaced in field by inserting a preformed ring in three equal circumferential segments. The welding was
accomplished by the shielded metal arc process ("stick welding") without preheating or postweld heat treating.

The ASME Code does not require preheating or postweld heat treatment for SA516 Gr 70 steel 1 inch thick or
less. However, this steel is slightly air hardenable during welding, depending on the welding process, position
and procedure employed. This material is classified as a P1, Group 2 material according to ASME Code
Section IX.



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The vessel was operated under the owner/user option of the Illinois Boiler and Pressure Vessel Safety Act and
received a certification inspection approximately every two years. Continuing corrosion problems in the lower
end of the vessel resulted in the installation of an internal Monel liner in 1976 covering the bottom head and
most of the first ring, stopping short of the replaced ring. Periodic internal inspections were mainly visual with
wall thickness determinations made by an ultrasonic thickness gauge.

Just prior to the rupture, an operator noted a horizontal crack about 6 inches long spewing a plume of gas.
While attempting to close off the main inlet valve, the operator noted the crack had increased in length to about
2 feet. As the operator was evacuating the area and as the firemen were arriving, the vessel ruptured releasing
a large quantity of flammable gas which ignited shortly thereafter creating a large fireball and the ensuing of
deaths and damage. The separation occurred along the lower girth weld joint made during the 1974 repair.
The upper portion of the vessel was propelled 3500 feet by the thrust of the escaping gas.

METALLURGICAL EXAMINATION

The fracture surfaces exhibited the presence of four major prerupture cracks in the heat affected zone (HAZ) of
the lower girth field repair weld. The cracks originated on the inside surface and had progressed nearly through
the wall over a period of time. The largest precrack was located in the same area as the prerupture leak
reported by the operator. In total, the four cracks encompassed a circumferential length of about 9 feet (33.7%
of circumference). The remainder of the fracture exhibited a fast running brittle separation.




Microscopic examination of various cross sections through the failed weld joint area showed the cracking
originated in a hard microstructure in the HAZ and progressed in a manner characteristic of hydrogen related
damage in hard steels (see figures above). The HAZ exhibited hardness of up to 45 HRC (Hardness Rockwell
"C") (450 Brinell), equivalent to a tensile strength of over 200,000 psi in the region of weld cracking. By
comparison, the base metal had a hardness value of less than 20 HRC (229 BHN [Brinell Hardness Number],
110,000 psi tensile strength). The following sections discuss technical factors contributing to in-service
cracking of weld joints under such conditions.




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Case Study: 2

        High temperature degradation in power plants and refineries

            Heloisa Cunha FurtadoI, *; Iain Le MayII, *

        I
        CEPEL, Centro de Pesquisas de Energia Elétrica C.P. 2754, Cidade Universitaria,
        20001-970 Rio de Janeiro - RJ, Brazil
        II
            Metallurgical Consulting Services Ltd. P.O. Box 5006, Saskatoon, SK S7K 4E3,
        Canada




        ABSTRACT

        Thermal power plants and refineries around the world share many of the same problems,
        namely aging equipment, high costs of replacement, and the need to produce more
        efficiently while being increasingly concerned with issues of safety and reliability. For
        equipment operating at high temperature, there are many different mechanisms of
        degradation, some of which interact, and the rate of accumulation of damage is not
        simple to predict. The paper discusses the mechanisms of degradation at high
        temperature and methods of assessment of such damage and of the remaining safe life
        for operation.

        Keywords: degradation mechanisms, high temperature, life assessment, power plants,
        refineries




        1. Introduction

        Thermal power plants and refineries around the world are aging and need to be
        assessed to ensure continued safe operation. Replacement is frequently not an option
        because of high capital costs, and the much lower cost of continuing the operation of the
        older plant. However, reliability and safety are issues that have become much more
        important in recent years, so the assessment of damage and of the risk associated with
        failure have become increasingly important. In order to make such assessments on a
        sound basis, it is necessary to know the potential mechanisms of degradation and the
        rate of accumulation of damage that may be expected with each.




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WELDING FACTORS

Welding procedures adopted must take into account not only the minimum requirements of ASME Code
Section IX and the appropriate design section, but must also be suitable for the specific service conditions
likely to be encountered. Stress corrosion cracking, hydrogen embrittlement and corrosion fatigue are typical of
material/environment interactions that are not fully accounted for in the ASME Code design rules. Appreciation
of such potential problems is left to the process designer, vessel designer, owner, contractor or inspector.
Reliance on only the ASME Code rules is not enough to assure safety of vessels operating in many corrosive
environments.

The weld HAZ contains potentially crack susceptible metallurgical structure, hardness variations and residual
stresses that can promote various types of unexpected service induced cracking depending on the chemical
environment and operating temperature. Industry experience has shown that steel having a hardness of 22
HRC maximum is resistant to cracking even under severe exposure conditions where hydrogen can be
absorbed by the steel. At hardness levels above 22 HRC, steel becomes less resistant to hydrogen induced
cracking and other environmental effects. At high hardness (above about 40 HRC), steel becomes quite
susceptible to cracking in the presence of hydrogen.

In potentially critical environments, the weld joint properties must be carefully controlled. Weld HAZ hardness
is a function of the cooling rate after welding. Preheating to at least several hundred degrees and maintaining
an interpass temperature during welding can warm the joint area sufficiently to prevent rapid cooling after
welding. Carbon content and alloy composition will dictate the appropriate temperature. Rapid cooling of even
mild steel can result in unacceptably high HAZ hardness for service in aggressive chemical environments.

Post weld heat treating (PWHT) is often necessary in critical weld joints to temper (soften) or stress relieve
weld joints in rugged duty or aggressive chemical environments. Higher carbon steels and more alloyed steels
are nearly always given PWHT. Even when not specifically called for in ASME Code Section IX, preheating or
PWHT may be necessary. In hydrogen environments, avoiding formation of a hard HAZ is crucial. Other
corrosive environments present similar concerns.

The specific weld procedure employed must be developed by individuals with pertinent knowledge of the
ASME Code (which should be viewed as the minimum guideline) as well as material behavior expertise in
aggressive environments.

CORROSION FACTORS

There are many specific ways that corrosion may contribute to unexpected failures. Often, corrosion problems
are handled simply by making the component thicker (a corrosion allowance). This is appropriate so long as
the corrosive conditions are known, the vessel is periodically inspected and if the corrosion is not highly
localized. Corrosion fatigue, pitting, stress corrosion and hydrogen attack are examples of metal/environment
problems that cannot be adequately handled by a corrosion allowance and superficial inspection methods
alone.




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                                                          Hydrogen-assisted cracking and stress corrosion
                                                          cracking will not always be readily apparent.
                                                          Carefully      preparing   the        surface   for     visual
                                                          examination, along with other techniques such as
                                                          dye penetrant, magnetic particle, or shear wave
                                                          ultrasonic inspection methods, may be required to
                                                          detect such defects. Corrosion-enhanced damage is
                                                          often associated with welds, nozzles, or areas of
                                                          unstable environmental conditions; places where
                                                          either the environment, stress, or metallurgical
                                                          condition may abruptly change.



                                                          High pressure hydrogen or acidic environments can
                                                          introduce damaging levels of hydrogen into steel,
                                                          particularly    hard   steels    or     hard    HAZs.      The
                                                          mechanism of hydrogen evolution and penetration
                                                          is illustrated above. The absorbed hydrogen atoms
                                                          are attracted to high stress regions in the structure,
                                                          such as crack-like defects. The combination of hard
                                                          steel   and     absorbed    hydrogen        leads     to   the
                                                          development of cracks. Once inside the steel, these
                                                          hydrogen atoms also migrate to inclusions or
                                                          laminations and create hydrogen fissures and
                                                          blisters.

                                                          Hydrogen sulfide, cyanide and arsenic, even in
trace deposits, are examples of materials that greatly increase the amount of hydrogen that becomes
absorbed by steel. Therefore, under acidic corrosive conditions, particularly those environments that also
contain hydrogen sulfide, cyanide or arsenic, hydrogen damage can be severe. Weld HAZ hardness must be
carefully controlled under these circumstances, regardless of whether or not the ASME Code or the National
Board Inspection Code specifically address the subject.

Welding procedures, repair methods, and inspection procedures must include careful consideration of potential
failure modes in corrosive environments. If pressure vessels or allied components are operating in an
aggressive environment, special steps should be taken to assure that individuals with pertinent expertise are
involved in the planning and review stages of design, inspections and repairs. When distress signals are
present, take the time to evaluate the cause and determine what special precautions are necessary.




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SUMMARY

The problems of in-service cracking of weld zones can be minimized by attention to the important factors
summarized below.

   •     Preheat or postweld heat treat weld joints that may develop a hard HAZ when corrosive conditions are
         met.

   •     Inspect weld HAZs for cracks by a suitable NDE method if hard HAZs are suspected.

   •     Field repair welds are likely to have hard HAZs unless proper preheat or PWHT is applied.

   •     Small welds on thick members and arc strikes are examples of conditions resulting in rapid heating
         and cooling and are likely areas for trouble.

   •     Shop welds made according to the ASME Code may also crack in service under severely corrosive
         conditions.

   •     Preheating field weld joints will help drive off the dissolved hydrogen that has been picked up by the
         steel in service.

   •     Be particularly cautious when inspecting critical components in unfamiliar corrosive service, especially
         when prior history reveals problems and when field repairs have been made.




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     2. Deterioration mechanisms

     The principal deterioration mechanisms in high temperature plant are creep damage,
     microstructural degradation, high temperature fatigue, creep-fatigue, embrittlement,
     carburization, hydrogen damage, graphitization, thermal shock, erosion, liquid metal
     embrittlement, and high temperature corrosion of various types. Additionally, stress
     corrosion cracking and aqueous corrosion may be problems although these damage
     mechanisms are not generally expected in high temperature components: however they
     may occur when components are cooled down and liquid is still present within or in
     contact with them. Aspects of each will be considered in turn.

     2.1. Creep

     Creep is one of the most serious high temperature damage mechanisms. It involves
     time-dependent deformation and high temperature creep cracking generally develops in
     an intercrystalline manner in components of engineering importance that fail over an
     extended time. These include boiler superheater and other components operating at
     high temperature, petrochemical furnace and reactor vessel components and gas
     turbine blades. At higher temperatures, as can occur with local overheating, deformation
     may be localized, with large plastic strains and local wall thinning. At somewhat lower
     temperatures and under correspondingly higher stress levels, fracture can be
     transgranular in nature. To characterize the type of deformation and the relevant fracture
     mechanisms to be expected or to correlate observed deformation and fracture
     characteristics with probable operating conditions, deformation and fracture mechanism
     maps as developed by Ashby1 and Mohamed and Langdon2 can be useful in this regard.

     Classification of creep damage in steam generators has been made using the largely
     qualitative approach of Neubauer and Wedel3 based on the distribution of creep voids
     and microcracks observed by in situ metallography, and illustrated schematically in Fig.
     1. However, as has been shown subsequently, the method is unreliable for CrMo steels,
     at least, as apparent voids may be developed during the polishing and etching
     sequence4-5.   Replica   metallography   is   useful,   however,   and   the   degree   of
     spheroidization of carbides in bainitic and pearlitic structures can provide a good
     indication of the degree of thermal exposure and can be correlated with the extent of
     creep damage6. Used in conjunction with hardness measurements, indicating loss of
     tensile strength, these semi-quantitative tools have served to allow estimates of
     remaining safe life to be made of components undergoing damage by creep.




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     2.2. Microstructural degradation

     Microstructural degradation is a damage mechanism that can lead to failure by some
     other process such as creep, fatigue or more rapid fracture. It is important that it is
     recognized as a mechanism of damage as it can result in a significant loss in strength in
     a material. It is appropriate to discuss this following directly upon the discussion of creep
     damage, because the two mechanisms are closely bound together and, indeed, are
     difficult to separate. It has already been noted that Cr-Mo steels that are liable to fail by
     creep in a short time may display spheroidization of the carbides but little, if any, void
     formation. The formation of voids appears, in many cases, to be a very local
     phenomenon occurring very close to the time of fracture. It is worth commenting that the
     approach of Kachanov7 to the accumulation of damage (the continuum damage
     approach), postulating a loss of effective area or a loss in resistance to deformation,
     does not require any actual voids or loss of cross-section, and microstructural damage
     may be the dominant aspect of reduction in creep strength. Thus, evaluation of the
     potential for creep failure and the extent of creep damage needs to take account of
     microstructural changes. This may be done directly or through a measurement of the
     change in hardness, as this quantity provides an indication of the resistance of a
     material to deformation. Recently, Dyson8 has discussed continuum damage mechanics
     modelling of creep in terms of several damage mechanisms, including microstructural
     degradation.

     Another example of microstructural degradation is decarburization of carbon or alloy
     steel when exposed to an oxidizing atmosphere at high temperature. There is a loss of
     strength in the surface layer of the steel.

     2.3. High temperature fatigue and thermal fatigue

     Fatigue, involving repeated stressing, can lead to failure at high temperature as it does
     at low temperature. In components operating at high temperature it often arises through
     temperature changes that can lead to cyclic thermal stresses. This can lead to thermal
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     fatigue cracking. The cracking tends to develop in areas of high constraint, and the
     detailed mechanism may be one of local creep deformation.




                                                                      Figure   2    shows         the
                                                                      initiation of cracks at
                                                                      the interface between
                                                                      CrMo ferritic steel and
                                                                      austenitic             stainless
                                                                      steel    tubes         at   the
                                                                      entrance to the outlet
                                                                      header            of        the
                                                                      secondary superheater
                                                                      of a boiler operating at
                                                                      540 °C and which had
                                                                      been     subjected            to
                                        9
     frequent shut-downs and start-ups . The unit was designed for continuous operation as
     are most steam generators, and the difference in the coefficients of thermal expansion
     between the ferritic and austenitic tubes has led to the cracking. Figure 3 shows cracking
     along the fusion line at a stub attached to the header.

                                                                     2.4. Creep-fatigue

                                                                     Creep-fatigue interaction
                                                                     is a complex process of
                                                                     damage involving creep
                                                                     deformation and cyclic
                                                                     stress        and            the
                                                                     predominant             damage
                                                                     mode can range from
                                                                     primarily fatigue crack
                                                                     growth        at          higher
                                                                     frequencies and lower
                                                                     temperatures                   to
     primarily creep damage where hold times are long and temperature is at the high end of
     the scale.




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     2.5. Embrittlement and carburization

     Embrittlement from precipitation can arise in a number of different ways. For example,
     sigma phase formation in austenitic stainless steels maintained at high temperature or
     cycled through the critical temperature range (approximately 565 to 980 °C) causes loss
     of ductility and embrittlement. Ferritic stainless steels may be subject to an embrittlement
     phenomenon when held at or cooled over the temperature range 550 to 400 °C10. If the
     temperature conditions are considered likely to lead to such effects, metallographic
     checks are advisable after extended exposure prior to an unexpected rupture developing.
     In addition to the embrittlement of ferritic steels exposed to high temperature during
     service, and of austenitic stainless steels through the formation of sigma phase,
     carburization can produce brittle material when a component is exposed to a carburizing
     atmosphere for extended time at high temperature. Figure 4 shows extensive carbide
     formation in the hot gas casing of a gas turbine used for peak load power generation
     after 18,000 h of operation, involving 1,600 operating cycles. With a gas-side
     temperature of 985 °C and an air side temperature of 204 °C, the 321 stainless steel had
     developed severe thermal (fatigue) cracking. The cracks had initiated at the brittle,
     carburized gas side surface, the material having little resistance to bending without
     cracks occurring.




     2.6. Hydrogen damage

     Hydrogen damage, arising particularly in petrochemical plant, can occur in carbon steels
     through diffusion of atomic hydrogen into the metal, where it combines with the carbon in
     the Fe3C to form methane and to eliminate the pearlite constituent. This is a special case
     of micro structural degradation, and is much less common today than in the past
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     because of the use of low-alloy steels containing elements that stabilize carbides. Figure
     5 shows carbon steel from a catalytic cracking unit. Carbide from the original pearlite has
     been converted to methane, producing voids. In fact, recrystallization of the ferrite was
     observed around some of the voids, produced by the combination of deformation under
     pressure of the methane and the elevated temperature. The steel had been subjected to
     a temperature during service that was higher than appropriate for the grade of steel
     employed.

     Hydrogen-assisted cracking is a potential problem in petroleum reactor pressure vessels
     in hydrogen service, and the concern is that such sub-critical cracks do not reach a
     critical size for failure. Relations are available to estimate crack growth rates, and the
     important matter is the ability to detect and measure accurately the depth of such cracks
     lying beneath stainless steel cladding so that accurate predictions can be made.




     2.7. Graphitization

     Graphitization can take place in ferritic steels after exposure to high temperature for
     extended time, owing to reversion of the cementite in the pearlite to the more stable
     graphite phase. It is a particular form of microstructural degradation that was formerly
     observed relatively frequently in petrochemical components. With the development of
     more stable CrMo steels, it is not often seen today, but occurs from time to time both in
     petrochemical plant and in steam generators in which the temperature is high and the
     material is not entirely stable.

     Figure 6 shows graphitization in a steam pipe of DIN 15Mo3 alloy steel at the exit of a
     superheater at a nominal operating temperature of 480 °C. The tube suffered a local
     failure in the form of a "window" after some 100,000 h of service. Clearly the
     temperature was in excess of that which the material could withstand without serious
     deterioration.




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     Fracture occurred along planes of graphite nodules, with decohesion between the
     graphite and the ferrite matrix, these regions linking together from the growth of creep
     cracks as shown in Fig. 7. The formation of graphite in local planes or lines is believed to
     be due either to banding in the original structure or to local cold working during tube
     straightening, as can occur when Lüder's bands are produced.


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     2.8. Thermal shock

     Thermal shock involves rapid temperature change producing a steep temperature
     gradient and consequent high stresses. Such loading can produce cracking, particularly
     if the shock loading is repetitive. Cracks generated in this manner progress by a process
     of thermal fatigue. Such conditions are not encountered in thermal generating plants and
     refineries under normal operating conditions, but may arise during emergencies or with
     an excursion in the operating conditions. Brittle materials are much more susceptible to
     thermal shock and ceramic components, as are becoming more common in advanced
     gas turbines for example, are susceptible to such damage.

     2.9. Erosion

     Erosion can occur in high temperature components when there are particles present in
     flowing gases. This is a not uncommon situation in coal-fired power plants in which
     erosion by fly-ash can lead to tube thinning and failure in economizers and reheaters,
     and sootblower erosion can produce thinning in superheaters and reheaters in those
     tubes that are in the paths of the blowers. The solution to fly ash erosion depends in part
     on improving boiler flue gas distribution, and cutting down on local excessively high gas
     velocities. The control of soot blower erosion depends on many factors including
     excessive blowing pressure, poor maintenance and the provision of effective tube
     protection where required.

     2.10. Liquid metal embrittlement (LME)

     The classic example of liquid copper metal embrittlement of steel is shown in Fig. 8,
     where the Cu has penetrated along the austenite grain boundaries when the carbon
     steel was at a temperature of 1100 °C.




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                                                                         Liquid                  metal
                                                                         embrittlement              can
                                                                         occur with a number of
                                                                         liquid-solid            metal
                                                                         combinations, and one
                                                                         that can have serious
                                                                         consequences for the
                                                                         refining industry is LME
                                                                         of austenitic stainless
                                                                         steel by zinc. Rapid
                                                                         embrittlement              can
                                                                         occur at temperatures
                                                                         above 750 °C, and has
                                                                         been         observed       to
     produce widespread cracking in stainless steel components after a fire when there is a
     source of Zn present such as galvanized steel structural parts, or when there is
     contamination from Zn-based paints11. This latter source led to considerable cracking at
     the time of the Flixborough disaster12. Cracking can be extremely rapid (m/s) and stress
     levels can be as low as 20 MPa for such cracking to take place13.

                                                                         Two types of attack are
                                                                         believed to occur in the
                                                                         process          of        Zn-
                                                                         embrittlement               of
                                                                         austenitic        stainless
                                                                                14
                                                                         steel , as illustrated in
                                                                         Fig.        9.   Type        1
                                                                         embrittlement         is    a
                                                                         relatively slow process,
                                                                         controlled by the rate of
                                                                         diffusion               along
                                                                         austenite               grain
                                                                         boundaries,                and
                                                                         involves                   the
                                                                         combination of Zn with
                                                                         Ni, this producing Ni-
                                                                         depleted zones along
                                                                         the boundaries. As a
     consequence, the FCC austenite structure transforms to BCC ferrite, producing
     expansion and a stress that initiates cracking. Type 2 embrittlement occurs at a much
     faster rate, requiring an external stress to facilitate crack initiation. Cracking will not
     occur in the presence of a substantial oxide film unless this is ruptured locally.
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     Figure 10 shows an example of LME cracking by Zn in an austenitic steel as a result of
     a fire in a refinery and the formation of molten Zn from a galvanized component on the
     stainless steel tubing. The resemblance to the crack morphology of stress corrosion
     cracking is obvious.

     2.11. High temperature corrosion

     Minimization of corrosion in alloys for high temperature applications depends on the
     formation of a protective oxide scale. Alternatively, for alloys with very high strength
     properties at high temperature, a protective coating may need to be applied. The oxides
     that are generally used to provide protective layers are Cr2O3 and Al2O3. Corrosion
     protection usually breaks down through mechanical failure of the protective layer
     involving spalling of the oxide as a result of thermal cycling or from erosion or impact.

                                                                        High          temperature
                                                                        corrosion     can    also
                                                                        occur by carburization
                                                                        or sulphidation. As has
                                                                        already been discussed,
                                                                        carburization       takes
                                                                        place    in   carbon-rich
                                                                        atmospheres such as in
                                                                        reformer      or    other
                                                                        furnaces      and        the
                                                                        surface layer of the
                                                                        alloy can become brittle,
                                                                        leading to the formation
                                                                        of cracks, particularly
     when there are severe or cyclic temperature changes and this can greatly reduce the
     strength of the component. Sulphidation can be a serious problem in nickel-based
     superalloys and austenitic stainless steels, with sulphides forming on grain boundaries
     and then being progressively oxidized, with the sulphides moving ahead along the grain
     boundaries, so causing embrittlement in the alloy.

     2.12. Stress corrosion cracking and aqueous corrosion

     As indicated earlier, these are not damage mechanisms that are normally associated
     with components operating at high temperature. However, when shutdown of a plant
     occurs, fluid may condense and there may be water containing contaminants within
     pipes or vessels in the plant. The corrosion or stress corrosion cracking that occurs at
     low temperature may lead to preferential damage at high temperature during later
     operation of the plant.




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     Cracking that initiated in the inlet header of a primary superheater at the stub
     attachments is shown in Fig. 11. The cracks are thought to have grown by a combination
     of stress corrosion cracking during shutdown periods as well as by thermal cycling of the
     boiler, although the initiation in this case is believed to have been caused by thermal
     fatigue cracking. This conclusion is supported by the higher magnification view, where
     the displacement of the inner surface of the header on opposite sides of the crack is
     seen clearly.

     3. Assessment of damage and of remaining life

     Assessment of the extent of damage depends on inspection, or on an estimation of the
     accumulation of damage based on a model for damage accumulation, or both. Sound
     planning of inspections is critical so that the areas inspected are those where damage is
     expected to accumulate and the inspection techniques used are such as will provide
     reliable estimates of the extent of damage. If the extent of the damage is known or can
     be estimated, a reduced strength can be ascribed to the component and its adequacy to
     perform safely can be calculated.

     The general philosophy for estimating fitness for service is outlined in the American
     Petroleum Institute (API) Recommended Practice 579, "Fitness-for-Service", the first
     edition of which was published in 2000. This document provides assessment procedures
     for the various types of defects to be expected in pressurized equipment in the refinery
     and chemical industry. The steps involved are as follows:

     • Step 1: Identification of flaws and damage mechanisms.

     • Step 2: Identification of the applicability of the assessment procedures applicable to the
     particular damage mechanism.

     • Step 3: Identification of the requirements for data for the assessment.

     • Step 4: Evaluation of the acceptance of the component in accordance with the
     appropriate assessment techniques and procedures.

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     • Step 5: Remaining life evaluation, which may include the evaluation of appropriate
     inspection intervals to monitor the growth of damage or defects.

     • Step 6: Remediation if required.

     • Step 7: In-service monitoring where a remaining life or inspection interval cannot be
     established.

     • Step 8: Documentation, providing appropriate records of the evaluation made.

     API 579 does not presently cover high temperature damage to components operating in
     the creep regime, this section still being under discussion and development. It should be
     noted in addition that the entire API 579 document is being re-developed in conjunction
     with the American Society of Mechanical Engineers (ASME) to provide a common
     document as a Standard issued by both societies.

     For equipment operating at high temperature in the creep range, the principles outlined
     above are followed. Creep damage can be assessed by various procedures including
     those described earlier. Life estimates can also be made based on the predicted life at
     the temperature and stress that are involved, by subtracting the calculated life used up,
     and making an allowance for loss of thickness by oxidation or other damage. Recently
     there has been increased use of the procedures of continuum damage mechanics7 for
     creep damage and remaining life assessment. These ideas were initially developed for
     practical use by Penny15, and have been advanced further by Penny and Marriott16 and
     through the application of the Omega method developed by the Materials Properties
     Council17

     The growth of cracks in components operating at high temperature that are detected can
     be estimated using established predictive methods as given, for example, by Webster
     and Ainsworth18. Additionally, various examples of simplified methods to predict safe life
     in petrochemical plant containing cracks have been published, for example in a reformer
     furnace19.




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Case Study: 3

Microbiologically Influenced Corrosion (MIC) Mitigation



Corrosion, including microbially influenced corrosion (MIC), negatively impacts the integrity, safety, and
reliability of natural gas pipeline operations throughout the world. Studies estimate that corrosion and
deterioration caused by various mechanisms in recovery wells and pipes carrying natural gas, water, and
chemicals cost U.S. companies $117.8 billion per year. The biocides that U.S. industries use cost at least $1.3
billion per year (1991 estimates), are toxic to humans and the environment, and face regulatory scrutiny and
restrictions in the future. In response, GTI is developing products and processes to detect, prevent, and
mitigate MIC in pipelines.


                                                                     Objective
                                                                     In a program sponsored by the U.S.
                                                                     Department of Energy's National Energy
                                                                     Technology Laboratory (DOE NETL) and
                                                                     others, GTI researchers are working to
                                                                     develop     one   or   more       biocides   and/or
                                                                     corrosion inhibitors based on the methods of
                                                                     "green" chemistry. These naturally occurring
                                                                     biocides will avoid most or all of the
                                                                     regulatory limitations facing existing biocides
                                                                     and corrosion-preventing chemicals. These
biocides/corrosion inhibitors are produced from plants, animals, microorganisms, or even waste materials so
they may be not only technically effective, but economically competitive. Current off-the-shelf products and
technologies to combat biofouling and biologically influenced corrosion involve high labor costs and can
require the shutdown and depressurization of large segments of pipeline for extended time periods. Many
technologies can only be applied to localized sections of pipeline for limited time periods; however, GTI's
proposed technology will be cost-effective, applicable without depressurizing the pipelines, environmentally
friendly, and multi-faceted in its uses (foam pigging, coatings, incorporation in the linings, etc.)


Background
Biodeterioration (including biocorrosion or MIC) is defined as any undesirable change in the properties of a
material caused by the vital activities of organisms. The activity of living organisms, especially microorganisms
(bacteria, yeast, fungi, etc.) can negatively impact the infrastructure in all facets of the production, refining,
transmission, and distribution of natural gas for commercial, industrial, and residential use. Biocorrosion,
biodeterioration, and biofouling, all components of materials biodegradation, are responsible for major natural
gas infrastructure degeneration in the U.S. This is especially true for natural gas pipelines, both in the
transmission and distribution area of the industry. The materials that can be impacted by biological activity
include: metals (e.g., iron, stainless steel, and high molybdenum austenitic stainless steel), concrete and
masonry, man-made materials, plastics, and fiber-reinforced polymeric composites.

                                                                                                            Pg: 219/ 220
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To control biocorrosion, various biocides are typically used; however, natural products have a number of
advantages over more traditional sources of biocides and other industrial chemicals. The majority of industrial
biocides are manufactured from fossil fuels, such as petroleum or natural gas. As the supplies of these
resources become limited, the cost of industrial or commodity chemicals derived from them will continue to rise.
In addition, most, if not all, petrochemical-derived biocides are extremely toxic to most other living organisms,
including man. This is especially true of metal-containing biocides, which usually contain tin, silver, or mercury.
Thus, the production, use, and disposal of these agents commonly lead to environmental threat or damage.
Organic biocidal compounds, including aldehydes such as glutaraldehyde, are very effective in control of
microorganisms in both the attached and planktonic states; however, these compounds are also toxic. This
potential damage to humans or the environment is one reason for the ongoing search for environmentally
benign MIC control agents.




Status


Numerous plant species generate oily coatings to block the adhesion and/or attachment of bacteria, fungi, etc.
to their leaf, stem, and root surfaces. Pepper plants are very effective in using this defense mechanism. Since
pepper oils are commercially available, volatile, and effective (at least for the plants), GTI scientists have been
extensively researching these substances for blocking the initial step in MIC-namely, the attachment of
"exploratory" bacteria that initiates biofilm formation. Results of GTI research conducted to date have shown
the ability of extracts obtained from various Capsicum species to both inhibit biofilm spread ("bacteriostatic"
effects) and kill planktonic bacteria prior to the initial formation of biofilms that leads to corrosion.


Benefits
Pepper oil, or its effective component(s), have significant potential advantages over existing biocides and MIC-
control agents. These oils:


> Inhibit microbial growth and attachment
> Are a readily available plant product (renewable)
> Have proven stability
> Are environmentally benign
> May contain numerous active compounds
> Concentration of active ingredient(s) can be controlled and produced by biotechnology.


In summary, naturally produced (or, "green") biocides have the potential to not only inhibit biodeterioration, but
also achieve this goal in a cost-effective manner while protecting the environment.
April 2003




                                                                                                            Pg: 220/ 220
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    Book 1:MIC
    An investigation of the mechanism of IGS/SCC
    of Alloy 600 in corrosion accelerating heated
    crevice environments.

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    Book 2: MIC
    Recent advances in the study of biocorrosion -
    an overview


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    Book 3:
    Microbiologically Influenced Corrosion of
    Stainless Steel


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    Book 4: HIC
    Microbiologically Influenced Corrosion of
    Stainless Steel


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    Book 5: General failure
    Metal failures: Mechanisms, analysis and
    prevention


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    Book 6: HIC
    Theoretical model for hydrogen-induced
    Cracking in steels in aqueous environments


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           Recommended Reading:

                                  Suggested links: may obsolete with time, or
       http://images.google.cn/images?hl=zh-CN&q=corrosion%20mihd&um=1&ie=UTF-8&sa=N&tab=wi




    http://rapidshare.com/files/6665816/Corrosion_Scienc   http://rapidshare.com/files/65485033/Corrosion_in_refin
                     e_and_Technology.rar                                           eries.rar




                                                           http://rapidshare.de/files/20320060/Electrochemical_Tec
    http://rapidshare.com/files/11921921/Corrosion_of_st   hniques_in_Corrosion_Science_and_Engineering.pdf.ht
    eel_in_concrete_-_Ubderstanding__investigation_an                                   ml
                         d_repair.pdf




    http://rapidshare.com/files/57479869...59246.rar.ht     http://rapidshare.com/files/67417695/0849382432.rar
                            ml                                       http://depositfiles.com/en/files/2256691
         http://www.mediafire.com/?1eimyjmjo7n
                  http://mihd.net/89erwl




    http://rapidshare.com/files/22542215/1432455.rar.htm   http://rapidshare.de/files/20323168/Roberge_P.R._-_Ha
                                l                          ndbook_of_Corrosion_Engineering__McGraw-Hill_1999
                                                                                       _.rar




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               Online website on corrosion
                                          Corrosionsource

                                           Pipeline safety

                                          Corr.Electrochem




MS402 -Corrosion

http://www.corrosionclinic.com/corrosion_online_lectures/ME303.HTM

Corrosion Control

http://www.cee.vt.edu/ewr/environmental/teach/wtprimer/corrosion/corrosion.html

Introduction to Materials and Processes

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/cc_mat_index.htm

Corrosion Doctors

http://corrosion-doctors.org/Modules/mod-prevention.htm

Corrosion and Degradation Engineering.

http://engnet.anu.edu.au/DEcourses/engn4520/

Corrosion Clinics

http://www.corrosionclinic.com/

Aluminum Corrosion

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=180&pageid=2144416690/

Multimedia Corrosion Guides

http://www.cdcorrosion.com/mode_corrosion/corrosion_uniform.htm

ESDEP Course
http://www.esdep.org/4ccr/members/master/toc.htm




                                    Learn Online – Use your own Creativity              The great thing about learning online is
that the courses are so flexible. You can do many of the courses at your home or work if you have access to the Internet. you
can learn at your own pace whenever and wherever it suits you. The only disadvantage is that it may accelerate your hair-drop
b’cos there is no instructor to assist you! It is therefore not recommended for BALD header, people like Pete.
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Corrosion type

 Mechanism
 Preferential local attack at grain boundaries in polycrystalline metals arises due to the higher internal energy of
 the grain boundary regions. This is enhanced by the segregation of impurities to the boundaries and by the
 precipitation of second phases which may be more noble and which may also lower the resistance of the
 surrounding matrix by denudation. The extent of intergranular corrosion will depend on the level of sensitisation
 and the aggressiveness of the corrosive environment.

 In austenitic stainless steel sensitisation due to grain boundary precipitation of Cr carbides can occur on heating
 in the temperature range 450-900°C, for example during annealing or stress relieving, at service in this range or
 during welding, when it is called Weld decay (see figure 1).




    Figure 1. Schematic views of intergranular corrosion in austenitic stainless steel, for example weld decay

 Sensitization

 Sensitisation can be reduced by use of very low carbon grades and by stabilisation by the addition of titanium or
                                                                                                      BACK     INDEX




 niobium. These elements have a greater affinity to form carbides than chromium, hence any carbide
 precipitation that occurs will not remove Cr from the matrix.




 Intergranular corrosion

                                                         The intergranular corrosion is hardly generated under the
                                                         general circumstance.
                                                         However, it may educes reactive impurity and passive
                                                         element like Cr can be exhausted because the intergranular
                                                         has strong reactivity under the certain condition
                                                         As a result of it, the corrosion is seriously generated to
                                                         intergranular    first   because     corrosion   resistance   of
                                                         intergranular and its neighbor region are reducing and it is
                                                         called intergranular corrosion.

                                                         The most general intergranular corrosion is when austenite
                                                         stainless steel is heated and chrome reacts with carbon, the
                                                         chrome in neighbor region of intergranular exhausts and
Sensitization that progresses when
                                                         corrosion resistance decreases.
chrome carbide is educed from intergranular




                          Surface temperature of welded area of Stainless Steel 304 stainless steel




Some compositions of stainless steel are prone to intergranular corrosion. When heated to around 700 °C,
chromium carbide forms at the intergranular boundaries, depleting the grain edges of chromium, impairing their
corrosion resistance. Steel in such condition is called sensitized. Steels with carbon content 0.06% undergo
sensitization in about 2 minutes, while steels with carbon content under 0.02% are not sensitive to it.
There is a possibility to reclaim sensitized steel, by heating it to above 1000 °C and then quenching it in water.
This process dissolves the carbide particles and keeps them in solution.
It is also possible to stabilize the steel to avoid this effect and make it welding-friendly. Addition of titanium,
                                                                                                BACK       INDEX




niobium and/or tantalum serves this purpose; titanium carbide, niobium carbide and tantalum carbide form
preferentially to chromium carbide, protecting the grains from chromium depletion. Use of extra-low carbon
steels is another method. Light-gauge steel also does not tend to display this behavior, as the cooling after
welding is too fast to cause effective carbide formation.

Stainless Steel - Heat Treatment
Background
Stainless steels are often heat treated; the nature of this treatment depends on the type of stainless steel and the
reason for the treatment. These treatments, which include annealing, hardening and stress relieving, restore
desirable properties such as corrosion resistance and ductility to metal altered by prior fabrication operations or
produce hard structures able to withstand high stresses or abrasion in service. Heat treatment is often performed
in controlled atmospheres to prevent surface scaling, or less commonly carburisation or decarburisation.
Annealing
The austenitic stainless steels cannot be hardened by thermal treatments (but they do harden rapidly by cold
work). Annealing (often referred to as solution treatment) not only recrystallises the work hardened grains but
also takes chromium carbides (precipitated at grain boundaries in sensitised steels) back into solution in the
austenite. The treatment also homogenises dendritic weld metal structures, and relieves all remnant stresses
from cold working. Annealing temperatures usually are above 1040°C, although some types may be annealed at
closely controlled temperatures as low as 1010°C when fine grain size is important. Time at temperature is often
kept short to hold surface scaling to a minimum or to control grain growth, which can lead to "orange peel" in
forming.

Quench Annealing

Annealing of austenitic stainless steel is occasionally called quench annealing because the metal must be cooled
rapidly, usually by water quenching, to prevent sensitisation (except for stabilised and extra-low carbon grades).

Stabilising Anneal

A stabilising anneal is sometimes performed after conventional annealing for grades 321 and 347. Most of the
carbon content is combined with titanium in grade 321 or with niobium in grade 347 when these are annealed in
the usual manner. A further anneal at 870 to 900°C for 2 to 4 hours followed by rapid cooling precipitates all
possible carbon as a titanium or niobium carbide and prevents subsequent precipitation of chromium carbide.
This special protective treatment is sometimes useful when service conditions are rigorously corrosive,
especially when service also involves temperatures from about 400 to 870°C, and some specifications enable
this treatment to be specified for the product.

Cleaning

Before annealing or other heat treating operations are performed on austenitic stainless steels, the surface must
be cleaned to remove oil, grease and other carbonaceous residues. Such residues lead to carburisation during
heat treating, which degrades corrosion resistance.

Process Annealing

All martensitic and most ferritic stainless steels can be subcritical annealed (process annealed) by heating into
the upper part of the ferrite temperature range, or full annealed by heating above the critical temperature into the
                                                                                              BACK      INDEX




austenite range, followed by slow cooling. Usual temperatures are 760 to 830°C for sub-critical annealing. When
material has been previously heated above the critical temperature, such as in hot working, at least some
martensite is present even in ferritic stainless steels such as grade 430. Relatively slow cooling at about
25°C/hour from full annealing temperature, or holding for one hour or more at subcritical annealing temperature,
is required to produce the desired soft structure of ferrite and spheroidised carbides. However, parts that have
undergone only cold working after full annealing can be sub-critically annealed satisfactorily in less than 30
minutes.

The ferritic types that retain predominantly single-phase structures throughout the working temperature range
(grades 409, 442, 446 and 26Cr-1Mo) require only short recrystallisation annealing in the range 760 to 955°C.

Controlled Atmospheres

Stainless steels are usually annealed in controlled atmospheres to prevent or at least reduce scaling. Treatment
can be in salt bath, but the best option is "bright annealing" in a highly reducing atmosphere. Products such as
flat rolled coil, tube and wire are regularly bright annealed by their producers, usually in an atmosphere of
nitrogen and hydrogen. The result is a surface requiring no subsequent scale removal; the product is as bright
after as before annealing. These products are often referred to as "BA".

Hardening

Martensitic stainless steels are hardened by austenitising, quenching and tempering much like low alloy steels.
Austenitising temperatures normally are 980 to 1010°C, well above the critical temperature. As-quenched
hardness increases with austenitising temperature to about 980°C and then decreases due to retention of
austenite. For some grades the optimum austenitising temperature may depend on the subsequent tempering
temperature.

Preheating before austenitising is recommended to prevent cracking in high-carbon types and in intricate
sections of low-carbon types. Preheating at 790°C, and then heating to the austenitising temperature is the most
common practice.

Cooling and Quenching

Martensitic stainless steels have high hardenability because of their high alloy content. Air cooling from the
austenitising temperature is usually adequate to produce full hardness, but oil quenching is sometimes used,
particularly for larger sections. Parts should be tempered as soon as they have cooled to room temperature,
particularly if oil quenching has been used, to avoid delayed cracking. Parts sometimes are frozen to
approximately -75°C before tempering to transform retained austenite, particularly where dimensional stability is
important, such as in gauge blocks made of grade 440C. Tempering at temperatures above 510°C should be
followed by relatively rapid cooling to below 400°C to avoid "475°C" embrittlement.

Some precipitation-hardening stainless steels require more complicated heat treatments than standard
martensitic types. For instance, a semi-austenitic precipitation-hardening type may require annealing, trigger
annealing (to condition austenite for transformation on cooling to room temperature), sub-zero cooling (to
complete the transformation of austenite) and aging (to fully harden the alloy). On the other hand, martensitic
precipitation-hardening types (such as Grade 630) often require nothing more than a simple aging treatment.
                                                                                                    BACK       INDEX




Stress Relieving

Stress relieving at temperatures below 400°C is an acceptable practice but results in only modest stress relief.
Stress relieving at 425 to 925°C significantly reduces residual stresses that otherwise might lead to stress
corrosion cracking or dimensional instability in service. One hour at 870°C typically relieves about 85% of the
residual stresses. However, stress relieving in this temperature range can also precipitate grain boundary
carbides, resulting in sensitisation that severely impairs corrosion resistance in many media. To avoid these
effects, it is strongly recommended that a stabilised stainless steel (grade 321 or 347) or an extra-low-carbon
type (304L or 316L) be used, particularly when lengthy stress relieving is required.

Full solution treatment (annealing), generally by heating to about 1080°C followed by rapid cooling, removes all
residual stresses, but is not a practical treatment for most large or complex fabrications.

Low Temperature Stress Relieving

When austenitic stainless steels have been cold worked to develop high strength, low temperature stress
relieving will increase the proportional limit and yield strength (particularly compressive yield strength). This is a
common practice for austenitic stainless steel spring wire. A two hour treatment at 345 to 400°C is normally used;
temperatures up to 425°C may be used if resistance to intergranular corrosion is not required for the application.
Higher temperatures will reduce strength and sensitise the metal, and generally are not used for stress relieving
cold worked products.

Annealing After Welding

Stainless steel weldments can be heated to temperatures below the usual annealing temperature to decrease
high residual stresses when full annealing after welding is impossible. Most often, stress relieving is performed
on weldments that are too large or intricate for full annealing or on dissimilar metal weldments consisting of
austenitic stainless steel welded to low alloy steel.

Stress relieving of martensitic or ferritic stainless steel weldments will simultaneously temper weld and heat
affected zones, and for most types will restore corrosion resistance to some degree. However, annealing
temperatures are relatively low for these grades, and normal subcritical annealing is the heat treatment usually
selected if the weldment is to be heat treated at all.

Surface Hardening

Only limited surface hardening treatments are applicable to the stainless steels. In most instances hardening of
carbon and low alloy steels is due to the martensitic transformation, in which the achievable hardness is related
to the carbon content - as most martensitic stainless steels have carbon contents ranging from fairly low to
extremely low, this hardening mechanism is of little use.

Nitriding

It is possible to surface harden austenitic stainless steels by nitriding. As in nitriding of other steels the hard layer
is very hard and very thin; this makes the process of limited use as the underlying stainless steel core is relatively
soft and unsupportive in heavily loaded applications. A further drawback is that the nitrided case has a
                                                                                                  BACK       INDEX




significantly lower corrosion resistance than the original stainless steel.

A number of alternative, proprietary surface hardening processes for austenitic stainless steels have been
developed but these have not as yet become commercially available in Australia.

Physical Vapour Deposition (PVD)

An interesting recent development is the PVD (Physical Vapour Deposition) process. This enables very thin but
hard layers to be deposited on many materials, including stainless steels. The most commonly applied coating is
Titanium Nitride "TiN", which in addition to being very hard is also an aesthetically pleasing gold colour. Because
of its appearance this coating has been applied, generally on No8 mirror polished surface, to produce gold mirror
finished architectural panels.

More on Stainless Steel:


Corrosion of Stainless Steels

Aside from steel, stainless steels are the most common construction metals. There are many different types of
stainless steels, divided into five major categories by crystal structure type. The austenitic stainless steel alloys,
with AISI numbers from 200 to 399, are usually nonmagnetic. The alloys with numbers of 300 or above contain
more nickel than those with numbers below 300, and have better seawater resistance. These 300-series alloys
are very corrosion resistant, and are used for architectural applications, boat topside fittings, and household
goods such as sinks and silverware. The 300-series alloys will usually show no appreciable corrosion in fresh
water or sea atmosphere. The 400-series ferritic and the martensitic alloys are usually magnetic, stronger, and
less corrosion resistant than the austenitic alloys. They are used for knife blades and certain hand tools. These
alloys will sometimes suffer from mild surface rusting when exposed to fresh water or sea atmosphere. Duplex
and precipitation hardenable stainless steels are specialty alloys. Some are very strong and not very corrosion
resistant, such as 17-4PH, and others have intermediate strength and corrosion resistance between the
austenitic and the ferritic or martensitic alloys. There are some specialty alloys that are very corrosion resistant
because they add more special elements to the alloy, and are consequently somewhat more expensive than
standard grades, such as the austenitic 6XN.




Stainless steels get their corrosion resistance by the formation of a very thin surface film, called the passive film,
which forms on the surface in the presence of oxygen. Therefore, stainless steels usually have poor corrosion
resistance in low-oxygen environments, such as under deposits, in mud, or in tight places, called crevices, where
structures or hardware are attached. This is particularly true in seawater, where the chlorides from the salt will
attack and destroy the passive film faster than it can reform in low oxygen areas. All of the stainless steels except
the best of the specialty alloys will suffer from pitting or crevice corrosion when immersed in seawater. One of the
best 300-series stainless steels is type 316. Even this alloy will, if unprotected, start corroding under soft washers,
in o-ring grooves, or any other tight crevice area in as little as one day, and it is not unusual to have penetration
of a tenth of an inch in a crevice area after only 30 days in seawater. If water flows fast past a stainless steel,
more oxygen is delivered to the stainless steel and it corrodes less. For this reason, stainless steels have been
successfully used for impeller blades and propellers. These need to be protected from corrosion when there is no
flow.
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Painting stainless steels usually does not stop the crevice corrosion; it will occur any place where there is a
scratch or nick in the paint. For this reason, I usually recommend against using any stainless steel except certain
specialty alloys in seawater for more than a few hours at a time. There is a strong tendency to use in seawater
the same materials that work well in fresh water or sea atmosphere, so that types 303, 304, and 316 stainless
steel are often used for undersea applications. They will also usually fail if the exposure is long enough, unless
they are in continuous solid electrical contact with a material that will provide them with cathodic protection such
as steel or aluminum. As soon as the electrical contact is broken, the steel will corrode.




Crevice corrosion of stainless steels happens irregularly, but when it occurs it is very destructive. For example, if
10 stainless steel screws are put in a plate in seawater, it may be that all but one will be un-attacked, as bright
and shiny as the day they were made. That one screw, however, may well have attack over one quarter inch
deep in only a few months. The attack will occur in crevices where it can not be seen, and will destroy the screw
from the inside out. This is because the corrosion starts inside the crevice between the screw and the metal,
where it cannot be seen, then proceeds inside the metal where there is no oxygen, sometimes hollowing out the
part or giving it the appearance of Swiss cheese.




Even the best of stainless steels may have its corrosion resistance affected by the way it is made. For example,
316 stainless steel is very corrosion resistant in fresh water, but when it is welded, the areas next to the welds
experience a thermal cycle that can cause that material to corrode. This is called sensitization, and can lead to
the appearance of knife-line attack next to welds. This is why certain heat treatments should be avoided with this
and similar alloys. On the other hand, a low-carbon version of 316, called 316L, will not be sensitized, and can be
welded with little effect on corrosion properties.




Austenitic stainless steels can suffer from stress corrosion cracking to various degrees when fully immersed in
seawater. Stress corrosion cracking is cracking without much metal loss in the presence of a continuous applied
load in the environment. If a susceptible material fails by cracking and has numerous side cracks besides the
one causing the failure, stress corrosion cracking should be suspected. The ferritic and duplex stainless steels
usually do not have this problem.




Questions and Answers

When buying stainless steels, some companies claim that they passivate them. What is passivation, why is it
done, and does it make the stainless steel corrode less?
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When a stainless steel is passivated, it is put into a bath of an oxidizing acid, such as nitric acid. Stainless steels
get their corrosion resistance from the formation of a very thin corrosion product film of uncertain composition
called the passive film. It was observed that when stainless steels were first treated with an oxidizing acid, they
would later appear to corrode less than if they had not been treated. It was thought that the oxidizing acid
somehow thickened the passive film on the stainless steel to make the steel more corrosion resistant. Therefore,
the treatment was called passivation. We now know that this treatment does not affect the passive film in a way
that lasts very long in water. The film will stabilize at the same thickness when exposed to the same water
whether or not passiviation has been done. Then why do stainless steels appear to corrode less after passivation?
The oxidizing acid treatment is essentially a cleaning process that removes small particles of iron and other
impurities that have gotten on the surface of the stainless steel during the rolling process, or are in the structure
of the stainless steel itself and happen to be protruding from the surface. These particles corrode in waters that
normally don 抰 corrode stainless steels, leaving behind rust or other corrosion products that are readily visible.
It looks like the stainless steel is corroding when, in fact, it is only the surface particles that corrode. Cleaning
these particles off with the acid treatment means that they will not later corrode and leave behind ugly rust spots.
It therefore seems that the stainless steel is corroding less. Some people believe that surface particle corrosion
can start pitting corrosion, but controlled tests show that pitting will still happen even if all of these particles are
removed.

The reason for the passivation treatment now becomes clear. It makes the stainless steel look prettier after it has
been exposed to the water for a while. It actually does not affect the corrosion of the stainless steel itself,
however. The treatment is fairly cheap, and usually does not hurt anything, so manufacturers usually go ahead
and do it, just to avoid later questions about "rust" spots forming on their stainless steel. Passivation can be a
problem for parts with tight crevices that can trap the acid used. Over time, these acids can cause crevice
corrosion. For parts without crevices, passivation does have a benefit if the stainless steel is to be given some
later treatment for which a clean surface is necessary. For example, it is prudent to use passivation before
painting or plating over the stainless steel.

Stainless Steel Grade 321: http://www.azom.com/Details.asp?ArticleID=967
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                                                           MIC of Piping

Microbiologically Influenced Corrosion (MIC) is a problem in many commercial and industrial properties simply due
to the fact that microbiological communities are such common inhabitants in our environment. MIC is most
commonly found in open condenser water and process cooling loops, although its presence has been identified in
most piping systems - from domestic water and fire sprinkler lines, to those serving hot water heating systems.


Corrosion Engineering and Metal Corrosion Testing Services - Example 1:
Carbon steel pipe fittings from a fire suppression system corroded due to
micro-biologically influenced corrosion (MIC), most likely due to anaerobic
sulfate reducing bacteria. Structures that appear to be tubercles (i.e. hollow
mounds of corrosion product and deposits that cap localized regions of metal
loss) form due to oxygen concentration cells. The oxygen gradient inside
tubercles can lead to the formation of anaerobic conditions and colonization by
sulfate reducing bacteria. Tubercles generally have shallow dish shaped
depressions caused by corrosion of the base metal. However, when sulfate
reducing bacteria are present, deep discrete hemispherical pits form.
(Scanning Electron Microscope (SEM) Photo, Mag: 100X)



   For open systems, the main entry point for MIC is via the cooling tower - which acts similar to a giant air
scrubber by washing large quantities of particulates, organic material, and microbes into the water. For closed
systems, the microbes present in the make-up water usually provide the initial source of the problem. Under
favorable conditions, even a small initial contamination can produce significant end result.


   MIC based corrosion is extremely aggressive, and in its worst form, will lead to piping failures within a short
period of time. Once established, MIC is extremely difficult to eliminate, and may elevate into a chronic
maintenance and operating problem for years following. The failure to totally remove MIC from deep pits and the
furthermost branches and dead legs of a piping system generally results in reinfection by the same
microorganisms within a short period of time.


   Most alloys including steel, cast iron, copper, and even stainless steel are known to be susceptible to MIC
corrosion - meaning that MIC can attack any piping system given the proper conditions. Of the many potential
corrosion problems which can plague any building or plant property, MIC is unquestionably the most feared, as
well as the most difficult to identify and correct.

                                                   Different of Types MIC Exist

   When a metal surface is exposed to water, the microorganisms typically resident in the water quickly attach
themselves to the surface to form a biofilm - which is a living biological mass composed of bacteria, algae and
other microorganisms. Those microorganisms grow, break free, and distribute throughout the piping system.
Chemical biocides are generally applied to prevent the growth of such microorganisms, although they are not
always effective. Even under well controlled conditions, MIC can develop within a short period of time due to a
variety of factors. Once MIC has gained a solid presence in the system, the reliance on biocides alone as a
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corrective measure becomes worthless.


   Many forms of MIC types exist to present different levels of threat. Some microorganisms are capable of
producing metal dissolving metabolic by-products such as sulfuric acid, and are often identified within a
classification termed sulfur reducing bacteria, or SRB. Whereas normal condenser water corrosion rates may
range between 1 to 5 mils per year (MPY), MIC attack often results in accelerated corrosion rates exceeding 20
MPY and more - causing penetration of some metal surfaces in as little as one or two years.

   The below close-up photographs well illustrate the deep penetration typical of an MIC infection. In many
examples, the surrounding area suffers only moderate deterioration, or little metal loss at all. We offer a number of
excellent resources with additional information regarding MIC in our reprints section.




                                              Most Pipes Vulnerable

   Microbiological activity should be assumed to exist to some degree in anything but a steam piping system - an
excellent indicator of which is always plate count monitoring. Whether a microbiological presence turns into a
severe corrosion loss, however, depends upon a number of special factors related to the piping system and service
involved.


   MIC can be found in domestic cold water systems comprised of copper pipe, and will similarly produce pinhole
leaks in short periods of time. Due to the optimal temperatures maintained in hot domestic water systems, the
possibility of encountering MIC is slightly higher - though still not a common occurrence. While MIC is a concern
due to its potential for damaging domestic water piping, it is still of secondary importance to other pathogenic
microorganisms such as Legionella Pneumophila - which can cause acute sickness to humans, and in isolated
cases, even death.

                                              Testing the First Step

   An understanding of any corrosion problem is an extremely important first step prior to attempting any cleanout
procedure. This requires a thorough assessment of remaining pipe condition, and most importantly - the
identification of any weak areas of the piping system.


   For most MIC problems, the greatest threat always exists at the threaded joints, at fixtures such as temperature
wells and pressure gauges, and at lower floors where higher pressures exist. Installing sufficient shut-off valves to
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isolate critically weakened areas is well recommended in the event a chemical cleanout produces further leaks - an
always present danger. Initiating a chemical cleanout program that results in producing an overhead lawn sprinkler
system is a nightmare no building owner or operator wants to ever be responsible.




Figure 2. Corrosion Scaling in Fire Sprinkler Pipe

   Corrosion coupons, ultrasound and other nondestructive testing methods are generally ineffective at showing
an MIC condition. Therefore, a full metallurgical and biological analysis of multiple representative samples of pipe
becomes another prerequisite step. Viable cell culture tests can determine both the types and approximate volume
of microbes present in the system. This is an extremely important tool since the presence of specific microbes and
their metabolic by-products are indicative of MIC. For example, the presence of ferrous iron, sulfide, and low pH at
the corrosion site would support a diagnosis of SRB or sulfur reducing MIC.


   New advances in DNA technology now allow the identification of the specific types of bacteria within a MIC
tubercular deposit and provide unquestionable proof of exactly what is causing the problem. See Technical Bulletin
# C-8 about new DNA identification methods for microbiological growths.

                                                     Prevention

   Prevention of MIC depends on constant vigilance and awareness of the many conditions that contribute to its
formation. Deposit covered metal surfaces, low flow conditions, interior surface pitting, high bacterial counts, the
absence of (or improperly applied) water treatment, as well as various other conditions contribute to the growth of
bacteria - thereby placing the entire system at risk. A measured corrosion rate exceeding 10 MPY always suggests
the possibility of MIC, while a rate of over 25 MPY almost confirms it.


   A fully automated chemical feed and bleed station is absolutely mandatory for any condenser water or open
process water system today. In addition, regular monitoring for correct inhibitor level, biological characterization,
testing for microbiological cell count, frequent visual inspection of any pipe access points, and the use of multiple
CorrView ® corrosion monitors are all highly recommended as a guard against MIC.


   Once it has been positively determined that a system is infected with MIC, the first decision that must be made
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relates to the method of cleaning. This is an often difficult decision which must take into account the remaining
condition of the pipe wall, physical layout of the piping system, deposit buildup, the relative level of MIC infection,
and system operating conditions, among other factors.

                                                Cleaning the Systems

   Resolving an MIC problem is a matter of repeated cleanings and sterilization, followed by testing. Generally,
microbiological growths exist hidden within other deposits in a stratification of layers. Removing only the surface
deposits, therefore, will not provide an effective solution, and it is necessary to clean the pipe down to the bare
metal if any success is expected. See Technical Bulletin # C-15 about an effective but rarely employed solution to
many MIC problems.


   Establishing a spool piece at a section of larger 3 in. to 6 in. pipe is well advised in order to periodically evaluate
cleanout effectiveness. Due to the high volume of rust and particulates typically associated with an MIC problem,
and the physical volume of material returned into solution through any cleanout procedure, an effective filtration
system is always recommended.


   Following the elimination or control of an MIC condition, added attention to the system is mandatory since under
deposit corrosion and pits will have provided the ideal environment for new microorganisms to collect and grow.
For any system which has undergone a vigorous cleaning down to the base metal, it is imperative to increase the
inhibitor level in order to discourage new corrosion activity while the surface metal is being passivated. Biocides
should be added regularly.

                                         Long Term Maintenance Problems

   Because the microbiological agents causing MIC are generally found at the boundary layer between the pipe
and interior deposits, it is often difficult to physically solve the problem with sterilizing chemicals alone. Increased
biocide use alone is generally useless, as they are only designed to suppress microbiological growths, not kill and
eradicate them. And the extended use of high concentrations of strongly oxidizing chemicals such as chlorine
leads to further metal damage.


   Often, a multi-stage program of repeated heavy duty chemical cleanings and high dosage level sterilizations
must be established. The use of chemical dispersants and chelating agents are some additional methods which
may be employed to remove the attached deposits. Mechanical cleaning using a high pressure water jet may be
applicable in some specific examples. See Technical Bulletin # M-3 about high pressure water jet pipe cleaning.


   The benefits of any proposed aggressive cleaning program must always be weighed against the potential
damage caused to the piping itself. Yet, it is important to realize that the failure to aggressively address an
established MIC problem will lead to advanced pipe failure anyway! Due to the fact that MIC produces intensive
corrosion rates at localized sites, it is critically important to first establish the extent throughout the piping system
and the depth of surface pitting prior to any cleaning program.




                                                  Treatment Options
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   While the elimination of an MIC problem is always preferred, it may not be possible for a variety of reasons. In
many cases, a severe MIC problem cannot be solved and will be recognized as such - therefore requiring some
consideration of alternative options. Different authorities hold differing viewpoints in addressing an MIC problem -
with five generalizations presented below:

                                                     Prevention:

The preferred view, obviously, is to prevent an MIC infection from even beginning. Attention to a strict water
treatment program is critical, as well as is a totally automated chemical feed and bleed system. Regularly
performing laboratory cultures of the water is important to verify biocide or chlorination effectiveness. Testing for
anaerobic microbes, while technically difficult, is strongly advised in dead or low flow areas.


Periodic cleaning and sterilization of the tower is recommended at least twice annually. Filtration is also a plus, as it
greatly reduces the particulate volume known to contribute to any MIC growth problem.


While an indication of biological activity can be easily determined by simple dip slides, they can not show what may
be attached and growing at the interior pipe wall surface. In such cases, electronic biofilm monitors may offer
added information.


Also quite valuable, 3 in. or 4 in. spool pieces offer an inside look into the piping system and provide opportunity to
sample any interior deposits for microbiological and specifically MIC analysis.

                                                     Elimination:

                             Once established, eliminating the MIC problem altogether is the preferred choice.
                             Aside from being an extremely difficult task, this is often not feasible due to the
                             damage already caused to the piping system, and due to the potential for any cleaning
                             action to cause further leaks and piping failures. Some of the largest piping failures we
                             are aware have been caused by acid cleanout procedures performed on weakened
                             pipe.


In many cases, extensive repairs must be made to the system before any cleanout is even attempted - especially
to the most vulnerable threaded pipe. This delays greatly any remedial measures and allows even further damage
to occur.


Once any vulnerable pipe is replaced, eliminating an MIC problem becomes an expensive exercise of repeated
chemical cleaning, sterilizing and draining the system. High pressure water jet cleaning is an excellent option in
many cases, and will remove both microbiological growths and the deposits in one quick action.


The use of ozone to sterilize the system is another excellent option. Although much more difficult to apply since it
requires on-site generation, ozone will effectively sterilize an MIC condition assuming any existing deposits have
been removed.
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                                                   Inhibits Growth:

Another view is to identify the corrosion mechanism involved and inhibit the corrosion process to the best degree
possible. Identifying a specific MIC organism responsible is often difficult, although new developments in DNA
analysis will provide most answers.


Identifying the corrosion mechanism is more difficult, though necessary in order to plan its remediation. By many
authoritative opinions, however, removing an MIC infection completely, once it has been firmly established, is
nearly an impossible task.


Of all sterilizing agents, ozone likely offers the highest probability of providing a cure for any piping system having a
severe MIC condition.

                                                 Minimize Damages:

The fourth view assumes the impossibility of eliminating MIC once present, and instead focuses on minimizing its
corrosive damage. In many cases, the higher 15-20 MPY corrosion rates can be significantly reduced to extend
system life, though random pockets of microbiological growths may produce periodic pipe failures.


Many corrosion and water treatment authorities consider that a piping system cannot be returned to normal
conditions once MIC has established itself system wide. Multiple chemical sterilizations and high expense can be
assumed necessary in any such cleaning effort.

                                                   Replace Pipes:

In many cases, a piping system seriously infected with MIC will require replacement. This occurs usually only after
MIC damage has resulted in multiple failures and the cost of another major failure is deemed to be an
unacceptable risk.


Replacing less then the entire piping system, without good reason to believe that any MIC infection in those
remaining areas has been eradicated, will generally reintroduce the microbiological agent into the new piping and
begin the problem all over. Intense chemical treatment and monitoring may reduce such a threat to any new piping
installed

In short, our obvious recommendation is to take the necessary precautions now to ensure that an MIC condition
does not begin in the first place. Aside from operating problems and equipment damage, an MIC infection is an
extremely costly - producing expenses from pipe testing, lab tests, maintenance overtime, chemicals cleanings,
and monitoring and services, etc. in the hundreds of thousands of dollars.
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                                                                  Microbiologically Influenced Corrosion:
                                                                  An Engineering Insight (Engineering Materials and
                                                                  Processes)
                                                                  By Reza Javaherdashti

                                                                  Publisher: Springer
                                                                  Number Of Pages: 164
                                                                  Publication Date: 2008-03-12
                                                                  ISBN-10 / ASIN: 1848000731
                                                                  ISBN-13 / EAN: 9781848000735
                                                                  Binding: Hardcover


                                                                  Microbiologically-influenced corrosion (MIC) is one of the
                                                                  greatest mysteries of corrosion science and engineering, due
                                                                  to the complexities resulting from the involvement of living
                                                                  things such as bacteria. Bacteria are not only able to affect our
                                                                  health, but are also capable of impacting upon everyday life
                                                                  through a wide range of industrial sectors and the economy.


                                                                  Microbiologically Influenced Corrosion: An Engineering Insight
                                                                  introduces a new approach to the basics of MIC and explains
                                                                  how to recognise, understand, mitigate and/or prevent this
                                                                  type of corrosion. Topics explored include stress corrosion
cracking and microbial corrosion, the pros and cons of biocides, the involvement of magnetic bacteria in microbial corrosion,
and a new interpretation of cathodic protection based on recent research in microbial environments.


The material covered by Microbiologically Influenced Corrosion: An Engineering Insight will be of benefit to professional and
consultant engineers in power generating, oil and gas, marine, and mining industries; as well as to researchers in the fields of
chemistry, chemical engineering, materials science, corrosion and mechanical engineering.




http://www.filefactory.com/file/d3b8c0/
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Predictive Maintenance for Fire Sprinkler Systems




                   Jeffrey D. Gentry
              Sonic Inspection Corporation




                      May 2005
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                                    Table of Contents

 TABLE   OF   CONTENTS ............................................................................ 2

 INTRODUCTION .................................................................................... 3
   Overview of Problem ....................................................................... 3
   Solution......................................................................................... 3

 FIRE SPRINKLER PROBLEMS WITH CORROSION AND MIC .................................. 4
   Microbiologically Influenced Corrosion (MIC) ......................................... 4

 SOLUTION: SONIC PREDICTIVE MAINTENANCE PROGRAM .................................. 6
  Predictive Maintenance .................................................................... 6
  Risk Mitigation................................................................................ 6
  Return on Investment...................................................................... 7

 NON-INVASIVE, ULTRASONIC INSPECTION TECHNOLOGIES ................................. 8
  Patented Guided Wave Pipe Corrosion Detection ................................. 8
  Conventional Ultrasonic Thickness Measurements ............................... 8
  Alternative Inspection Techniques ...................................................... 9
  Analysis and Reporting .................................................................... 9

 SUMMARY REMARKS .......................................................................... 10

 REFERENCES .................................................................................... 10




Sonic Inspection Corporation                                                Phone (303) 308-3000
2070 Kahala Circle                                                            Fax (720) 733-9975
Castle Rock, CO 80104                                                      www.SonicInspection.com
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                                               Introduction

Overview of Problem
Corrosion of Fire Sprinkler piping can lead to potentially hazardous system malfunctions, as well as costly water
damage and repair costs. Microbiologically Influenced Corrosion (MIC) can rapidly accelerate corrosive growth
leading to these problems even in buildings less than five years old [1]. Unfortunately, inspections for MIC and
Corrosion are often overlooked until expensive problems such as damaging leaks occur or the corrosion is so
prevalent that large areas of the entire Fire Sprinkler system have to be replaced. This corrective maintenance
approach is a retro-active strategy. The task of the maintenance team in this scenario is usually to effect repairs as
soon as possible. Costs associated with corrective maintenance include repair costs (replacement components,
labor, and consumables), lost production and lost sales.

Solution
A new, proactive approach to fire sprinkler maintenance is available using completely non-invasive, ultrasonic
technologies that form the basis of a predictive maintenance approach. This approach provides a cost-effective
means of detecting the presence and monitoring progression of corrosion and creating a digital record of the system
state that can be used to schedule replacement of localized sections of the system before leaks or operation
failures occur.




                                Figure 1. Typical Sections of Fire Sprinkler System




Sonic Inspection Corporation                                                            Phone (303) 308-3000
2070 Kahala Circle                                                                        Fax (720) 733-9975
Castle Rock, CO 80104                                                                  www.SonicInspection.com
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                Fire Sprinkler Problems with Corrosion and MIC
The most common Fire Sprinkler Pipes are constructed using steel pipes sized according to hydraulic requirements
but typically ranging from 1.0 inch diameter to 8.0 inch diameter pipes in Schedule 5, 10 or 40 (with Schedule 40
having a significantly thicker wall than Schedule 5 or 10).

There are numerous types of corrosive reactions that can occur with steel and various methods for combating or
trying to slow the corrosive activity. Corrosion in Wet fire sprinkler systems is not usually a problem IF all of the air
is removed from the system after filling the system with water unless MIC is present (see below). Even a small
amount trapped air can cause the onset of corrosive activity.




                                  Figure 2. Corrosion Scaling in Fire Sprinkler Pipe

Microbiologically Influenced Corrosion (MIC)
MIC is the term used for corrosion influenced by microbes in the water. The primary concern is that the influence of
these microbes is often an extremely accelerated rate of corrosion. MIC is not caused by a single microbe, but is
attributed to many different microbes. These are often categorized by common characteristics such as by-products
(i.e., sludge producing) or compounds they effect (i.e. sulfur oxidizing). In a general sense, they all fall into one of
two groups based upon their oxygen requirements; one being aerobic (requires oxygen) such as sulfur oxidizing
bacteria, and the other being anaerobic, (requires little or no oxygen), such as sulfate reducing bacteria [2].

Although there have been regions of the United States, such as the Phoenix, Arizona area, where a large number
of MIC cases have been reported and documented, there is presently no indication that MIC is confined
to any specific geographical area. Reports of MIC have been received from throughout the United States
and also from abroad [1].



Sonic Inspection Corporation                                                                Phone (303) 308-3000
2070 Kahala Circle                                                                            Fax (720) 733-9975
Castle Rock, CO 80104                                                                      www.SonicInspection.com
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                            Figure 3. Large MIC Nodules in a Wet Fire Sprinkler System

MIC almost always occurs concurrently with other corrosion mechanisms, and it is virtually impossible to separate
them. This is in part due to the fact that microbes help create conditions under which other corrosion mechanisms
can occur, such as crevice corrosion, pitting, and under-deposit corrosion [1].

In a Dry system, water often collects in low spots in the piping after the pipe is periodically flushed (per NFPA
requirements for Dry systems). As the water sits in the bottom of the pipe, MIC can begin to rapidly eat through the
wall thickness, as most Dry systems incorporate thinner Schedule 5 or 10 pipes.




                          Figure 4. Wall Thinning & Pitting in a Dry Fire Sprinkler System




Sonic Inspection Corporation                                                           Phone (303) 308-3000
2070 Kahala Circle                                                                       Fax (720) 733-9975
Castle Rock, CO 80104                                                                 www.SonicInspection.com
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               Solution: Sonic Predictive Maintenance Program
Sonic Inspection has developed a comprehensive inspection service and predictive maintenance program for
facility managers and building owners. The basis of this program is a completely non-invasive, ultrasonic inspection
technique that provides a quick and accurate measurement of internal pipe corrosion and MIC. Sonic’s proprietary
software permanently stores the analyzed results and ties the measurements to copies of the facilities blueprints.

Predictive Maintenance
Predictive maintenance refers to maintenance based on the actual condition of a component. Maintenance is not
performed according to fixed preventive schedules but rather when a certain change in characteristics is noted.
Periodically inspecting fire sprinkler systems for the presence of MIC or Corrosion allows the facility manager to
accurately monitor the condition of the system, schedule localized replacement and significantly reduce the risk and
costs associated with corrective maintenance.

Using the non-invasive, ultrasonic inspection techniques described in the next section, a cost-effective predictive
maintenance program can be implemented to detect the presence and the progression of corrosion or MIC in the
sprinkler piping. The density of inspection locations and the frequency of inspections should be chosen based on
the risk associated with a leak or operational failure, history of the system, and condition of the sprinkler system
water supply.

Risk Mitigation
The risk of MIC or Corrosion in fire sprinkler piping can be broken into two general categories: (1) loss of life or
property damage caused by fire that spreads due to an operational failure; and (2) significant property damage
caused by a leak from corrosive pitting.

Almost any facility that is required to have a fire sprinkler system is subject to the first risk, but several types of
facilities rely on the sprinkler system to extinguish or slow the spread of fire more so than other structures. These
include military and commercial ships at sea, correctional facilities, petroleum refineries, chemical plants, power
plants (oil, coal, and especially nuclear).




                  Figure 5. Fire sprinkler operation is critical for both military and commercial ships


The potential of fire sprinkler leaks may not seem especially risky, but for facilities that house sensitive electronics
and equipment such as clean rooms and computer data centers a single small leak can produce potentially
catastrophic financial losses.


Sonic Inspection Corporation                                                               Phone (303) 308-3000
2070 Kahala Circle                                                                           Fax (720) 733-9975
Castle Rock, CO 80104                                                                     www.SonicInspection.com
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                        Figure 6. Leaks above Data Centers like these could be disastrous

Return on Investment
Calculating the Return on Investment in a predictive maintenance program for MIC and corrosion in the fire sprinkler
piping requires assessing the risk of either type of system failure, estimating the potential cost of such a failure,
estimating the cost of a corrective maintenance approach once a problem is discovered. Once these costs are
estimated they need to be weighed against the cost of inspecting the system using a non-invasive, ultrasonic
technique and monitoring the level of corrosion at suitable intervals for the associated level of risk.




Sonic Inspection Corporation                                                           Phone (303) 308-3000
2070 Kahala Circle                                                                       Fax (720) 733-9975
Castle Rock, CO 80104                                                                 www.SonicInspection.com
                                                                                               BACK       INDEX




               Non-invasive, Ultrasonic Inspection Technologies
Sonic uses two separate ultrasonic inspection technologies can be used to quickly detect and monitor the level of
corrosion and MIC in a fire sprinkler system.

Patented Guided Wave Pipe Corrosion Detection
Sonic Inspection uses a patented Guided Wave Ultrasonic technique to rapidly identify areas of pipe that show
indications of internal corrosion. The technique uses a specialized ultrasonic scanning head placed on the exterior
of the pipe to excite guided waves that propagate around the circumference of the pipe.




                                         Figure 7. Guided Wave Scan Head

Guided Wave signatures for brand new, pristine pipe have been stored in software for all of the possible pipe
diameters and schedules, for both Wet and Dry systems. The measured signature is compared to a pristine pipe.
The more corrosion (presence of nodules attached to the interior of the pipe and amount of wall thinning) the more
the received signal is affected.




                           Figure 8. No Corrosion (left) versus Corrosion Indication (right)



Conventional Ultrasonic Thickness Measurements
Any areas of pipe that show indications of corrosion are investigated further with highly accurate wall thickness
measurements made around the circumference of the pipe.

Sonic Inspection Corporation                                                             Phone (303) 308-3000
2070 Kahala Circle                                                                         Fax (720) 733-9975
Castle Rock, CO 80104                                                                   www.SonicInspection.com
                                                                                                   BACK       INDEX




                             Figure 9. Conventional Ultrasonic Thickness Measurements

Alternative Inspection Techniques
Some areas of pipe may be inaccessible and therefore cannot be measured using the ultrasonic techniques
described above. One alternative method for inspecting hard to reach pipe includes feeding a digital video
boroscope into the pipe and recording the visual condition of the pipe interior. This method may be appropriate for
limited use in high risk areas but is too intrusive and expensive for a general recurring inspection of an entire facility.

Analysis and Reporting
The measurements are permanently stored for each location and a report showing the current level of corrosion can
be produced using the sprinkler system blueprints.




                          Figure 10. Corrosion measurements are tracked for each location




Sonic Inspection Corporation                                                                Phone (303) 308-3000
2070 Kahala Circle                                                                            Fax (720) 733-9975
Castle Rock, CO 80104                                                                      www.SonicInspection.com
                                                                                               BACK        INDEX




Summary Remarks
There are four general approaches to maintaining any system: (1) Corrective Maintenance; (2) Preventative
Maintenance; (3) Reliability Centered Maintenance (RCM); and (4) Predictive Maintenance. Because of the nature
of MIC and corrosion and expense of Fire Sprinkler Systems, neither Preventative Maintenance (i.e. simply
replacing the pipes on a scheduled basis before corrosion can occur) nor RCM are good choices. Corrective
maintenance refers to the practice that is common today of waiting until the corrosion causes a leak or operational
problem and then reacting to the problem with some sort of corrective action.

Until recently, facility managers and building owners had little choice but to wait for corrosive problems to arise
before implementing costly corrective maintenance in a totally reactionary mode. Under these circumstances, a lot
of pipe is either replaced unnecessarily (at a very high cost), or corroded pipe is left in place to cause a future
problem (which is also costly).

Now, with Sonic’s Predictive Maintenance Program, the presence of MIC and corrosion can be quickly identified,
and tracked to provide cost-effective risk mitigation for both pin-hole leaks and operational failure of the system.
Facility managers and building owners now have the means to create a database (see Figure 10) with the current
level of corrosion and MIC in their fire sprinkler system piping and use this information to proactively schedule
replacement of only the pipe deemed unacceptable.




References
1. FM Global Property Loss Prevention Data Sheet for Internal Corrosion in Automatic Sprinkler Systems.
May 2001.

2. Huggins, Roland. “Microbiologically Influenced Corrosion: What It Is and How It Works”, Article on American Fire
Sprinkler Association Web Site.




Sonic Inspection Corporation                                                             Phone (303) 308-3000
2070 Kahala Circle                                                                         Fax (720) 733-9975
Castle Rock, CO 80104                                                                   www.SonicInspection.com
                                                                                                          BACK       INDEX




   Corrosion type
 Stress Corrosion Cracking (SCC)




                                     Progress of SCC on stainless steel in austenite system


The SCC is a type of corrosion when it receives environmental influence and mechanical stress at the same time and cracks

and its impact transfers.

The stress corrosion of stainless is mainly generates from the liquid including chloride like a pitting and crevice corrosions and it

is caused more than 50° C. As its density of chloride is low, it is generated to the environment where pitting and crevice

corrosion are not generated. Above figure shows generation and propagation processes of SCC on austenite stainless steel

with various factors that affects SCC.

The pitting corrosion is generated when the film is broken by chlorine ion or slip step, the pitting corrosion grows to crack when

the volume of hydrogen ion in pitting corrosion increases and crack grows according to continuous increase of hydrogen ion

and its reduction reaction.

It is big problem because SCC forms passive film and it is generated from the material with excellent corrosion resistance under

the lower stress than designed stress. Even though there is no external stress, residue stress from material manufacturing and

processing such as molding and welding can cause stress corrosion.

The chloride that causes stress corrosion exists in the water with various densities under the natural environment and it is

caused by gasket or insulating material that includes chloride. In case of water pipe, intergranular-stress corrosion cracking is

largely generated because it becomes sensitive to intergranular corrosion based on residue stress from welding and

sensitization of HZA.

To prevent it, residue stress has to be removed with heat treatment under appropriate temperature and it is better to use
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STS604L or STS316L that reduce the content of carbon.




Polythionic acid stress corrosion cracking of type 310 stainless steel. The item was exposed to sulfur containing
natural gas in a continuous flare. (100X)

Stress corrosion cracking is a rapid and severe form of stainless steel corrosion. It forms when the material is

subjected to tensile stress and some kinds of corrosive environments, especially chloride-rich environments (sea water)

at higher temperatures. The stresses can result of the service loads, or can be caused by the type of assembly or

residual stresses from fabrication (eg. cold working); the residual stresses can be relieved by annealing. This limits the

usefulness of stainless steel for containing water with higher than few ppm content of chlorides at temperatures above

50 °C.

Stress corrosion cracking applies only to austenitic stainless steels and depends on the nickel content.
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   Corrosion type
 Stress Corrosion Cracking (SCC)




                                     Progress of SCC on stainless steel in austenite system


The SCC is a type of corrosion when it receives environmental influence and mechanical stress at the same time and cracks

and its impact transfers.

The stress corrosion of stainless is mainly generates from the liquid including chloride like a pitting and crevice corrosions and it

is caused more than 50° C. As its density of chloride is low, it is generated to the environment where pitting and crevice

corrosion are not generated. Above figure shows generation and propagation processes of SCC on austenite stainless steel

with various factors that affects SCC.

The pitting corrosion is generated when the film is broken by chlorine ion or slip step, the pitting corrosion grows to crack when

the volume of hydrogen ion in pitting corrosion increases and crack grows according to continuous increase of hydrogen ion

and its reduction reaction.

It is big problem because SCC forms passive film and it is generated from the material with excellent corrosion resistance under

the lower stress than designed stress. Even though there is no external stress, residue stress from material manufacturing and

processing such as molding and welding can cause stress corrosion.

The chloride that causes stress corrosion exists in the water with various densities under the natural environment and it is

caused by gasket or insulating material that includes chloride. In case of water pipe, intergranular-stress corrosion cracking is

largely generated because it becomes sensitive to intergranular corrosion based on residue stress from welding and

sensitization of HZA.

To prevent it, residue stress has to be removed with heat treatment under appropriate temperature and it is better to use
                                                                                                         BACK       INDEX




STS604L or STS316L that reduce the content of carbon.




Polythionic acid stress corrosion cracking of type 310 stainless steel. The item was exposed to sulfur containing
natural gas in a continuous flare. (100X)

Stress corrosion cracking is a rapid and severe form of stainless steel corrosion. It forms when the material is

subjected to tensile stress and some kinds of corrosive environments, especially chloride-rich environments (sea water)

at higher temperatures. The stresses can result of the service loads, or can be caused by the type of assembly or

residual stresses from fabrication (eg. cold working); the residual stresses can be relieved by annealing. This limits the

usefulness of stainless steel for containing water with higher than few ppm content of chlorides at temperatures above

50 °C.

Stress corrosion cracking applies only to austenitic stainless steels and depends on the nickel content.
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Wet corrosion
Stress corrosion cracking




Stress corrosion cracking of a tube.

STRESS CORROSION CRACKING




Cracks across the grains (transgranular SCC) or along the grain boundaries (intergranular SCC).




Stress corrosion cracking (SCC) results from the combined action of three factors: Tensile stresses in the
material, a corrosive medium (esp. chloride-bearing or hydrogen-sulphide environment) and elevated
temperature (normally above 60°C for chloride-induced SCC). Cases where chloride induced SCC has occurred
at lower temperatures than 60°C exist. The most common media where stress corrosion cracking occurs are
chloride containing solutions, but in other environments, such as caustics and polythionic acid, problems with
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SCC may also appear. Some enviroments that may cause stress corrosion cracking of stainless steels are listed
below.


Some environments where stainless steels are prone to stress corrosion cracking:

    •    Acid chloride solutions
    •    Seawater
    •    Condensing steam from chloride waters
    •    H 2 S + chlorides
    •    Polythionic acid (sensitised material)
    •    NaCl-H 2 O 2
    •    NaOH-H 2 S

The mechanism of stress corrosion cracking is not well understood. This is mainly due to the specific features of
SCC being the result of a complex interplay of metal, interface and environment properties. As a result of this
different combinations of solution and stress are seldom comparable and the most reliable information is
obtained from empirical experiments. During SCC the material does not undergo general corrosion and the
phenomenon is sometimes considered to be one of activation/passivation interaction. It has been found that
cracks often initiate in trenches or pits on the surface, which can act as stress raisers. The isolated times for pit
initiation, pit growth, crack initiation and fracture may, however, differ considerably between different materials.


In some cases crack initiation has been associated with the formation of a brittle film at the surface. The film
developed at grain boundaries might, for instance, have lower ductility due to a different metal composition than
the bulk material. At a certain film thickness and under stress this brittle film will crack and expose the underlying
metal. New film growth will proceed with subsequent continued crack growth and so forth. The developed crack
tip has a small radius and will develop a very high stress concentration. Even so, the stress condition alone is not
sufficient for crack growth, but corrosion still plays a very large part. It has been shown experimentally that stress
corrosion cracking can be stopped when applying cathodic protection, i.e. when corrosion is stopped but the
stress conditions remain unchanged.


Cracking may be either transgranular (TGSCC) or intergranular (IGSCC) or, perhaps most usual, a combination
of both. The material microstructure and alloying components are of major importance for crack paths as well as
for SCC resistance. Alloying with Ni can make materials less prone to SCC and the duplex microstructure of the
austenitic-ferritic grades is also beneficial. Standard austenitic stainless steels, like AISI 304 and AISI 316, are
generally prone to SCC in chloide containing environments at temperatures above 60°C, except at very low
chloride contents, and therefore higher alloyed austenitics or duplex stainless steels should be used.
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                                                 Transgranular stress corrosion
                                                 crack in Sandvik grade 2RE69
                                                 after
                                                 autoclave testing in 1000 ppm
                                                 chloride at 250°C.




HYDROGEN EMBRITTLEMENT
Hydrogen embrittlement (HE) is sometimes stated to be a kind of SCC. This might, however, lead to serious
misunderstandings as many discrepancies exist. Perhaps most important is that HE cannot be reduced by
cathodic protection, but might instead increase under such circumstances. The reason for this is that HE is
caused by the penetration of atomic hydrogen into the metal structure. This, in turn, might occur when reduction
of H + is taking place on the metal surface, e.g. during cathodic protection in acidic environments. Several
deposition techniques, such as electroplating, also involve reduction processes at the metal surface with the
following risk of hydrogen penetration and embrittlement. To avoid this, treated articles are often baked before
use to remove the hydrogen. The risk for HE is increased for harder metals, but the tendency to hydrogen
cracking decreases with increasing temperature. Some differences between HE and SCC are illustrated in figure
14.



SULPHIDE STRESS CRACKING
Sulphide stress cracking (SSC) might be defined as a variant of HE, but is sometimes treated as a special
corrosion type. Sulphides are hydrogen-evolution poisons and as such prevent the hydrogen atoms formed on
the metal surface from pairing up and dissolving as H2 into the surrounding solution.


SSC has been found to cause severe problems especially in the oil and gas industry. A standard for material
requirements in so-called sour environments has therefore been developed: NACE MR0175. Among the
acceptable steel grades are SAF 2205, SAF 2507 and Sanicro 28. New grades can be accepted in NACE
MR0175 after successful testing according to one of four methods described in NACE TM 0177.


Chloride-induced SCC
The best way of solving the problem of SCC is by selecting a suitable material. Type 304L and 316L austenitic
steels have limited resistance to SCC, even at very low chloride contents and temperatures. The following steels,
on the contrary, are highly resistant:

      •   Ferritic steels (also carbon steels)
      •   Austenitic-ferritic (duplex) steels
      •   Austenitic steels with high Ni content.



To some extent the risk of SCC can be avoided by proper design. It is especially important to avoid stress
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concentration, which will occur at sharp edges and notches.


Testing can be carried out in e.g. 40% CaCl2 or in chloride-containing water. The diagram below shows results
from chloride solutions containing 8 ppm oxygen. Note that no cracking was observed in SAF 2507.


H2S-induced SCC
Within the oil and gas industry, the process fluids often contain a certain amount of hydrogen sulphide, H2S.
Applications involving exposure to this type of process fluids are often referred to as sour service. When
considering the corrosivity of a sour process fluid, the partial pressure of H2S is to be considered besides the pH
value, the temperature, the oxygen and chloride contents as well as the presence of solid particles (such as
sand). It has been shown that this type of corrosion attack is worst at temperatures around 80°C, but cracking
may occur also at temperatures below 60°C.


A high nickel content is favourable for a good resistance to this form of SCC and for most sour environments high
nickel alloys are to be used. A Sandvik grade with very good resistance to sulphide-induced cracking is Sanicro
28. The duplex grades SAF 2205 and SAF 2507 have not as good resistance as the high nickel alloys, but can
successfully be used at intermediate hydrogen-sulphide partial pressures.


Testing can be carried out according to NACE TM0177 (5% NaCl and 0.5% acetic acid saturated with H2S). The
diagram below shows results from this type of testing with SAF 2205 and SAF 2507. No cracking was observed
on the SAF 2507 samples after the 720-hours test period. Note: Testing in NACE solution is carried out at an
external laboratory, and it is both time consuming and expensive. Several of our standard grades as well as SAF
2205, SAF 2507 and Sanicro 28 are covered by the standard MR0175 and should not normally need further
testing. Read more about the test in S-133.




1. SCC resistance in oxygen-bearing neutral solutions with various chloride contents. Testing time 1,000 hours.
Applied stress equal to the 0.2% proof strength at testing temperature.
2. Constant-load SCC tests in NACE solution at room temperature (NACE TM0177).
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Although looking as a piece of art, this SCC attack was devastating for the tube.




                                The photo was taken in a scanning electron microscope (SEM) and it shows a
                                SCC crack with a magnification of 45 times.
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"Stress Corrosion Cracking in Stainless Steel"


Question:


"We have experienced repeated failures on seal flush piping on the naphtha and distillate

reflux pumps in our Crude Fractionation Unit. The piping is currently constructed of 316L

tubing. The process stream in low in organic chlorides, but high in hydrogen sulphide.

Inspection of the failures(cracks) suggests stress corrosion cracking; likely sulphide induced. I

am considering replacing the stainless steel piping with either carbon steel, or 5% chromium

1/2 molybdenum. Do you have any thoughts or suggestions?I was not aware that h3S

increases the susceptability of austenitic stainless steels to chloride induced stress corrosion

cracking. This relates to another persistent problem that we have experienced; cracking of 347

valves in hydrotreating service. The valves that fail are typically small diameter, A182 TP347

forged steel valves. The service conditions are about 800°F and 2500psig. The fluid in the

piping circuits is heavy oil; high in sulfur, hydrogen and hydrogen sulphide. We currently

neutralize the piping circuits during turnarounds using a soda ash/sodium nitrite wash as per

NACE recomendations. This procedure was developed to prevent polythionic acid attack on

the stainless steels when the piping is exposed to oxygen. Although this does not specifically

address chloride contamination problems, it does help to flush contaminants high in chlorides

from the system. It also leaves a thin protective layer of crystalline soda ash/sodium nitrite on

the piping which helps to limit oxygen exposure to the piping. The reactor circuits(feed and

effluent) in our plant have been constructed with A297 HF Modified piping(cast and machined

347SS). The smaller diameter piping is typically A312 TP347 with A182 F347 fittings. We have

seen chloride induced stress corrosion cracking in valves, forged fittings and butt-welded

connections. The cracking of small diameter forged valve bodies(drains and vents) has been

the most common failure. Do you have any suggestions that may help to eliminate the

problems that we are experiencing?"
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Answer:


You are probably correct with respect to chloride stress corrosion cracking being responsible

for failure of the 316 piping. Hydrogen sulfide significantly decreases the threshold quantities

of chlorides need to promote chloride SCC. (A laboratory analysis would easily verify your

theory of chloride SCC). The question of replacement metallurgy depends on the nature of

your process stream. The proper selection of carbon steel or a chromium-molybdenum low

alloy steel depends several factors, including amounts of hydrogen sulfide, sulfur and

temperature. The McConomy curves are a widely used reference for materials selection in

h3S environments. If the cracking has been correctly diagnosed as chloride stress corrosion

cracking, and the problem is confined to small drain, flush fittings, etc. you might consider

upgrading those specific components to an alloy not susceptible to chloride SCC while still

maintaining resistance to polythionic acid SCC, i.e., alloy 825 or something similar..
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Stainless steel
In metallurgy, stainless steel is defined as a ferrous alloy with a minimum of 10.5% chromium content.[1]
The name originates from the fact that stainless steel does not stain, corrode or rust as easily as ordinary
steel. This material is also called corrosion resistant steel when it is not detailed exactly to its alloy type
and grade, particularly in the aviation industry. As such, there are now different and easily accessible grades
and surface finishes of stainless steel, to suit the environment to which the material will be subjected to in its
lifetime. Common uses of stainless steel are the everyday cutlery and watch straps.

Stainless steels have higher resistance to oxidation (rust) and corrosion in many natural and man made
environments; however, it is important to select the correct type and grade of stainless steel for the particular
application.

High oxidation resistance in air at ambient temperature is normally achieved with additions of a minimum of
13% (by weight) chromium, and up to 26% is used for harsh environments.[2] The chromium forms a
passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible,
meaning the metal remains lustrous. It is, however, impervious to water and air, protecting the metal beneath.
Also, this layer quickly reforms when the surface is scratched. This phenomenon is called passivation and is
seen in other metals, such as aluminium and titanium. When stainless steel parts such as nuts and bolts are
forced together, the oxide layer can be scraped off causing the parts to weld together. When disassembled,
the welded material may be torn and pitted, an effect that is known as galling.

Nickel also contributes to passivation, as do other less commonly used ingredients such as molybdenum
and vanadium.

Commercial value of stainless steel

Stainless steel's resistance to corrosion and staining, low maintenance, relative inexpense, and familiar
luster make it an ideal base material for a host of commercial applications. There are over 150 grades of
stainless steel, of which fifteen are most common. The alloy is milled into sheets, plates, bars, wire, and
tubing to be used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial
equipment, a structural alloy in automotive and aerospace assembly and building material in skyscrapers
and other large buildings. The one most noted automotive with stainless steel is the Delorean DMC-12,
which was also featured in the hit film, Back To The Future.

Stainless steel is 100% recyclable. In fact, an average stainless steel object is composed of about 60%
recycled material, 25% originating from end-of-life products and 35% coming from manufacturing
processes.[4]




Corrosion
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Even a high-quality alloy can corrode under certain conditions. Because these modes of corrosion are more
exotic and their immediate results are less visible than rust, they often escape notice and cause problems
among those who are not familiar with them.




Pitting corrosion

Passivation relies upon the tough layer of oxide described above. When deprived of oxygen (or when a salt
such as chloride competes as an ion), stainless steel lacks the ability to re-form a passivating film. In the
worst case, almost all of the surface will be protected, but tiny local fluctuations will degrade the oxide film in
a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of
several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme
circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is
naturally deprived of oxygen. In extreme cases, the sharp tips of extremely long and narrow pits can cause
stress concentration to the point that otherwise tough alloys can shatter, or a thin film pierced by an invisibly
small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are
difficult to detect before a part or structure fails. Pitting remains among the most common and damaging
forms of corrosion in stainless alloys, but it can be prevented by ensuring that the material is exposed to
oxygen (for example, by eliminating crevices) and protected from chlorides wherever possible.

Pitting corrosion can occur when stainless steel is subjected to high concentration of Chloride ions (for
example, sea water) and moderately high temperatures. A textbook example for this was a replica of the Jet
d'Eau fountain in Geneva, ordered by an Arab Sheikh for installation in the Red Sea. The replica did not last
long, because the engineers responsible failed to take into account the difference between the freshwater of
Lake Geneva and the saltwater of the sea.

Rouging

Rouging is a very peculiar phenomenon, which occurs only on polished stainless steel surfaces with very low
surface roughness in a pure water environment. This effect is mostly common in pharmaceutical industries.
It is caused by the simple fact that pure water is lacking any ions and pulls the metal ions of the passive
stainless steel surface into solution. Iron ions do not dissolve at neutral pH and will precipitate as an iron
hydroxide film, which has a reddish colour, hence the name rouging.
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Intergranular corrosion

Some compositions of stainless steel are prone to intergranular corrosion when exposed to certain
environments. When heated to around 700 °C, chromium carbide forms at the intergranular boundaries,
depleting the grain edges of chromium, impairing their corrosion resistance. Steel in such condition is called
sensitized. Steels with carbon content 0.06% undergo sensitization in about 2 minutes, while steels with
carbon content under 0.02% are not sensitive to it.




Intergranular corrosion

A special case of intergranular corrosion is called 'weld decay' or 'knifeline attack'(KLA). Due to the elevated
temperatures of welding the stainless steel can be sensitized very locally along the weld. The chromium
depletion creates a galvanic couple with the well-protected alloy nearby in highly corrosive environments. As
the name 'knifeline attack' implies, this is limited to a small zone, often only a few micrometres across, which
causes it to proceed more rapidly. This zone is very near the weld, making it even less noticeable[5].

It is possible to reclaim sensitized steel by heating it to above 1000 °C and holding at this temperature for a
given period of time dependent on the mass of the piece, followed by quenching it in water. This process
dissolves the carbide particles, then keeps them in solution.

It is also possible to stabilize the steel to avoid this effect and make it welding-friendly. Addition of titanium,
niobium and/or tantalum serves this purpose; titanium carbide, niobium carbide and tantalum carbide form
preferentially to chromium carbide, protecting the grains from chromium depletion. Use of extra-low carbon
steels is another method and modern steel production usually ensures a carbon content of <0.03% at which
level intergranular corrosion is not a problem. Light-gauge steel also does not tend to display this behavior,
as the cooling after welding is too fast to cause effective carbide formation.

Crevice corrosion

In the presence of reducing acids or exposure to reducing atmosphere, the passivation layer protecting steel
from corrosion can break down. This wear can also depend on the mechanical construction of the parts, eg.
under gaskets, in sharp corners, or in incomplete welds. Such crevices may promote corrosion, if their size
allows penetration of the corroding agent but not its free movement. The mechanism of crevice corrosion is
similar to pitting corrosion, though it happens at lower temperatures.

Stress corrosion cracking
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Stress corrosion cracking can be a severe form of stainless steel corrosion. It forms when the material is
subjected to tensile stress and some corrosive environments, especially chloride-rich environments (sea
water) at higher temperatures. The stresses can be a result of service loads, or can be caused by the type of
assembly or residual stresses from fabrication (eg. cold working); residual stresses can be relieved by
annealing. This limits the usefulness of stainless steels of the 300 series (304, 316) for containing water with
higher than few ppm content of chlorides at temperatures above 50 °C. In more aggressive conditions,
higher alloyed austenitic stainless steels (6% Mo grades) or Mo containing duplex stainless steels may be
selected.

Stress corrosion cracking depends on the nickel content. High nickel content austenitic (non-magnetic)
steels, which are the most resistant to other forms of corrosion, tend to be the most susceptible to stress
corrosion.

Chlorine catalyzes the formation of hydrogen which hardens and embrittles the metal locally, causing
concentration of the stress and a microscopic crack. The chlorine moves into the crack, continuing the
process.

Sulphide stress cracking

Sulphide stress cracking is an important failure mode in the oil industry, where the steel comes into contact
with liquids or gases with considerable hydrogen sulfide content, e.g., sour gas. It is influenced by the tensile
stress and is worsened in the presence of chloride ions. Very high levels of hydrogen sulfide apparently
inhibit the corrosion. Rising temperature increases the influence of chloride ions, but decreases the effect of
sulfide, due to its increased mobility through the lattice; the most critical temperature range for sulphide
stress cracking is between 60-100 °C.

 Galvanic corrosion

Galvanic corrosion occurs when a galvanic cell is formed between two dissimilar metals. The resulting
electrochemical potential then leads to formation of an electric current that leads to electrolytic dissolving of
the less noble material. This effect can be prevented by electrical insulation of the materials, e.g. by using
rubber or plastic sleeves or washers, keeping the parts dry so there is no electrolyte to form the cell, or
keeping the size of the less-noble material significantly larger than the more noble ones (e.g. stainless-steel
bolts in an aluminum block won't cause corrosion, but aluminum rivets on stainless steel sheet would rapidly
corrode.)

If these options are not available to protect from galvanic corrosion, a sacrificial anode can be used to protect
the less noble metal. For example, if a system is composed of 316 SS, a very noble alloy with a low galvanic
potential, and a mild steel, a very active metal with high galvanic potential, the mild steel will corrode in the
presence of an electrolyte such as salt water. If a sacrificial anode is used such as a Mil-Spec A-18001K zinc
alloy, Mil-Spec A-24779(SH) aluminum alloy, or magnesium, these anodes will corrode instead, protecting
the other metals in the system. The anode must be electrically connected to the protected metal(s) in order
to be able to preserve them. This is common practice in the marine industry to protect ship equipment. Boats
and vessels that are in salt water use either zinc alloy or aluminum alloy. If the boats are only in fresh water,
a magnesium alloy is used. Magnesium has one of the highest galvanic potential of any metal. If it is used in
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a saltwater application on a steel or aluminum hull boat, hydrogen bubbles will form under the paint, causing
blistering and peeling.

Contact corrosion

Contact corrosion is a combination of galvanic corrosion and crevice corrosion, occurring where small
particles of suitable foreign material are embedded to the stainless steel. Carbon steel is a very common
contaminant here, coming from nearby grinding of carbon steel or use of tools contaminated with carbon
steel particles. The particle forms a galvanic cell, and quickly corrodes away, but may leave a pit in the
stainless steel from which pitting corrosion may rapidly progress. Some workshops therefore have separate
areas and separate sets of tools for handling carbon steel and stainless steel, and care has to be exercised
to prevent direct contact between stainless steel parts and carbon steel storage racks.

Particles of carbon steel can be removed from a contaminated part by passivation with dilute nitric acid, or by
pickling with a mixture of hydrofluoric acid and nitric acid.

Types of stainless steel

There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron
is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For
higher hardness and strength, carbon is added. When subjected to adequate heat treatment these steels are
used as razor blades, cutlery, tools etc.

Significant quantities of manganese have been used in many stainless steel compositions. Manganese
preserves an austenitic structure in the steel as does nickel, but at a lower cost.

Stainless steels are also classified by their crystalline structure:

    •   Austenitic, or 300 series, stainless steels comprise over 70% of total stainless steel production. They
        contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or
        manganese to retain an austenitic structure at all temperatures from the cryogenic region to the
        melting point of the alloy. A typical composition of 18% chromium and 10% nickel, commonly known
        as 18/10 stainless is often used in flatware. Similarly 18/0 and 18/8 is also available.
        “Superaustenitic” stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to
        chloride pitting and crevice corrosion due to high Molybdenum contents (>6%) and nitrogen
        additions and the higher nickel content ensures better resistance to stress-corrosion cracking over
        the 300 series. The higher alloy content of "Superaustenitic" steels means they are fearsomely
        expensive and similar performance can usually be achieved using duplex steels at much lower cost.

    •   Ferritic stainless steels are highly corrosion resistant, but less durable than austenitic grades. They
        contain between 10.5% and 27% chromium and very little nickel, if any. Most compositions include
        molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo,
        29Cr-4Mo, and 29Cr-4Mo-2Ni.
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•   Martensitic stainless steels are not as corrosion resistant as the other two classes, but are extremely
    strong and tough as well as highly machineable, and can be hardened by heat treatment. Martensitic
    stainless steel contains chromium (12-14%), molybdenum (0.2-1%), zero to less than 2% nickel, and
    about 0.1-1% carbon (giving it more hardness but making the material a bit more brittle). It is
    quenched and magnetic. It is also known as "series-00" steel.

•   Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to
    austenitic varieties, but can be precipitation hardened to even higher strengths than the other
    martensitic grades. The most common, 17-4PH, uses about 17% chromium and 4% nickel. There is
    a rising trend in defence budgets to opt for an ultra-high-strength stainless steel if possible in new
    projects as it is estimated that 2% of the US GDP is spent dealing with corrosion. The
    Lockheed-Martin JSF is the first aircraft to use a precipitation hardenable stainless steel - Carpenter
    Custom 465 - in its airframe.

•   Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to
    produce a 50:50 mix although in commercial alloys the mix may be 40:60 respectively. Duplex steel
    have improved strength over austenitic stainless steels and also improved resistance to localised
    corrosion particularly pitting, crevice corrosion and stress corrosion cracking. They are characterised
    by high chromium (19-28%) and molybdenum (up to 5%) and lower nickel contents than austenitic
    stainless steels.
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Comparison of standardized steels


  EN-standard           EN-standard                ASTM/AISI
                                                                          UNS
  Steel no. DIN          Steel name                Steel type

                                           440A                  S44002

1.4112                                     440B                  S44004

1.4125                                     440C                  S44003

                                           440F                  S44020

1.4016            X6Cr17                   430

1.4512            X6CrTi12                 409

1.4310            X10CrNi18-8              301

1.4318            X2CrNiN18-7              301LN

1.4307            X2CrNi18-9               304L                  S30403

1.4306            X2CrNi19-11              304L                  S30403

1.4311            X2CrNiN18-10             304LN                 S30453

1.4301            X5CrNi18-10              304                   S30400

1.4948            X6CrNi18-11              304H                  S30409

1.4303            X5CrNi18 12              305

1.4541            X6CrNiTi18-10            321                   S32100

1.4878            X12CrNiTi18-9            321H                  S32109

1.4404            X2CrNiMo17-12-2          316L                  S31603

1.4401            X5CrNiMo17-12-2          316                   S31600

1.4406            X2CrNiMoN17-12-2         316LN                 S31653

1.4432            X2CrNiMo17-12-3          316L                  S31603

1.4435            X2CrNiMo18-14-3          316L                  S31603

1.4436            X3CrNiMo17-13-3          316                   S31600

1.4571            X6CrNiMoTi17-12-2        316Ti                 S31635

1.4429            X2CrNiMoN17-13-3         316LN                 S31653

1.4438            X2CrNiMo18-15-4          317L                  S31703

1.4539            X1NiCrMoCu25-20-5        904L                  N08904

1.4547            X1CrNiMoCuN20-18-7                             S31254


Stainless steel Grades [list is not exhaustive]

    •    200 Series—austenitic chromium-nickel-manganese alloys
    •    300 Series—austenitic chromium-nickel alloys
             o    Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical
                  working. Good weldability. Better wear resistance and fatigue strength than 304.
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           o   Type 302—same corrosion resistance as 304, with slightly higher strength due to additional
               carbon.
           o   Type 303—easier machining version of 304 via addition of sulfur and phosphorus. Also
               referred to as "A1" in accordance with International Organization for Standardization ISO
               3506[6].
           o   Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as
               "A2" in accordance with International Organization for Standardization ISO 3506[7].
           o   Type 309— better temperature resistance than 304
           o   Type 316—the second most common grade (after 304); for food and surgical stainless steel
               uses; Alloy addition of molybdenum prevents specific forms of corrosion. Also known as
               "marine grade" stainless steel due to its increased resistance to chloride corrosion
               compared to type 304. SS316 is often used for building nuclear reprocessing plants. Most
               watches that are made of stainless steel are made of this grade. Rolex is an exception in
               that they use Type 904L. 18/10 stainless often corresponds to this grade.[1] Also referred to
               as "A4" in accordance with International Organization for Standardization ISO 3506[8].
           o   Type 321— similar to 304 but lower risk of weld decay due to addition of titanium. See also
               347 with addition of niobium for desensitization during welding.
   •   400 Series—ferritic and martensitic chromium alloys
           o   Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
           o   Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
           o   Type 410—martensitic (high-strength iron/chromium). Wear resistant, but less corrosion
               resistant.
           o   Type 416— easy to machine due to additional sulfur
           o   Type 420—"Cutlery Grade" martensitic; similar to the Brearley's original "rustless steel".
               Also known as "surgical steel". Excellent polishability.
           o   Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced
               temperature and corrosion resistance.
           o   Type 440—a higher grade of cutlery steel, with more carbon in it, which allows for much
               better edge retention when the steel is heat treated properly. It can be hardened to Rockwell
               58 hardness, making it one of the hardest stainless steels. Also known as "razor blade steel".
               Available in three grades 440A, 440B, 440C (more common) and 440F (free machinable).
   •   500 Series—heat resisting chromium alloys
   •   600 Series—martensitic precipitation hardening alloys
           o   Type 630—most common PH stainless, better known as 17-4; 17% chromium, 4% nickel




Stainless steel finishes
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316L stainless steel, with an unpolished, mill finish.

Standard mill finishes can be applied to flat rolled stainless steel directly by the rollers and by mechanical
abrasives. Steel is first rolled to size and thickness and then annealed to change the properties of the final
material. Any oxidation that forms on the surface (scale) is removed by pickling, and the passivation layer is
created on the surface. A final finish can then be applied to achieve the desired aesthetic appearance.

    •     No. 0 - Hot Rolled Annealed, thicker plates
    •     No. 1 - Hot rolled, annealed and passivated
    •     No, 2D - cold rolled, annealed, pickled and passivated
    •     No, 2B - same as above with additional pass through polished rollers
    •     No, 2BA - Bright Anealed (BA) same as above with highly polished rollers
    •     No. 3 - coarse abrasive finish applied mechanically
    •     No. 4 - brushed finish
    •     No. 6 - matte finish
    •     No. 7 - reflective finish
    •     No. 8 - mirror finish




History

A few corrosion-resistant iron artifacts survive from antiquity. A famous (and very large) example is the Iron
Pillar of Delhi, erected by order of Kumara Gupta I around the year AD 400. However, unlike stainless steel,
these artifacts owe their durability not to chromium, but to their high phosphorus content, which together with
favorable local weather conditions promotes the formation of a solid protective passivation layer of iron
oxides and phosphates, rather than the non-protective, cracked rust layer that develops on most ironwork.

The corrosion resistance of iron-chromium alloys was first recognized in 1821 by the French metallurgist
Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery.
However, the metallurgists of the 19th century were unable to produce the combination of low carbon and
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high chromium found in most modern stainless steels, and the high-chromium alloys they could produce
were too brittle to be of practical interest.

This situation changed in the late 1890s, when Hans Goldschmidt of Germany developed an aluminothermic
(thermite) process for producing carbon-free chromium. In the years 1904–1911, several researchers,
particularly Leon Guillet of France, prepared alloys that would today be considered stainless steel.

In Germany, Friedrich Krupp Germaniawerft built the 366-ton sailing-yacht "Germania" featuring a
chrome-nickel steel hull in 1908. [2] In 1911, Philip Monnartz reported on the relationship between the
chromium content and corrosion resistance. On October 17, 1912 Krupp engineers Benno Strauss and
Eduard Maurer patented austenitic stainless steel. [3]

Similar industrial developments were taking place contemporaneously in the United States, where Christian
Dantsizen and Frederick Becket were industrializing ferritic stainless.

However Harry Brearley of the Brown-Firth research laboratory in Sheffield, England is most commonly
credited as the "inventor" of stainless steel, but many historians feel this is disputable. In 1913, while seeking
an erosion-resistant alloy for gun barrels, he discovered and subsequently industrialized a martensitic
stainless steel alloy.




Use in sculpture, building facades and building structures

    •    Stainless steel was particularly in vogue during the art deco period. The most famous example of
         this is the upper portion of the Chrysler Building (illustrated above). Diners and fast food restaurants
         feature large ornamental panels, stainless fixtures and furniture. Owing to the durability of the
         material, many of these buildings still retain their original and spectacular appearance.
    •    In recent years the forging of stainless steel has given rise to a fresh approach to architectural
         blacksmithing. The work of Giusseppe Lund illustrates this well. [4]
    •    Also pictured above, the Gateway Arch is clad entirely in stainless steel: 886 Tons (804 metric
         tonnes) of 1/4" (6.3 mm) plate, #3 Finish, Type 304. [5]
    •    Type 316 stainless is used on the exterior of both the Petronas Twin Towers and the Jin Mao
         Building, two of the world's tallest skyscrapers. [6]
    •    Stainless Steel is the fourth common material used in metal wall tiles, and is used for its corrosion
         resistance properties in kitchens and bathrooms. [7]
    •    Edmonton, Alberta, Canada uses North America's largest stainless steel building for its composter
         facility. The building is the size of 14 NHL hockey rinks.
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Postweld heat treatment to avoid intergranular stress corrosion cracking of
supermartensitic stainless steels

  Abstract


 Supermartensitic stainless steels (SMSS) are attractive materials for flowlines transporting produced fluids

 with high levels of CO   2   and low levels of H   2   S. However, recent cracking of lean grade material in service

 and both lean and high-alloy grades during qualification testing have revealed sensitivity to intergranular

 stress corrosion cracking (IGSCC) at some girth welds although all flowlines in high alloy SMSS have

 apparently had no such problems in service. One potential solution is to use a brief postweld heat treatment,

 typically at around 630-650°C for five minutes, which has been shown to overcome susceptibility to IGSCC in

 laboratory tests. The paper considers existing information on the effects of brief PWHT on welded SMSS,

 presents additional data for a range of pipes and weld types and discusses the likely mechanism by which

 PWHT may be effective in preventing IGSCC. It is concluded that a microstructural effect is probably

 dominant. Based on this preliminary conclusion and a consideration of the potential detrimental effects of an

 inappropriate PWHT cycle, the necessary control of the PWHT process is addressed and recommendations

 are made with respect to application of PWHT, highlighting best practice based on current knowledge.


1. Introduction


 Intergranular SCC of SMSS pipe girth welds represents an obstacle to the exploitation of these materials in

 flowlines for some applications, although they are still being used extensively by some operators. It is

 recognised that a reliable way to prevent susceptibility to IGSCC of as-welded SMSS via control of welding

 parameters and without PWHT may still take some time to develop, assuming that it is possible. However,

 experimental evidence to date suggests that the use of brief PWHT at around 650°C will eliminate sensitivity

 to IGSCC. No examples of IGSCC have been reported after PWHT at around 650°C for 5 minutes. Such

 PWHT is therefore an attractive interim solution to the problem, albeit one that will add to the cost of

 producing welded fabrications. Nevertheless some flowlines are operating successfully without PWHT,

 although the ability to do this will probably depend on the operating environment. It is noted that welded

 SMSS is also susceptible to cracking in sour environments but this is by a different mechanism to the IGSCC

 discussed here and PWHT does not prevent cracking in sour environments, although it may improve

 resistance.
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 Several authors have previously examined the effects of PWHT on the properties of SMSS welds, although

 only more recently has its effect on sensitivity to IGSCC been explored. The range of PWHT treatments

 studied on actual welds is from 600-700°C for 3-5 minutes, although simulated HAZ studies have suggested

 that a wider range of thermal cycles, from 550-700°C for 1 to 17 minutes may also be effective at eliminating

 sensitivity to IGSCC. However, it should be noted that the heat treatment required to eliminate sensitivity to

 IGSCC will presumably depend its severity and the composition of the steel, notably C and perhaps N content,

 and the levels of other carbide/nitride forming elements such as Ti, Nb and Mo.

 When specifying a PWHT cycle in practice, it is essential that it should not only provide acceptable IGSCC

 properties but other mechanical and corrosion properties must also be acceptable after PWHT. Therefore this

 study also examines effects of PWHT on microstructure, hardness and toughness.


2. Experimental work


2.1 Materials


 Six low carbon martensitic stainless steel pipes with 10.9-13.5%Cr were selected, all of which could broadly

 be considered as 'supermartensitic' but not all representing currently commercially available grades, Table 1.

 Two of the steels were variants of the same grade, with very similar composition (C1 and C2). Nickel content

 varied from 1.55-6.4% and Mo ranged from 0-2.5% for the steels examined.
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 Table 1

 Chemical analyses of the materials used


                                                            Element, wt%


                   Pipe code
                                    C       N        Si    Mn     Cr     Ni    Mo      Cu     Ti


              A (12Cr5Ni2Mo)        0.013   0.009   0.12   0.54   11.8   5.1   2.03    0.04   <0.005


              B (12Cr6Ni2Mo)        0.010   0.011   0.17   0.18   12.4   5.8   2.18    0.03   0.020


              C1 (12Cr6Ni2.5MoTi)   0.009   0.005   0.20   0.43   12.2   6.4   2.51    0.03   0.12


              C2 (12Cr6Ni2.5MoTi)   0.010   0.007   0.26   0.46   12.2   6.5   2.48    0.03   0.09


              D (13Cr5Ni1Mo)        0.013   0.006   0.16   0.65   13.5   5.1   0.78    0.03   0.088


              E (11Cr1.5Ni0.5Cu)    0.010   0.006   0.18   1.14   10.9   1.6   <0.01   0.49   0.01


              NA = not analysed




2.2 Welding


 Three types of girth weld were examined, (i) three automatic pulsed MIG welds made with superduplex solid

 filler wire throughout (W1-W3), (ii) two automatic pulsed MIG welds made with approximately matching

 composition metal cored filler wires (W4 and W5) and (iii) two manual welds made using the TIG process for

 the root and second pass and the MMA process for the fill and cap passes, using superduplex consumables

 (W6 and W7).

 Table 2 lists the welding consumable analyses, which were either direct analyses of the solid wires or were

 from all-weld metal pads deposited using the coated electrodes and the metal cored wires. Analyses were

 obtained by OES and inert gas fusion for O and N.
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Table 2 Welding consumable analyses

                                                                 Element, wt%


        Consumable code      Dia (mm)
                                         C        Si     Mn     Cr     Mo     Ni    Cu      W       N


        C1 (SMSS, MCW)       1.2         0.009    0.67   1.22   11.9   1.49   6.6   0.48    NA      0.009


        C2 (SMSS, MCW)       1.2         0.008    0.39   1.77   12.1   2.51   6.4   0.58    NA      0.009


        C3 (SDSS, SW)        1.2         0.027    0.40   0.41   26.1   3.90   9.3   0.12    <0.05   0.23


        C4 (SDSS, SW)        1.2         0.015    0.30   0.40   25.0   4.00   9.5   NA      NA      0.24


        C5 (SDSS, SW)        2.4         0.018    0.39   0.69   24.8   3.80   9.3   0.60    0.61    0.22


        C6 (SDSS, CE)        2.5         0.030    0.32   0.90   24.9   3.65   9.4   0.79    0.68    0.24


        C7 (SDSS, CE)        3.2         0.030    0.34   0.90   25.4   3.61   9.0   0.75    0.67    0.21


        SMSS = supermartensitic stainless steel

        SDSS = superduplex stainless steel

        MCW = metal cored wire

        SW = solid wire

        CE = coated electrode

        NA - not analysed


Table 3 summarises the welding matrix. All welding was in the 5G position (pipe horizontal, fixed). For the

automatic pulsed MIG welding, a copper backing shoe and Ar backing gas were used and interpass

temperature was restricted to <150°C, whilst pre-heat was just sufficient to remove moisture. Travel speed

was in the range 350-500mm/min and heat input approximately 0.5kJ/mm. An Ar/He/CO 2 /N 2 shielding gas

mixture was used for welding with the superduplex wires and Ar+0.5%CO               2    was used for the matching

composition wires. A narrow gap J preparation was used. For the manual welding, interpass temperature was

again <150°C, the heat input for the root and second pass was 1.1-1.5kJ/mm and for the fill and cap passes it

was 0.5-1.4kJ/mm. Argon shielding gas was used for TIG welding and Ar back purge gas was used

throughout. A 30° bevel was used with no root face and a 4mm root gap.
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 Table 3 Girth welding matrix

                                Welding process       Welding consumable


Weld                                                                              Shielding
                                        Fill and
code     Pipe code                                                                gas             PWHT
                            Root        cap        Root            Fill and cap


W1       A (12Cr5Ni2Mo)     Pulsed      Pulsed     25%Cr wire C3                  Ar/He/CO 2 /N   None

                            MIG         MIG                                       2




W1P      A (12Cr5Ni2Mo)     Pulsed      Pulsed     25%Cr wire C3                  Ar/He/CO 2 /N   650°C/5min*

                            MIG         MIG                                       2




W2       B (12Cr6Ni2Mo)     Pulsed      Pulsed     25%Cr wire C4                  Ar/He/CO 2 /N   None

                            MIG         MIG                                       2




W2P      B (12Cr6Ni2Mo)     Pulsed      Pulsed     25%Cr wire C4                  Ar/He/CO 2 /N   650°C/5min*

                            MIG         MIG                                       2




W3       C1                 Pulsed      Pulsed     25%Cr wire C3                  Ar/He/CO 2 /N   None

         (12Cr6Ni2.5MoTi)   MIG         MIG                                       2




W3P      C1                 Pulsed      Pulsed     25%Cr wire C3                  Ar/He/CO 2 /N   650°C/5min*

         (12Cr6Ni2.5MoTi)   MIG         MIG                                       2




W4       D (13Cr5Ni1Mo)     Pulsed      Pulsed     1.5%Mo SMSS wire (C1)          Ar+0.5%CO 2     None

                            MIG         MIG


W4P      D (13Cr5Ni1Mo)     Pulsed      Pulsed     1.5%Mo SMSS wire (C1)          Ar+0.5%CO 2     650°C/5min*

                            MIG         MIG


W5       B (12Cr6Ni2Mo)     Pulsed      Pulsed     2.5%Mo SMSS wire (C2)          Ar+0.5%CO 2     None

                            MIG         MIG


W5P      B (12Cr6Ni2Mo)     Pulsed      Pulsed     2.5%Mo SMSS wire (C2)          Ar+0.5%CO 2     650°C/5min*

                            MIG         MIG
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W6          E (11Cr1.5Ni0.5Cu)     Manual       MMA           SDSS wire   SDSS CE   Ar              None

                                   TIG                        (C5)        (C6,C7)


W6P         E (11Cr1.5Ni0.5Cu)     Manual       MMA           SDSS wire   SDSS CE   Ar              650°C/5min

                                   TIG                        (C5)        (C6,C7)                   **


W7          C2                     Manual       MMA           SDSS wire   SDSS CE   Ar              None

            (12Cr6Ni2.5MoTi)       TIG                        (C5)        (C6,C7)


W7P         C2                     Manual       MMA           SDSS wire   SDSS CE   Ar              650°C/5min

            (12Cr6Ni2.5MoTi)       TIG                        (C5)        (C6,C7)                   **


* induction heat treatment of whole pipe girth weld

** furnace heat treatment of piece cut from pipe girth weld




2.3 PWHT


 Examples of each weld type were subjected to brief PWHT. For the pulsed MIG welds W1-W5, the PWHT

 was applied on the whole weld by induction heating, whilst pieces from welds W6 and W7 were heat treated

 in a furnace. The specified heat treatment cycle was rapid heating to 650°C, followed by holding for 5 minutes

 and air cooling. A volume of metal around 40mm wide, including the weld metal and approximately 15-20mm

 of pipe either side of the root and 10-15mm either side of the weld cap was heated by the induction coil.

 Temperature was controlled via thermocouples on the weld metal cap and measurements were made also on

 the root. Heating was fairly rapid to 600°C and then temperature rose to 650°C over about two minutes.

 During the five minute hold period, the cap temperature remained between 640 and 657°C. The root

 temperature was typically 15-35°C less than the cap temperature, i.e. 620-640°C, depending on the pipe wall

 thickness. For furnace heat treatment, heating was fairly slow taking about 10 minutes to reach 650°C.

 Temperature was again monitored by thermocouples, this time on the root weld metal. The welds were given

 a 'P' designation after PWHT.
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2.4 Weld characterisation


 Sections were taken through the welds for microstructural examination and Vickers hardness measurement

 (HV10) in the weld metal and HAZ both before and after PWHT.


2.5 Toughness testing


 Charpy V-notch impact tests were performed on through-thickness notched specimens from weld W7

 (12Cr6Ni2.5MoTi pipe welded with superduplex consumables) before and after PWHT, with the notch on the

 weld metal centreline or at the fusionline mid-thickness position. Tests were performed over the temperature

 range -80 to +40°C.

 In addition, fracture toughness tests were performed to BS7448 part 1 at -20°C on Bx2B (11.5x23mm)

 specimens from weld W7 both before and after PWHT, both given 1% local compression to reduce the effects

 of residual stress. Specimens were through-thickness notched on the weld metal centreline or on the fusion

 line mid-thickness position. A loading rate of 0.4mm/min was employed.


2.6 Corrosion testing


 Four point bend tests were performed in two environments (i.e. 25%NaCl solutions acidified to calculated pH

 = 3.3 and 4.5 respectively, Table 4) on 100x15x3mm specimens from each of the girth weld types W1-W6, in

 both as-welded and PWHT conditions. Two specimen types were examined (i) with the root machined flush

 and ground to a 600 grit finish and (ii) with the root in the as-welded condition. Both specimen types had

 strain gauges applied (i) on the test face for flush ground specimens and (ii) on the non-test face for

 specimens with the profile intact. Specimens were deflected to give a strain equivalent to 100% of the parent

 material 0.2% proof stress in the HAZ. After deflection the specimens were placed in a nitrogen-blanketed

 autoclave filled with deoxygenated test solution. The vessel was then heated to test temperature and finally

 pressurised with the test gas. Test exposure was for 30 days. After test, specimens were examined visually

 under a binocular microscope, photographed and, if cracking was not observed, they were sectioned

 transverse to the weld at the mid-width position, to look for small cracks.
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 Table 4 Corrosion test environments

                 Total pressure   ppH 2 S   ppCO 2    NaCl   NaHCO 3   Temp

        Code     (bar)            (bar)     (bar)     (%)    (ppm)     (°C)   Calculated pH


        Env. A   21.5             0         10        25     0         110    3.3


        Env. B   21.5             0         10        25     500       120    4.5



3. Results


3.1 Weld characterisation


 The HAZs were generally visibly tempered by PWHT, i.e. showed slightly greater etching response,

 particularly for the lean grades D and E, but no other microstructural changes were observed optically. In

 some areas, precipitation on HAZ prior austenite boundaries in steel C1 was visible at high magnification

 after PWHT. Under a light microscope, this leads to a clear definition of the HAZ prior austenite grain

 boundaries in the high temperature HAZ within about 150µm of the fusionline, Fig.1. The superduplex weld

 metals showed evidence of precipitation of very fine secondary austenite after PWHT but no intermetallic

 phases were observed, Fig.2.

 Fig.1. HAZ of weld W3P (pipe C1,                   Fig.2. Superduplex weld metal of weld W7P after PWHT.

 12Cr6Ni2.5MoTi) after PWHT                         The secondary austenite appears as very fine particles

                                                    between the larger primary austenite units
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In general, the weld root/mid thickness gave higher maximum HAZ hardness than the weld cap, reflecting

effects of reheating/straining, Table 5. Maximum HAZ values, as-welded, were in the range 332-351 HV5,

with the highest hardness always being about 2mm from the fusion line. After brief induction PWHT at

620-660°C for 5 minutes, peak HAZ hardness was typically reduced at the root position, by up to 54 HV5 but

more typically by 10-15 HV5 for the higher alloy grades. The weld cap HAZs showed a mixed response with

hardening observed in some cases (up to +12 HV5) and softening (up to -26 HV5) in others.

Table 5 Maximum HAZ hardness change after induction PWHT at 650°C for five minutes

                                                 Maximum HAZ hardness

                                                 (HV5)                     Change in max hardness

   Weld code    Pipe code             Position                             (HV5)
                                                 As-welded    After PWHT


   W1/W1P       A (12Cr5Ni2Mo)        Cap HAZ    330          330          0


                                      Root HAZ   345          345          0


   W2/W2P       B (12Cr6Ni2Mo)        Cap HAZ    303          315          +12


                                      Root HAZ   332          319          -13


   W3/W3P       C1 (12Cr6Ni2.5MoTi)   Cap HAZ    315          304          -11


                                      Root HAZ   327          313          -14


   W4/W4P       D (13Cr5Ni1Mo)        Cap HAZ    345          330          -15


                                      Root HAZ   351          327          -24


   W5/W5P       B (12Cr6Ni2Mo)        Cap HAZ    341          315          -26


                                      Root HAZ   347          312          -35


   W6/W6P       E (11Cr1.5Ni0.5Cu)    Root HAZ   306          254          -54


                                      Root WM    332          319          -13
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    W7/W7P       C2 (12Cr6.5Ni2.5MoTi)     Root HAZ    355             315        -40


                                           Root WM     308             301        -7



3.2 Toughness testing


 Table 6 and Fig.3 present the results of the fracture toughness and impact tests respectively. Neither showed

 a substantial reduction of properties after PWHT, although it is noted that the lowest impact values at -50°C

 for both weld metal centreline and fusionline notch positions were after PWHT. Impact toughness for the

 fusionline was 65-82J over the range -50 to 0°C, whilst CTOD at maximum load was 0.15-0.29mm at 20°C.

 The corresponding figures for the weld metal centreline were 25-70J and 0.15-0.23mm.

 Table 6 Fracture mechanics test results for superduplex stainless steel weld metal (all from weld W7,

 tested at -20°C)




                                                              Measured CTOD*
                Samples       Condition     Notch position
                                                              (   m), mm


                W7-01 to 03   As-welded     WMCL              0.21, 0.15, 0.22,

                                                              (0.19)


                W7-04 to 06   PWHT          WMCL              0.12, 0.20, 0.23

                                                              (0.18)


                W7-07 to 09   As-welded     FLMT              0.15, 0.20, 0.19

                                                              (0.18)


                W7-10 to 12   PWHT          FLMT              0.22, 0.29, 0.21

                                                              (0.24)


                * Presented as individual values with average in parenthesis

                WMCL = weld metal centreline

                FLMT = fusionline mid-thickness
                                                                                          BACK      INDEX




               PWHT = 650°C for 5 minutes




                                                         Fig.3. Effect of PWHT at nominally 650°C for 5

                                                         minutes on impact toughness of HAZ and

                                                         superduplex weld metal in weld W7/W7P (pipe

                                                         C2, 12Cr6Ni2.5MoTi)




3.3 Corrosion testing


 Table 7 lists the results of the SCC tests. None of the specimens with the root machined flush showed any

 evidence of cracking in the environment A (calculated pH=3.3, 110°C) and no tests on such specimens were

 performed in environment B. When specimens were tested with the root surface intact, most of the

 specimens showed intergranular cracking in the HAZ, at a variety of locations in the HAZ ranging from

 immediately adjacent to the fusionline (e.g. W3, steel C1, 12Cr6Ni2.5MoTi, superduplex wire) to about

 0.5mm from the fusionline (W1, Steel A, 12Cr5Ni2Mo). No such cracking was found in any weld after PWHT.

 There was also some variation in crack depth between specimens. In particular, W2 (pipe B, 12Cr6Ni2Mo,

 welded with superduplex wire) showed very shallow cracking (25-30µm). Weld W5 (also pipe B, welded with

 SMSS wire) showed no cracking on the section examined. The other specimens cracked through most of the

 thickness. A second environment was examined (environment B), with pH raised to a calculated value of 4.5

 by an addition of 500mg/l NaHCO 3 . Again similar trends were observed, i.e. as-welded specimens tended to

 crack and PWHT specimens did not. Crack location was similar to that in the environment A. Weld W2 (pipe

 B, 12Cr6Ni2Mo welded with superduplex wire) and W4 (pipe D, 13Cr5Ni1Mo, welded with matching

 composition wire) showed no cracks and only shallow cracks were found in W5 (pipe B, 12Cr6Ni2Mo welded

 with approximately matching composition wire).
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 Table 7 Results of stress corrosion cracking tests.

                                            Max

                                            root               Machined            Root intact:   Root intact:

Weld                      Root              HAZ                surface: Env. A     Env. A (pH =   Env. B (pH =

code   Parent pipe        consumable        HV10       PWHT    (pH = 3.3, 110°C)   3.3, 110°C)    4.5, 120°C)


W1     A (12Cr5Ni2Mo)     25Cr              345        No      No cracks           Cracks         Cracks


W1P    A (12Cr5Ni2Mo)     25Cr              345        Yes     No cracks           No cracks      No cracks


W2     B (12Cr6Ni2Mo)     25Cr              332        No      No cracks           Small crack    No cracks


W2P    B (12Cr6Ni2Mo)     25Cr              319        Yes     No cracks           No cracks      No cracks


W3     C1                 25Cr              327        No      No cracks           Cracks         Cracks

       (12Cr6Ni2.5MoTi)


W3P    C1                 25Cr              313        Yes     No cracks           No cracks      No cracks

       (12Cr6Ni2.5MoTi)


W4     D (13Cr5Ni1Mo)     12Cr6.5Ni1.5Mo    351        No      No cracks           Cracks         No cracks


W4P    D (13Cr5Ni1Mo)     12Cr6.5Ni1.5Mo    327        Yes     No cracks           No cracks      No cracks


W5     B (12Cr6Ni2Mo)     12Cr6.5Ni2.5Mo    347        No      No cracks           No cracks      Shallow

                                                                                                  cracks


W5P    B (12Cr6Ni2Mo)     12Cr6.5Ni2.5Mo    312        Yes     No cracks           No cracks      No cracks


 Extensive, very shallow surface penetrations (about 5-10µm deep), with an intergranular morphology ( Fig.4),

 were found in the HAZ and parent steel of welds W1 and W1P (pipe A, 12Cr5Ni2Mo, welded with

 superduplex wire), the first of which was as-welded and the second had been given PWHT. Similar shallow

 intergranular corrosion was observed in the HAZ and parent steel of W2, W5 and W5P (all in pipe B,

 12Cr6Ni2Mo).
                                                                                                BACK      INDEX




                                               Fig.4. Shallow intergranular features on the surface of

                                               specimen from weld W1P (pipe A, 12Cr5Ni2Mo) after test in

                                               environment B




4.0 Discussion


4.1 Current Understanding of the Mechanism of IGSCC of Supermartensitic Stainless Steel


 Cracking was at least partly intergranular with respect to prior austenite grain boundaries in most cases, e.g.

 Fig.5, but some cracks had areas with an apparently transgranular morphology. Figure 6 shows a crack

 running through an area with retained delta ferrite in the HAZ of weld W3, where the morphology appears to

 be more transgranular although it is noted that the prior austenite grain structure is not clearly defined. All

 other authors who have reported this cracking phenomenon have indicated an intergranular morphology. The

 intergranular crack appearance suggests that the sensitisation mechanism is a consequence of the formation

 of Cr-carbides and adjacent Cr-depleted zones, as in austenitic and ferritic stainless steels. However, there

 are a number of differences between the sensitisation of the austenitic stainless steels and the ferritic

 stainless steels, which are essentially single-phase throughout the welding thermal cycle, and the

 supermartensitic grades, which undergo several phase changes during welding. For SMSS, IGSCC has been

 reported to only occur in weld roots, [4] where multiple thermal cycles are experienced. This is not the case for

 austenitic and ferritic grades, where only one thermal cycle is required. However, this observation does

 support a Cr-carbide precipitation sensitisation mechanism for SMSS, as little or no carbide formation would

 be expected to occur during one weld thermal cycle in material that has been transformed to austenite during

 welding. This is due to the very low M s temperatures (around 200°C for the highest alloy grades).
                                                                                           BACK        INDEX




Fig.5. Intergranular cracking in the HAZ of            Fig.6. Cracking close to the fusion boundary in

weld W1 (pipe A, 12Cr5Ni2Mo) tested in                 the HAZ of weld W3 (pipe C, 12Cr6Ni2.5MoTi)

environment A                                          tested in environment A




The phenomenon seems to occur at specific HAZ locations, suggesting a critical combination of thermal

cycles is required, i.e. to put carbon back into solution and then to form chromium carbides and associated

Cr-depleted zones without subsequent 'healing'. Some welds showed cracking at two specific locations in the

HAZ, one of which was very close to the fusionline. This suggests that there may be more than one critical

location for cracking. This may be rationalised by considering the carbon (and perhaps nitrogen) that may be

present in solution at the various locations. In most of the transformed HAZ, carbon and nitrogen in solution

after one thermal cycle will depend on the levels present in the steel and the extent of carbide/nitride

dissolution during the first thermal cycle. Complete dissolution of Cr and Mo carbides occurs above about

720-800°C. In addition, adjacent to the fusionline for duplex and superduplex weld metals there will be a

fairly narrow band where diffusion of carbon and nitrogen into the HAZ may occur from superduplex weld

metal, which has higher levels of both elements (especially nitrogen) compared to the parent steel.

Experience with duplex stainless steels indicates that this zone may be about 50-100µm wide. This zone may

subsequently sensitise at a higher rate than the remainder of the HAZ due to the N and C enrichment.

It was noted that steels C and D, with a Ti addition, cracked particularly close to the fusion boundary.

Titanium would be expected to form carbides and nitrides preferentially and tend to lower the C and N content

in solution and hence act as a stabilising element as the same way that it does in austenitic and ferritic

stainless steels. At very high temperatures, above about 1300°C, stabilised austenitic and ferritic stainless

steels show dissolution of the stable Ti-carbides and may subsequently sensitise in such regions if reheated
                                                                                              BACK       INDEX




 to temperatures around 500-600°C, which promote Cr-carbide formation, leading to so-called 'knifeline'

 corrosion. Therefore, the location of cracking close to the fusionline in SMSS grades that contain Ti is

 consistent with the temperature range over which such dissolution of Ti-carbides might be expected. These

 facts all suggest a sensitisation mechanism that is related to the formation of Cr-depleted zones associated

 with Cr-carbides. It is noted also that the region close to the fusion boundary also typically contains a small

 fraction of retained delta ferrite within 100-200µm of the fusion boundary, which could have contributed to

 cracking susceptibility, perhaps via precipitation of Cr-carbides on the ferrite-martensite boundaries. However,

 no strong correlation between the location of delta ferrite and the IGSCC crack path was found.

 This observation is supported by very fine scale chemical analysis in a transmission electron microscope,

 which has confirmed the presence of Cr-carbides and Cr-depleted zones in lean grades. However, no such

 evidence has been found for the high grades, so it is impossible at present to be conclusive for these grades.

 Nevertheless, it seems unlikely that IGSCC of these two classes of SMSS would be a result of widely differing

 mechanisms. In the absence of evidence that is inconsistent with such a mechanism, it is postulated that

 sensitisation of high alloy SMSS grades is also a consequence of Cr-carbide precipitation, whilst recognising

 that no positive proof has been obtained to date.

 A mechanism of localised near-surface sensitisation has also been observed, associated with the formation

 of Cr-oxide on the weld surface. The formation of the oxide is associated with prior austenite grain boundary

 diffusion of chromium, which leads to the development of Cr-depleted regions adjacent to the near-surface

 prior austenite grain boundaries. Shallow intergranular corrosion associated with this sensitised layer has

 been observed in high alloy SMSS grades.

 Bend specimens tested in hot acidic chloride media only cracked with the as-welded root and, hence, with

 surface oxide and a stress concentrator. This has been observed by other authors although in highly acidic

 solutions, smooth specimens have been found to crack. This indicates that the weld surface oxide or stress

 concentration or both encourage crack initiation but are not essential. The effect of the oxide is presumably

 related to the Cr-depletion adjacent to grain boundaries immediately beneath the surface. Not all weld

 specimens cracked in the higher pH environment (B), suggesting a fairly strong effect of pH on IGSCC,

 similar to the situation for austenitic stainless steels.


4.2 The beneficial effect of PWHT with respect to IGSCC
                                                                                                             BACK      INDEX




 Postweld heat treatment clearly has a beneficial effect on the resistance of SMSS girth welds to IGSCC.

 However, testing here was for a fairly short duration and longer term data are required to confirm its

 applicability to long term service, particularly in the light of reservations expressed by one end user with a 30

 day test duration for qualification of SMSS for sweet service. In the present work, one PWHT cycle has been

 examined on girth welds, namely a nominal 650°C for five minutes although actual temperatures were

 ~620-660°C, with 620-640°C at the root. Assuming that the Cr-carbide precipitation theory of sensitisation of

 SMSS to IGSCC is correct, the most likely mechanism by which PWHT is effective in eliminating sensitivity to

 IGSCC is by allowing chromium back-diffusion into the chromium-depleted zones. The chromium-depleted

 zone width has been estimated to be up to 20nm in lean grade material but may be <5nm in steel with about

 6%Ni and 2%Mo. Hence, in order for PWHT to be effective, the time and temperature must be sufficient for

 chromium to diffuse over a distance of this magnitude. Use of a simple x=                           Dt calculation, based on
                                                               -14               -13
 published matrix diffusion coefficients in the range 4.9x10         to 1.5x10         cm 2 s   -1
                                                                                                     for chromium in iron with

 10-20%Cr, extrapolated from higher temperature data, which presumably relates to an austenitic

 microstructure, indicates a diffusion distance of about 40-70nm for five minutes at 650°C. Higher diffusion

 rates would be expected in the martensite and ferrite phases. Hence, this very simple calculation supports

 the proposed Cr-diffusion explanation of the effect of PWHT on eliminating sensitisation to IGSCC.


4.3 Avoiding potential detrimental effects of PWHT


 In order for a PWHT cycle to be successfully applied to a SMSS weld, in addition to eliminating sensitivity to

 IGSCC, it must also be such that it does not have any significantly detrimental effects on other weld

 properties.

 One undesirable effect of PWHT on the HAZ would be associated with heating to a temperature such that an

 excessive amount of austenite re-forms, leading to formation of un-tempered martensite on subsequent

 cooling. Un-tempered martensite has high hardness and low toughness in conventional martensitic stainless

 steels, which have carbon contents in excess of 0.03%, although for the low carbon SMSS grades, these

 effects are not pronounced and may not be significant. Examination of the effect of PWHT in simulated HAZs

 showed that 650°C was typically the temperature giving most hardness reduction of the steels studied but

 also showed substantial variation in response between SMSS grades, with some giving more hardness

 reduction at 625°C. This indicates the importance of choosing PWHT for the specific steel in question,
                                                                                                 BACK      INDEX




although broadly similar behaviour is expected for all SMSS grades based on the data obtained here. The

reformed, stable austenite content was generally found to increase on tempering at 600-650°C, indicating

that Ac   1   was exceeded over this range, hence some virgin martensite formation is possible if the upper

temperature during PWHT is above this range. With induction heating, a temperature gradient develops

through the pipe wall thickness, with the outside being hotter than the inside. For wall thicknesses of

11-18mm, induction PWHT trials indicated that the root was typically 15-35°C cooler than the cap. The

greatest risk of un-tempered martensite formation and associated hardening is therefore in the weld cap,

whilst it is essential for eliminating sensitivity to IGSCC in the internal environment that the temperature at the

root is controlled. This requires that both root and cap temperatures are held within an acceptable range

during PWHT. The limiting upper temperature will vary from grade to grade but based on the current data,

which only extends to a cap temperature of up to 660°C, it is recommended that temperatures in excess of

660°C should be avoided. Further work is required to explore the suitability of PWHT temperatures exceeding

660°C.

Another potential detrimental effect of PWHT is that it will tend to increase oxidation of the weld area.

Oxidation during welding has been demonstrated to have a detrimental influence on the pitting resistance of

SMSS HAZs in mildly sour media and hence any further oxidation from PWHT might also be detrimental.

However, published work has indicated that PWHT at 650°C may be beneficial for service under mildly sour

conditions, presumably by lowering hardness, but it does not give immunity to cracking in sour media. Further

work is required to explore this issue, although use of an inert gas shield during PWHT would eliminate the

concern.

Detrimental microstructural effects at the edge of the PWHT zone, where intermediate temperatures will be

experienced, are not anticipated, provided that the whole of the weld HAZ is heat treated, i.e. that the

intermediate temperatures are experienced by parent steel. This assumes that the parent steel will have been

tempered such that the carbon content in solution is very low. Detrimental microstructural effects in the HAZ

and weld metal are of greater concern. These may include precipitation of (i) further carbides, e.g. on prior

austenite boundaries or within or on the interface of any delta ferrite retained in the HAZ and (ii) intermetallic

phase, secondary austenite or alpha prime phase in the delta ferrite in weld metal deposited with a duplex or

superduplex consumable. These precipitation reactions may act to lower corrosion resistance and toughness

in the weld metal or HAZ very close to the fusion line, although the present study showed that the toughness

effects are not significant for a high grade SMSS HAZ and superduplex weld metal subject to PWHT at 650°C
                                                                                              BACK       INDEX




 for 5 minutes. To avoid loss of toughness, it is recommended that the PWHT duration should not be

 substantially longer than 5 minutes whilst recognising that longer PWHT may still give acceptable results for

 many applications. Substantially shorter PWHT periods are not recommended due to the absence of data.

 Sensitisation is not expected provided that the whole of the HAZ sees the intended PWHT temperature. No

 loss of corrosion resistance associated with precipitation on delta ferrite in SMSS HAZs has been noted to

 date, although one reference cites it as an issue for conventional 13%Cr 4%Ni steels, but does not indicate

 the precise temperature range of concern, although it does state that tempering at around 600°C gives good

 corrosion resistance, and hence problems are only likely to occur below this. Based on the results of the

 present work, a suitable lower temperature limit of 620°C is suggested for the HAZ.

 Although some precipitation occurred in superduplex weld metal during PWHT, this was apparently restricted

 to the formation of secondary austenite. Secondary austenite tends to reduce corrosion resistance but this

 should not be a problem when welding SMSS. This implies that, although PWHT of superduplex weld metal is

 not normally considered advisable, in this case 5 minutes PWHT at 650°C does not seem to be detrimental. If

 longer PWHT times or higher temperatures were used, some loss of toughness in superduplex weld metal

 might occur, although this was not studied here.


5. Conclusions


   1. The sensitisation of lean grade SMSS HAZs has been linked to the formation of Cr-carbides on

       prior-austenite grain boundaries and adjacent Cr-depleted zones but this link has not been established

       for the high alloy grades. Formation of Cr-depleted zones on prior-austenite boundaries immediately

       underneath the welding oxide has been observed in high alloy grades. Hence some uncertainty remains

       over the mechanism of IGSCC of high grade supermartensitic stainless steel and the effect of PWHT.

       Nevertheless, there is a substantial body of information supporting a consistent beneficial effect of brief

       PWHT for a broad range of supermartensitic grades.


   2. It is recommended that PWHT should be applied to welds in supermartensitic stainless steel where there

       is a risk of intergranular SCC in service, i.e. in hot acidic environments. A PWHT temperature of

       620-650°C at the root is recommended and the heat treated zone should encompass the whole of the

       weld metal and HAZ. The maximum allowable cap temperature has not been established but the current

       work extended up to 660°C. Heating and cooling should be fairly rapid. The most appropriate PWHT
                                                                                              BACK      INDEX




    duration has not been established but there is fairly common agreement that 5 minutes is an appropriate

    duration.


3. Whilst the beneficial effect of PWHT with respect to IGSCC has been demonstrated for 30 day exposure

    tests, longer term data are required to confirm the applicability of the effect to long term service.


4. Due to the limited information available, the use of welded supermartensitic stainless steel in the PWHT

    condition will require qualification on a case by case basis. The qualification programme should consider

    the effects of PWHT on toughness and sour service performance, in addition to IGSCC. The qualification

    process should consider the extremes of the range of PWHT thermal cycles that may be experienced, as

    the acceptable range has not been established.


5. No substantial change in toughness of superduplex weld metal was observed for PWHT at 650°C for 5

    minutes, although secondary austenite was formed. A small reduction of root HAZ hardness was

    generally associated with PWHT.


6. Postweld heat treatment may also have detrimental effects if not adequately controlled, e.g. (i)

    thickening of weld area oxides and associated loss of general/pitting corrosion resistance, (ii) formation

    of virgin martensite in the HAZ and increased hardness leading to reduced toughness and resistance to

    sour environments, (iii) loss of toughness in superduplex stainless steel weld metal, (iv) tempering of

    HAZ at temperatures that could induce sensitisation to intergranular SCC if the heat treated area is not

    wide enough.


7. The precise response to PWHT is specific to each individual grade of supermartensitic steel, although

    the data indicate that all steels examined here were fairly similar and the beneficial effect of 5 minutes at

    620-650°C, with respect to IGSCC, is applicable to 'lean' grades, with <1%Mo and 'high' grades with

    >2%Mo both with and without Ti addition.
                                                                                           BACK       INDEX




Corrosion of Aluminum and Its Alloys: Forms of Corrosion
Abstract:
Corrosion is the chemical reaction of a metal, in this case aluminum, with its environment, which leads to the
deterioration of the properties of metals, aluminum in this case. Aluminum is a very reactive metal, but it is
also a passive metal. This contradictory nature is explainable because nascent aluminum reacts with
oxygen or water and forms a coherent surface oxide which impedes further reaction of aluminum with the
environment.
Corrosion is the chemical reaction of a metal, in this case aluminum, with its environment, which leads to the
deterioration of the properties of metals, aluminum in this case. Aluminum is a very reactive metal, but it is
also a passive metal. This contradictory nature is explainable because nascent aluminum reacts with
oxygen or water and forms a coherent surface oxide which impedes further reaction of aluminum with the
environment.

Aluminum is chemically very reactive. For example, powdered aluminum is used as rocket propellant for
propulsion of the space shuttle's solid fuel rockets. Additionally, the reaction of aluminum with water
releases a tremendous amount of energy:

AI + 3H2O → AI(OH)3 + 3H2 ↑

Corrosion is the reaction of aluminum with water and the subsequent deterioration of its properties.
Corrosion, by definition, is a slow process, requiring days or years to occur to a noticeable extent, as
opposed to similar electrochemical reactions such as etching, brightening, or anodizing which occur in
minutes or less.

Aluminum alloys may corrode via several different pathways. Recognizing the pathway or the forms of
aluminum corrosion is an important step to determine the appropriate remedy for each probe.

Atmospheric Corrosion

Atmospheric corrosion is defined as the corrosion or degradation of material exposed to the air and its
pollutants rather than immersed in a liquid. This has been identified as one of the oldest forms of corrosion
and has been reported to account for more failures in terms of cost and tonnage than any other single
environment. Many authors classify atmospheric corrosion under categories of dry, damp, and wet, thus
emphasizing the different mechanisms of attack under increasing humidity or moisture.

Corrosivity of the atmosphere to metals varies greatly from one geographic location to another, depending
on such weather factors as wind direction, precipitation and temperature changes, amount and type of urban
and industrial pollutants, and proximity to natural bodies of water. Service life may also be affected by the
design of the structure if weather conditions cause repeated moisture condensation in unsealed crevices or
in channels with no provision for drainage.
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Uniform Corrosion

General corrosion, or uniform corrosion, occurs in the solutions where pH is either very high or very low, or
at high potentials in electrolytes with high chloride concentrations. In acidic (low pH) or alkaline (high pH)
solutions, the aluminum oxide is unstable and thus non-protective.

Galvanic Corrosion

Economically, galvanic corrosion creates the largest number of corrosion problems for aluminum alloys.
Galvanic corrosion, also known as dissimilar metal corrosion, occurs when aluminum is electrically
connected to a more noble metal, and both are in contact with the same electrolyte.

Crevice Corrosion

Crevice corrosion requires the presence of a crevice, a salt water environment, oxygen (Fig. 1). The crevice
can result from the overlap of two parts, or gap between a bolt and a structure. When aluminum is wetted
with the saltwater and water enters the crevice, little happens initially. Over time, inside the crevice oxygen is
consumed due to the dissolution and precipitation of aluminum.




Figure 1: Crevice corrosion can occur in a saltwater environment if the crevice becomes deaerated, and the
    oxygen reduction reaction occurs outside of the crevice mouth. Under these conditions, the crevice
                        becomes more acidic, and corrosion occurs at an increasing rate.

Pitting Corrosion

Corrosion of aluminum in the passive range is localized, usually manifested by random formation of pits. The
pitting-potential principle establishes the conditions under which metals in the passive state are subject to
corrosion by pitting.

Pitting corrosion is very similar to crevice corrosion. Pitting of aluminum alloys occurs if the electrolyte
contains a low level of chloride anions, and if the alloy is at a potential above the "pitting potential." Pitting
initiates at defects on the surface of the aluminum, such as at second phase particles or on grain
boundaries.

Deposition Corrosion
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In designing aluminum and aluminum alloys for satisfactory corrosion resistance, it is important to keep in
mind that ions of several metals have reduction potentials that are more cathodic than the solution potential
of aluminum and therefore can be reduced to metallic form by aluminum. For each chemical equivalent of
so-called heavy-metal ions reduced, a chemical equivalent of aluminum is oxidized. Reduction of only a
small amount of these ions can lead to severe localized corrosion of aluminum, because the metal reduced
from them plates onto the aluminum and sets up galvanic cells.

The more important heavy metals are copper, lead, mercury, nickel, and tin. The effects of these metals on
aluminum are of greatest concern in acidic solutions; in alkaline solutions, they have much lower solubilities
and therefore much less severe effects.

Intergranular Corrosion

Intergranular (intercrystalline) corrosion is selective attack of grain boundaries or closely adjacent regions
without appreciable attack of the grains themselves. Intergranular corrosion is a generic term that includes
several variations associated with different metallic structures and thermomechanical treatments.
Intergranular corrosion is caused by potential differences between the grain-boundary region and the
adjacent grain bodies.

The location of the anodic path varies with the different alloy systems. In 2xxx series alloys, it is a narrow
band on either side of the boundary that is depleted in copper; in 5xxx series alloys, it is the anodic
constituent Mg2AI3 when that constituent forms a continuous path along a grain boundary; in copper-free
7xxx series alloys, it is generally considered to be the anodic zinc- and magnesium-bearing constituents on
the grain boundary. The 6xxx series alloys generally resist this type of corrosion, although slight
intergranular attack has been observed in aggressive environments.

Exfoliation Corrosion




Exfoliation corrosion in an aluminum alloy exposed to tropical marine environment. Also note the paint failures
caused by corrosion of aluminium at the coating/aluminium interface.
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Exfoliation corrosion is a special form of intergranular corrosion which occurs when the grains are flattened
by heavy deformation during hot or cold rolling, and where no recrystallization has occurred. Exfoliation is
characteristic for the 2000 (Al-Cu), 5000 (Al-Mg), and 7000 (Al-Zn-Mg) series alloys which have grain
boundary precipitation or depleted grain boundary regions.

The remedy for exfoliation is similar to above for IG corrosion. To prevent the exfoliation of alloy 7075-T6,
the newer alloy 7150-T77 can be substituted wherever 7075-T6 is used.

Erosion-Corrosion

Erosion-corrosion of aluminum occurs in high velocity water and is similar to jet-impingement corrosion.
Erosion-corrosion of aluminum is very slow in pure water, but is accelerated at pH > 9, especially with high
carbonate and high silica content of the water.

Aluminum is very stable is neutral water; however it will corrode in either acidic or alkaline waters. To
prevent erosion-corrosion, one may change the water chemistry or reduce the velocity of the water, or both.
For the water chemistry, the pH must be below 9, and the carbonate and the silica levels must be reduced.

Stress Corrosion Cracking (SCC)

Stress corrosion cracking (SCC) is the bane of aluminum alloys. SCC requires three simultaneous
conditions, first a susceptible alloy, second a humid or water environment, and third a tensile stress which
will open the crack and enable crack propagation. SCC can occur in two modes, intergranular stress
corrosion cracking (IGSCC) which is the more common form, or transgranular SCC (TGSCC). In IGSCC, the
crack follows the grain boundaries. In transgranular stress corrosion cracking (TGSCC), the cracks cut
through the grains and are oblivious to the grain boundaries.

The general trend to use higher strength alloys peaked in 1950 with alloy 7178-T651 used on the Boeing
707, then the industry changed to using lower strength alloys. The yield strength of the upper wing skin did
not exceed the 1950 level until the Boeing 777 in the 1990s. The reason lower strength alloys were selected
for the Boeing 747 and the L-1011 was that the aircraft designers chose an alloy with better SCC resistance
rather than the higher yield strength.

Corrosion Fatigue

Corrosion fatigue can occur when an aluminum structure is repeatedly stressed at low stress levels in a
corrosive environment. A fatigue crack can initiate and propagate under the influence of the crack-opening
stress and the environment. Similar striations may sometimes be found on corrosion fatigued samples, but
often the subsequent crevice corrosion in the narrow fatigue crack dissolves them.

Fatigue strengths of aluminum alloys are lower in such corrosive environments as seawater and other salt
solutions than in air, especially when evaluated by low-stress long-duration tests. Like SCC of aluminum
alloys, corrosion fatigue requires the presence of water. In contrast to SCC, however, corrosion fatigue is not
appreciably affected by test direction, because the fracture that results from this type of attack is
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predominantly transgranular.

Filiform Corrosion

Filiform corrosion (also known as wormtrack corrosion) is a cosmetic problem for painted aluminum.
Pinholes or defects in the paint from scratches or stone bruises can be the initiation site where corrosion
begins with salt water pitting. Filiform corrosion requires chlorides for initiation and both high humidity and
chlorides for the propagation of the track.

The propagation depends on where and how the alloy is used. The filament must be initiated by chlorides,
and then it proceeds by a mechanism similar to crevice corrosion. The head is acidic, high in chlorides, and
deaerated and is the anodic site. Oxygen and water vapor diffuse through the filiform tail, and drive the
cathodic reaction. Filiform corrosion can be prevented by sealing defects with paint or wax, and keeping the
relative humidity low.




Microbiological Induced Corrosion

Microbiological Induced Corrosion (MIC) applies to a corrosive situation which is caused or aggravated by
the biological organisms. A classic case of MIC is the growth of fungus at the water/fuel interface in
aluminum aircraft fuel tanks. The fungus consumes the high octane fuel, and excretes an acid which attacks
and pits the aluminum fuel tank and causes leaking. The solution for this problem is to control the fuel quality
and prevent water from entering or remaining in the fuel tanks. If fuel quality control is not feasible, then
fungicides are sometimes added to the aircraft fuel.
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                                        * Publisher: Elsevier Science
                                        * Number Of Pages: 700
                                        * Publication Date: 2004-12-16
                                        * Sales Rank: 246595
                                        * ISBN / ASIN: 0080444954
                                        * EAN: 9780080444956
                                        * Binding: Hardcover
                                        * Manufacturer: Elsevier Science
                                        * Studio: Elsevier Science
                                        * Average Rating:
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Book Description:


This book highlights the practical and general aspects of the corrosion of aluminium alloys with many
illustrations and references. In addition to that, the first chapter allows the reader who is not very familiar with
aluminium to understand the metallurgical, chemical and physical features of the aluminium alloys.


The author Christian Vargel, has adopted a practitioner approach, based on the expertise and experience
gained from a 40 year career in aluminium corrosion This approach is most suitable for assessing the
corrosion resistance of aluminium- an assessment which is one of the main conditions for the development of
many uses of aluminium in transport, construction, power transmission etc.


* 600 bibliographic references provide a comprehensive guide to over 100 years of related study
* Providing practical applications to the reader across many industries
* Accessible to both the beginner and the expert


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                                                                                             BACK        INDEX




Corrosion of Copper and Copper Alloys

In normal atmospheric exposure to carbonic acid (H2CO3) copper and copper alloys form a layer of
copper carbonate - a green substance also called patina. This patina (CuCO3) actually serves to
protect the copper underneath from further corrosion. The insoluble copper carbonate tightly
adheres to the surface preventing further contact with acid rain. However, the patina is a pale and
unsightly green that detracts from the appearance of many copper structures and monuments. If you
have ever visited Charlottetown, Prince Edward Island you may be familiar with this as the War
Memorial in the downtown area is heavily coated in patina.
This reaction begins with the release of gaseous carbon dioxide into the atmosphere from respiration
or in the form of emissions from industrial processes. The CO2 dissolves in atmospheric moisture to
form carbonic acid:

CO2(gas) + H2O(liquid)         H2CO3(aq)
The acidic properties of carbonic acid have no effect here as the H+(aq) do not take an active role in
the reaction. The carbonate (CO32-) polyatomic ion is the reactive species that oxidizes the copper
into patina and allows the hydrogen gas to escape:


H2CO3(aq) + Cu(solid)         CuCO3(solid) + H2(gas)
Even though this reaction would slowly occur naturally, as the concentration of CO2 in the atmosphere
increases from emissions the rate of this reaction increases. This shortens the longevity of the
lustrous metal and hastens the restoration process that can be costly.



COPPER AND COPPER ALLOYS are widely used in many environments and applications because of
their excellent corrosion resistance, which is coupled with combinations of other desirable properties, such
as superior electrical and thermal conductivity, ease of fabricating and joining, wide range of attainable
mechanical properties, and resistance to biofouling.

Copper corrodes at negligible rates in unpolluted air, water, and deaerated nonoxidizing acids. Copper
alloy artifacts have been found in nearly pristine condition after having been buried in the earth for
thousands of years, and copper roofing in rural atmospheres has been found to corrode at rates of less
than 0.4 mm in 200 years.

Copper alloys resist many saline solutions, alkaline solutions, and organic chemicals. However, copper is
susceptible to more rapid attack in oxidizing acids, oxidizing heavy-metal salts, sulfur, ammonia (NH3), and
some sulfur and
NH3 compounds.

Copper and copper alloys provide superior service in many of the applications included in the following
general classifications:
                                                                                              BACK         INDEX




        Applications requiring resistance to atmospheric exposure, such as roofing and other architectural
        uses, hardware, building fronts, grille work, hand rails, lock bodies, doorknobs, and kick plates
        Freshwater supply lines and plumbing fittings, for which superior resistance to corrosion by
        various types of waters and soils is important
        Marine applications - most often freshwater and seawater supply lines, heat exchangers,
        condensers, shafting, valve stems, and marine hardware - in which resistance to seawater,
        hydrated salt deposits, and biofouling from marine organisms is important
        Heat exchangers and condensers in marine service, steam power plants, and chemical process
        applications, as well as liquid-to-gas or gas-to-gas heat exchangers in which either process
        stream may contain a corrosive contaminant
        Industrial and chemical plant process equipment involving exposure to a wide variety of organic
        and inorganic chemicals
        Electrical wiring, hardware, and connectors; printed circuit boards; and electronic applications that
        require demanding combinations of electrical, thermal, and mechanical properties, such as
        semiconductor packages, lead frames, and connectors




Effects of alloy compositions on corrosion

Coppers and high-copper alloys (C 10100 - C 19600; C 80100 - C 82800) have similar corrosion
resistance.
They have excellent resistance to seawater corrosion and biofouling, but are susceptible to
erosion-corrosion at high water velocities. The high-copper alloys are primarily used in applications that
require enhanced mechanical performance, often at slightly elevated temperature, with good thermal or
electrical conductivity. Processing for increased strength in the high-copper alloys generally improves their
resistance to erosion-corrosion.

Brasses (C 20500 - C 28580) are basically copper-zinc alloys and are the most widely used group of
copper alloys. The resistance of brasses to corrosion by aqueous solutions does not change markedly as
long as the zinc content does not exceed about 15%. Above 15% Zn, dezincification may occur.

Susceptibility to stress-corrosion cracking (SCC) is significantly affected by zinc content; alloys that
contain more zinc are more susceptible. Resistance increases substantially as zinc content decreases
from 15% to 0%. Stress-corrosion cracking is practically unknown in commercial copper. Elements such as
lead, tellurium, beryllium, chromium, phosphorus, and manganese have little or no effect on the corrosion
resistance of coppers and binary copper-zinc alloys. These elements are added to enhance such
mechanical properties as machinability, strength, and hardness.

Tin Brasses (C 40400 - C 49800; C 90200 - C 94500). Tin additions significantly increase the corrosion
resistance of some brasses, especially resistance to dezincification.
                                                                                             BACK       INDEX




Cast brasses for marine applications are also modified by the addition of tin, lead, and, sometimes, nickel.
This group of alloys is known by various names, including composition bronze, ounce metal, and valve
metal.

Aluminum Brasses (C66400-C69900). An important constituent of the corrosion film on a brass that
contains few percents of aluminum in addition to copper and zinc is aluminum oxide (A1203), which
markedly increases resistance to impingement attack in turbulent high-velocity saline water.

Phosphor Bronzes (C 50100 - C 52400). Addition of tin and phosphorus to copper produces good
resistance to flowing seawater and to most nonoxidizing acids except hydrochloric (HCl). Alloys containing
8 to 10% Sn have high resistance to impingement attack. Phosphor bronzes are much less susceptible to
SCC than brasses and are similar to copper in resistance to sulfur attack. Tin bronzes-alloys of copper and
tin-tend to be used primarily in the cast form, in which they are modified by further alloy additions of lead,
zinc, and nickel.

Copper Nickels (C 70000 - C 79900; C 96200 - C 96800). Alloy C71500 (Cu-30Ni) has the best general
resistance to aqueous corrosion of all the commercially important copper alloys, but C70600 (Cu-3ONi) is
often selected because it offers good resistance at lower cost. Both of these alloys, although well suited to
applications in the chemical industry, have been most extensively used for condenser tubes and
heat-exchanger tubes in recirculating steam systems. They are superior to coppers and to other copper
alloys in resisting acid solutions and are highly resistant to SCC and impingement corrosion.

Nickel Silvers (C 73200 - C 79900; C 97300 - C 97800). The two most common nickel silvers are C75200
(65Cu-18Ni-17Zn) and C77000 (55Cu-18Ni-27Zn). They have good resistance to corrosion in both fresh
and salt waters. Primarily because their relatively high nickel contents inhibit dezincification, C75200 and
C77000 are usually much more resistant to corrosion in saline solutions than brasses of similar copper
content.

Copper-silicon alloys (C 64700 - C66100; C 87300 - C 87900) generally have the same corrosion
resistance as copper, but they have higher mechanical properties and superior weldability. These alloys
appear to be much more resistant to SCC than the common brasses. Silicon bronzes are susceptible to
embrittlement by high-pressure steam and should be tested for suitability in the service environment
before being specified for components to be used at elevated temperature.

Aluminum bronzes (C 60600 - C 64400; C 95200 - C 95810) containing 5 to 12% Al have excellent
resistance to impingement corrosion and high-temperature oxidation. Aluminum bronzes are used for
beater bars and for blades in wood pulp machines because of their ability to withstand mechanical
abrasion and chemical attack by sulfite solutions.

In the most of practical commercial applications, the corrosion characteristics of aluminum bronzes are
primarily related to aluminum content. Alloys with up to 8% Al normally have completely face-centered
cubic structures and a good resistance to corrosion attack. As aluminum con tent increases above 8%,
 
   duplex structures appear.
                                                                                             BACK       INDEX




Depending on specific environmental conditions,   phase or eutectoid structure in aluminum bronze can
be selectively attacked by a mechanism similar to the dezincification of brasses. Proper
quench-and-temper treatment of duplex alloys, such as C62400 and C95400, produces a tempered ( 
structure with reprecipitated acicular a crystals, a combination that is often superior in corrosion resistance
to the normal annealed structures.

Nickel-aluminum bronzes are more complex in structure with the introduction of the K phase. Nickel
appears to alter the corrosion characteristics of the   phase to provide greater resistance to dealloying
and cavitation-erosion in most liquids.

Aluminum bronzes are generally suitable for service in nonoxidizing mineral acids, such as phosphoric
(H3PO4), sulfuric (H2SO4), and HCl; organic acids, such as lactic, acetic (CF3COOH), or oxalic; neutral
saline solutions, such as sodium chloride (NaCI) or potassium chloride (KCl); alkalies, such as sodium
hydroxide (NaOH), potassium hydroxide (KOH), and anhydrous ammonium hydroxide (NH4OH); and
various natural waters including sea, brackish, and potable waters. Environments to be avoided include
nitric acid (HNO3); some metallic salts, such as ferric chloride (FeCl3) and chromic acid (H2CrO4); moist
chlorinated hydrocarbons; and moist HN3. Aeration can result in accelerated corrosion in many media that
appear to be compatible.
                                                                                      BACK     INDEX




1-1. Metal Corrosion

                         The open recirculating cooling water system for a building is
                         mainly for air-conditioning system and the main construction
                         materials are mild steel and copper alloys. Corrosion of metals
                         reduces the life of the cooling systems. The corrosion products
                         also precipitate on the heat exchangers and reduce heat transfer
                         efficiency to increase energy loss.
                         Corrosion of copper and copper alloys produces heat insulating
                         thick black copper oxide on the surfaces. This copper oxide increases
                         energy costs also.


1-2. Scale

                         In addition to the corrosion products which was mentioned
                         before, some components in cooling water, such as calcium,
                         magnesium and silica, precipitate on the heat transfer surfaces
                         and cause serious problem of low heat transfer problem and
                         excessive energy loss. Most of the heat exchangers for air
                         -conditioning systems show 0.2 to 1mm thickness of scale before
                         their annual chemical cleaning if they are not treated regularly by
                         a proper scale inhibitor chemical product.



1-3. Microbial Fouling

                         The microbial fouling in an open recirculating cooling water
                         system is caused by algae, slime bacteria, legionellar and fungi.
                         Legionellar control is particullarly important for building cooling
                         water treatment. Cooling efficiency of a cooling tower is
                         significantly reduced by bio-fouling. Adhesion of algae or slime
                         bacteria on the heat exchanger surfaces reduces heat transfer
                         and can cause under-deposit corrosion.
                                                                                                                                      BACK           INDEX




Corrosion Resistance of Copper and Copper Alloys

Reprinted with permission by the Copper Development Association



An R indicates that the material is resistant to the named chemical up to the temperature shown, subject to
limitations indicated by the footnotes.

An X indicates that the material is NOT RECOMMENDED.


                           Aluminium Bronze        Brass (a)              Copper                           Copper-Nickel        90/10 Gunmetal           and

                                                                                                           alloys (b)                Bronze (c)

Temperature, Celcius       20      60      100     20      60      100    20      60         100           20       60      100      20      60      100

Acetaldehyde               R       R       R       R       R       R      R       R          R             R        R       R        R       R       R

Acetic acid (10%)          R       R       R       X       X       X      R       R          R             R        R       R        R       R       R

Acetic acid (glac./anh.)   R       R       R       X       X       X      R       R          R             R        R       X        R       R       R

Acetic anhydride           R       R       R       X       X       X      R       R          R             R        R       R        R       R       X

Aceto-acetic ester         R       R       R       R (82) X        X      R       R          R             R        R       R        R       R       R

Acetone                    R       R       R       R       R       R      R       R          R             R        R       R        R       R       R

Other ketones              R       R       R       R       R       R      R       R          R             R        R       R        R       R       R

Acetonitrile               R (36) X        X       X       X       X      R (36) X           X             R (36)   X       X        R (36) X        X

Acetylene                  X       X       X       R       R       R (82) X       X          X             X        X       X        X       X       X

Acetyl salicylic acid                              No      No      No
                           R       R       R                              R (36) X           X             R        R       R        R       R       R
                                                   data    data    data

Acid fumes                 R (2)   R (2)   R (2)   X       X       X      R (2)   R (2)      R (2)         R (2)    R (2)   R (2)    X       X       X

Alcohols (mostly fatty)    R       R       R       R       R       R      R       R          R             R        R       R        R       R       R

Aliphatic esters           R       R       R       R       R       R      R       R          R             R        R       R        R       R       R

Alkyl chlorides            No      No      No
                                                   X       X       X      R       R          R             R        R       R        R       R       R
                           data    data    data

Alum                       R       R       R       X       X       X      R       R          R             R        R       R        R       R       R

Aluminium chloride         R (20) R (20) X         X       X       X      R       R          R             R        R       X        R       R       R

Aluminium sulphate                                 R       R              R       R      (20, R     (20,                             R       R       R
                           R       R       R                       X                                       R        R       R
                                                   (119)   (119)          (119)   119)       119)                                    (119)   (119)   (119)

Ammonia, anhydrous         R       R       R       X       X       X      R       R          R (83)        R        R       R        R       R       R

Ammonia, aqueous           X       X       X       X       X       X      X       X          X             X        X       X        X       X       X

Ammonium chloride          X       X       X       X       X       X      X       X          X             X        X       X        X       X       X

Amyl acetate               R       R       R       X       X       X      R       R          R             R        R       R        R       R       R

Aniline                    X       X       X       X       X       X      X       X          X             X        X       X        X       X       X

Antimony trichloride       No      No      No                             No
                                                   X       X       X              No data No data No data No data No data R                  X       X
                           data    data    data                           data
                                                                                                                         BACK         INDEX




Aqua regia               X      X      X        X      X      X      X      X       X        X        X        X        X      X      X

Aromatic solvents        R      R      R        R      R      R      R      R       R        R        R        R        R      R      R

Ascorbic acid            X      X      X        X      X      X      X      X       X        X        X        X        X      X      X

Beer                     R      R      R        R      R      R      R      R       R        R        R        R        R      R      R

Benzaldehyde                                    No     No     No
                         R      R      R                             R      R       R        R        R        R        R      R      R
                                                data   data   data

Benzene, pure            R      R      R        R      R      R      R      R       R        R        R        R        R      R      R

Benzoic acid             R      R      R        R      R      R      X      X       X        R        R        R        X      X      X

Benzoyl peroxide         No     No     No       No     No     No     No                                                 No     No     No
                                                                            No data No data No data No data No data
                         data   data   data     data   data   data   data                                               data   data   data

Boric acid               R      R      R        R      R      R      R      R       R        R        R        R        R      R      R

Brines, saturated        R      R      R        X      X      X      R      R       R (20)   R        R        R        R      R      X

Bromide (K) solution     R      R      R        X      X      X      R      R       X        R        R        R        R      R      X

Bromine                  R (20) X      X        X      X      X      X      X       X        R (11)   R (11)   R (11)   X      X      X

Bromine liquid, tech.    R      X      X        R      X      X      R      X       X        R        X        X        R      X      X

Bromine water, sat.aq.   R      X      X        R      X      X      X      X       X        X        X        X        X      X      X

Butyl acetate            R      R      R        R      R      R      R      R       R        R        R        R        R      R      R

Calcium chloride         R      R      R        X      X      X      R      R       R        R        R        R        R      R      R

Carbon disulphide        R      X      X        R      R      R      R      R       R        R        R        X        R      R      R

Carbonic acid            R      R      R        X      X      X      X      X       X        R        R        R        X      X      X

Carbon tetrachloride     R      R      R        R      R      R      R      R       R        R        R        R        R      R      R

Caustic soda & potash    R      X      X        X      X      X      R      R       R        R        R        X        R      R      R

Cellulose paint          R      R      R        R      R      R      R      R       R        R        R        R        R      R      R

Chlorates of Na, K, Ba   R      R      R        X      X      X      R      R       R        R        R        X        R      R      R

Chlorine, dry            R      R      R        R      R      R      R      R       R        R        R        R        R      R      R

Chlorine, wet            X      X      X        X      X      X      X      X       X        X        X        X        X      X      X

Chlorides of Na, K, Mg   R      R      R        X      X      X      R      R       R (20)   R        R        R        R      R      R

Chloroacetic acids       No     No     No                            No                                                 No     No     No
                                                X      X      X             No data No data No data No data No data
                         data   data   data                          data                                               data   data   data

Chlorobenzene                                   No     No     No     No
                         R      R      R                                    No data No data R         R        R        R      R      R
                                                data   data   data   data

Chloroform               R      R      R        R      R      R      R      R       R        R        R        R        R      R      R

Chlorosulphonic acid                            No     No     No
                         R (20) R (20) R (20)                        X      X       X        R        R        R        X      X      X
                                                data   data   data

Chromic acid (80%)       X      X      X        X      X      X      X      X       X        X        X        X        X      X      X

Citric acid              R      R      R        X      X      X      R      R       R        R        R        R        R      R      R

Copper salts (most)      R      R      R        X      X      X      X      X       X        R        X        X        R      R      R

Cresylic acids (50%)     R      R      R        X      X      X      R      R       R        R        R        X        R      R      R
                                                                                                                   BACK         INDEX




Cyclohexane                   R      R      R      R      R      R      R     R    R   R        R        R        R      R      R

Detergents, synthetic         No     No     No
                                                   R      R      R      R     R    R   R        R        R        R      R      R
                              data   data   data

Emulsifiers (all conc.)                            No     No     No                                               No     No     No
                              R      R      R                           R     R    R   R        R        R
                                                   data   data   data                                             data   data   data

Esters                        R      R      R      R      R      R      R     R    R   R        R        R        R      R      R

Ether                         R      R      R      R      R      R      R     R    R   R        R        R        R      R      R

Fatty acids (>C6)             R      R      R      X      X      X      R     R    R   R        R        R        R      R      R

Ferric chloride               X      X      X      X      X      X      X     X    X   X        X        X        X      X      X

Ferrous sulphate              R (20) R (20) R (20) X      X      X      X     X    X   R        X        X        X      X      X

Fluorinated refrigerants      R      R      R      R      R      R      R     R    R   R        R        R        R      R      R

Fluorine, dry                 R      R      R (11) X      X      X      R     R    R   R        R        R        R      R      R

Fluorine, wet                 X      X      X      X      X      X      X     X    X   X        X        X        X      X      X

Fluorosilic acid              X      X      X      X      X      X      X     X    X   X        X        X        X      X      X

Formaldehyde (40%)            R      R      R      R      X      X      R     R    R   R        R        R        R      R      R

Formic acid                                        No     No     No
                              R      R      R                           R     R    R   R        R        R        R      R      R
                                                   data   data   data

Fruit juices                  R      R      R      X      X      X      R     R    R   R        R        R        R      R      R

Gelatine                      R      R      R      R      R      R      R     R    R   R        R        R        R      R      R

Glycerine                     R      R      R      R      R      R      R     R    R   R        R        R        R      R      R

Glycols                       R      R      R      R      R      R      R     R    R   R        R        R        R      R      R

Glycol, ethylene              R      X      X      R      X      X      R     R    R   R (175) R (175) R (175) R         R      R

Glycollic acid                R (36) X      X      R (36) X      X      R (36) X   X   R (36)   X        X        R (36) X      X

Hexamethylene diamine X              X      X      X      X      X      X     X    X   X        X        X        X      X      X

Hexamine                      X      X      X      X      X      X      X     X    X   X        X        X        X      X      X

Hydrazine                     X      X      X      X      X      X      X     X    X   X        X        X        X      X      X

Hydrobromic acid (50%) X             X      X      X      X      X      X     X    X   X        X        X        X      X      X

Hydrochloric acid (10%) R            X      X      X      X      X      X     X    X   R        X        X        X      X      X

Hydrochloric acid (conc.) R (62) X          X      X      X      X      X     X    X   X        X        X        X      X      X

Hydrocyanic acid              R (20) R (20) R (20) X      X      X      X     X    X   X        X        X        X      X      X

Hydrofluoric acid (40%)       R (62) X      X      X      X      X      X     X    X   X        X        X        X      X      X

Hydrofluoric acid (75%)       R (62) X      X      X      X      X      X     X    X   X        X        X        X      X      X

Hydrogen           peroxide
                              X      X      X      X      X      X      X     X    X   R        X        X        X      X      X
(30%)

Hydrogen           peroxide
                              X      X      X      X      X      X      X     X    X   X        X        X        X      X      X
(30-90%)

Hydrogen sulphide             R (11) R      R      R (11) R      R      R (11) R   R   R (11)   R (11)   R (11)   R (11) R      R

Hypochlorites                 R      X      X      X      X      X      X     X    X   X        X        X        X      X      X
                                                                                                                   BACK           INDEX




Hypochlorite           (Na
                             R      R      R      R      R      R      R      R        R   X        X        X    X       X       X
12-14%)

Iso-butyl acetate            R      R      X      X      X      X      R      R        X   R        R        R    R       R       R

Lactic acid (90%)            No     No     No
                                                  X      X      X      X      X        X   R        R        X    R (4)   R (4)   X
                             data   data   data

Lead acetate                 X      X      X      X      X      X      X      X        X   R        R        X    X       X       X

Lead perchlorate             R      R      R      R      R      R      R      R        R   X        X        X    X       X       X

Lime (CaO)                   No     No     No     No     No     No
                                                                       R      R        R   R        R        R    R       R       R
                             data   data   data   data   data   data

Maleic acid                                                                                                       No      No      No
                             R      X      X      R      X      X      R (60) R (60)   X   R        R        X
                                                                                                                  data    data    data

Manganate, pot (K)           R      R      R      X      X      X      X      X        X   X        X        X    R (60) R (60) X

Meat juices                  X      X      X      X      X      X      X      X        X   No data No data No data X      X       X

Mercuric chloride            X      X      X      X      X      X      X      X        X   X        X        X    X       X       X

Mercury                      R      R      R      R      R      R      R      R        R   X        X        X    X       X       X

Methanol                     R      R      R      R (82) R (82) R (82) R      R        R   R        R        R    R       R       R

Methylene chloride           R      R      X      X      X      X      X      X        X   R        R        R    R       R       R

Milk & milk products         R      R      R      X      X      X      R      R        R   R        R             R       R       R

Moist air                    R (30) R (30) R (30) R (30) R      X      R (30) R        R   R        R        R    R       R       R

Molasses                     X      X      X      X      X      X      X      X        X   R        R        R    R (30) R        R

Monoethanolamine             R      R      R      R      R      R      R      R        R   X        X        X    X       X       X

Naphtha                      No     No     No     No     No     No
                                                                       R      R        R   R        R        R    R       R       R
                             data   data   data   data   data   data

Naphthalene                  No     No     No                                                                     No      No      No
                                                  X      X      X      X      X        X   R        R        R
                             data   data   data                                                                   data    data    data

Nickel salts                 R (73) R (73) R (73) X      X      X      X      X        X   R        R        R    R       R       R

Nitrates of Na, K, NH3       X      X      X      X      X      X      X      X        X   R (73)   R (73)   X    X       X       X

Nitric acid (<25%)           X      X      X      X      X      X      X      X        X   X        X        X    X       X       X

Nitric acid (50%)            X      X      X      X      X      X      X      X        X   X        X        X    X       X       X

Nitric acid (90%)            X      X      X      X      X      X      X      X        X   X        X        X    X       X       X

Nitric acid, fuming          R      R      R      R      R      R      R      R        R   X        X        X    X       X       X

Nitrite (Na)                 R      R      R      R      R      R      R      R        R   R        R        R    R       R       R

Nitrobenzine                 R      X      X      R      X      X      R      X        X   R        R        R    R       R       R

Oil, diesel                  R      R      R      R      R      R      R      R        R   R        X        X    R       X       X

Oils, essential              R      R      X      R      R      X      R      R        X   R        R        R    R       R       R

Oils, lube + aromatic
                             R      R      R      R      R      R      R      R        R   R        R        X    R       R       X
ads.

Oils, mineral                R      R      R      R      R      R      R      R        R   R        R        R    R       R       R
                                                                                                                        BACK         INDEX




Oils, vegetable & animal                         No     No     No
                           R      R      R                            R      R      R        R       R       R         R      R      R
                                                 data   data   data

Oxalic acid                No     No     No      No     No     No     No
                                                                             No data No data R       R      R          R      R      R
                           data   data   data    data   data   data   data

Ozone                                                                                                                  No     No     No
                           R      R      R       R      R      R      R      R       R       No data No data No data
                                                                                                                       data   data   data

Paraffin wax                                     No     No     No
                           X      X      X                            X      X      X        R       R       R         R      R      R
                                                 data   data   data

Perchloric acid            R      R      R       R      R      R      R      R      R        X       X      X          X      X      X

Petroleum spirits          R      R      R       R      R      R      R      R      R        R       R      R          R      R      R

Phenol                     R      R      R       X      X      X      R      R      R        R       R      R          R      R      R

Phosphoric acid (20%)      R      R      R       X      X      X      X      X      X        R       R       X         X      X      X

Phosphoric acid (50%)      R      R      R       X      X      X      X      X      X        R       R       X         X      X      X

Phosphoric acid (95%)      R (11) R (11) R (11) X       X      X      X      X      X        R       X       X         X      X      X

Phosphorus chlorides       No     No     No
                                                 X      X      X      X      X      X        R       X       X         X      X      X
                           data   data   data

Phosphorous pentoxide                            No     No     No
                           R      R      R                            R      R      R        No data No data No data X        X      X
                                                 data   data   data

Phthalic acid              X      X      X       X      X      X      X      X      X        R       R       R         R      R      R

Picric acid                No     No     No
                                                 X      X      X      X      X      X        R       R       R         R      R      R
                           data   data   data

Pyridine                   No     No     No      No     No     No     No
                                                                             No data No data No data No data No data X        X      X
                           data   data   data    data   data   data   data

Salicyl aldehyde                                                                                                       No     No     No
                           R      R      R       R (62) R      R      R      R      R        No data No data No data
                                                                                                                       data   data   data

Sea water                                        No     No     No
                           R      R      R                            R      R      R        R       R       R         R      R      R
                                                 data   data   data

Silicic acid                                                                                                           No     No     No
                           R      R      R       R      R      R      R      R      R        X       X       X
                                                                                                                       data   data   data

Silicone fluids            X      X      X       X      X      X      X      X      X        R       R      R          R      R      R

Silver nitrate             R      R      R (4)   R      R      R      R      R      R        X       X      X          X      X      X

Sodium carbonate           X      X      X       X      X      X      X      X      X        R       R      R          R      R      R

Sodium peroxide            R      R      R       R      R      R      R      R      R        R       X      X          X      X      X

Sodium silicate            X      X      X       X      X      X      X      X      X        R       R       R         R      R      R

Sodium sulphide            R (11) X      X       X      X      X      X      X      X        X       X       X         X      X      X

Stannic chloride                                 No     No     No
                           R      R      X                            R      R      R        X       X       X         X      X      X
                                                 data   data   data

Starch                     R      R      R       R      R      R      R      R      R        R       R       R         R      R      R

Sugar soln, syrups, jams No       No     No      X      X      X      X      X      X        R       R      R          R      R      R
                                                                                                                    BACK         INDEX




                         data    data   data

Sulphamic acid           R       R      R      R      R      R       R      R        R   No data No data No data X        X      X

Sulphates (Na, K, Mg,
                         R       R      R      X      X      X       R      R        R   R        R        R       R      R      R
Ca)

Suphites                 No      No     No     No     No     No
                                                                     X      X        X   R        R        R       R      R      R
                         data    data   data   data   data   data

Sulphonic acids                                                                                                    No     No     No
                         X       X      X      X      X      X       X      X        X   No data No data No data
                                                                                                                   data   data   data

Sulphur                  R       R      R      R      R      R       R      R        R   X        X        X       X      X      X

Sulphur dioxide, dry     R       R      R      X      X      X       X      X        X   R        R        X       R      R      R

Sulphur dioxide, wet     R       R      R      X      X      X       R (20) R (20)   X   X        X        X       X      X      X

Sulphur dioxide, (96%)   R
                                 R      R      R (11) R      R       R (11) R        R   R        R (20)   X       R (20) R (20) R (20)
                         (11)b

Sulphur trioxide         R       R      R      X      X      X       R      R        R   R (11)   R        X       R (11) R      R

Sulphuric acid (<50%)    R       R (62) X      X      X      X       X      X        X   R        X        X       X      X      X

Sulphuric acid (70%)     R (62) X       X      X      X      X       X      X        X   R        X        X       X      X      X

Sulphuric acid (95%)     X       X      X      X      X      X       X      X        X   R        X        X       X      X      X

Sulphuric acid, fuming   X       X      X      X      X      X       X      X        X   X        X        X       X      X      X

Sulphur chlorides                              No     No     No
                         R       R      R                            R      R        R   X        X        X       X      X      X
                                               data   data   data

Tallow                                                                                                             No     No     No
                         R       R      R      R      R      R       R      R        R   R        R        R
                                                                                                                   data   data   data

Tannic acid (10%)        R       R      R      R      R      R       R      R        R   R        R        R       R      R      R

Tartaric acid            R       R      R      R      R      R       R      R        R   R        R        R       R      R      R

Trichlorethylene         R       R      X      R      R      X       R      R        X   R        R        R       R      R      R

Urea (30%)                                                                                                                       No
                         R       R      R      X      X      X       X      X        X   R        R        X       R      R
                                                                                                                                 data

Vinegar                  R (53) R       X      X      X      X       R (53) R        X   R        R        R       X      X      X

Water, distilled         RR      R      R      R      R      R       R      R        R   R        R        R       R (53) R      R

Water, soft              R       R      R      R      R      R       R      R        R   R        R        R       R      R      R

Water, hard              R       R      R      R      R      R       R      R        R   R        R        R       R      R      R

Wetting agents (to 5%)   No      No     No     No     No     No
                                                                     R      R        R   R        R        R       R      R      R
                         data    data   data   data   data   data

Yeast                    R       R      R      X      X      X       X      X        X   No data No data No data R        R      R

Zinc chloride            X       X      X      X      X      X       X      X        X   X        X        X       X      X      X




                                                                  Back to Top
                                                                                                 BACK   INDEX




Footnotes:


(a) Brass: Some type of brass have less corrosion resistance than is shown on the chart, others have more, e.g.
Aluminium brass.
(b) Copper-nickel alloys: Based on behaviour of Cu/Ni 90/10; 70/30 may be generally more resistant.
(c) Gunmetal: The data refer only to high tin gunmetals.
(2) Depending on the acid.
(4) Fair resistance.
(11) Anhydrous
(20) Not aerated solutions
(30) Depending on composition
(36) Over 85%.
(53) In absence of dissolved O2 and CO2
(60) May discolour liquid/ product
(62) Depending on type.
(73) Not ammonium.
(82) Provided more than 70% copper.
(83) Water less than 150ppm.
(119) Pure solution.
(175) With stabilizer


More reading:
http://www.hghouston.com/coppers/copper.html


Content: Copper and Copper Alloys


    •    Copper Corrosion Resistance Data
    •    Aluminum Bronze
    •    Brasses
    •    Copper Nickel Alloys
    •    Corrosion of Copper in Downhole Environments


http://www.copper.org/resources/properties/protection/homepage.html
                                                                                      BACK     INDEX




Corrosion of Titanium and Titanium Alloys

Titanium alloys were originally developed in the early 1950s for aerospace applications, in which
their high strength-to-density ratios were especially attractive. Although titanium alloys are still
vital to the aerospace industry for these properties, recognition of the excellent resistance of
titanium to many highly corrosive environments, particularly oxidizing and chloride-containing
process streams, has led to widespread non-aerospace (industrial) applications.

Because of decreasing cost and the increasing availability of titanium alloy products, many
titanium alloys have become standard engineering materials for a host of common industrial
applications. In fact, a growing trend involves the use of high-strength aerospace-founded
titanium alloys for industrial service in which the combination of strength to density and corrosion
resistance properties is critical and desirable.

The excellent corrosion resistance of titanium alloys results from the formation of very stable,
continuous, highly adherent, and protective oxide films on metal surfaces. Because titanium metal
is highly reactive and has an extremely high affinity for oxygen, these beneficial surface oxide
films form spontaneously and instantly when fresh metal surfaces are exposed to air and/or
moisture. In fact, a damaged oxide film can generally reheal itself instantaneously if at least traces
of oxygen or water are present in the environment. However, anhydrous conditions in the absence
of a source of oxygen may result in titanium corrosion, because the protective film may not be
regenerated if damaged.

The nature, composition, and thickness of the protective surface oxides that form on titanium
alloys depend on environmental conditions. In most aqueous environments, the oxide is typically
TiO2, but may consist of mixtures of other titanium oxides, including TiO2, Ti2O3, and TiO.
High-temperature oxidation tends to promote the formation of the chemically resistant, highly
crystalline form of TiO, known as rutile, whereas lower temperatures often generate the more
amorphous form of TiO, anatase, or a mixture of rutile and anatase.

Although these naturally formed films are typically less than 10 nm thick and are invisible to the
eye, the TiO; oxide is highly chemically resistant and is attacked by very few substances, including
hot, concentrated HCl, H2SO4, NaOH, and (most notably) HF. This thin surface oxide is also a
highly effective barrier to hydrogen.

The methods of expanding the corrosion resistance of titanium into reducing environments
include:

   •   Increasing the surface oxide film thickness by anodizing or thermal oxidation
   •   Anodically polarizing the alloy (anodic protection) by impressed anodic current or galvanic
       coupling with a more noble metal in order to maintain the surface oxide film
   •   Applying precious metal (or certain metal oxides) surface coatings
   •   Alloying titanium with certain elements
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   •   Adding oxidizing species (inhibitors) to the reducing environment to permit oxide film
       stabilization

Titanium alloys, like other metals, are subject to corrosion in certain environments. The primary
forms of corrosion that have been observed on these alloys include general corrosion, crevice
corrosion, anodic pitting, hydrogen damage, and SCC.

In any contemplated application of titanium, its susceptibility to degradation by any of these forms
of corrosion should be considered. In order to understand the advantages and limitations of
titanium alloys, each of these forms of corrosion will be explained. Although they are not common
limitations to titanium alloy performance, galvanic corrosion, corrosion fatigue, and
erosion-corrosion are included in the interest of completeness.

General corrosion is characterized by a relatively uniform attack over the exposed surface of the
metal. At times, general corrosion in aqueous media may take the form of mottled, severely
roughened metal surfaces that resemble localized attack. This often results from variations in the
corrosion rates of localized surface patches due to localized masking of metal surfaces by process
scales, corrosion products, or gas bubbles; such localized masking can prevent true uniform
surface attack.

Titanium alloys may be subject to localized attack in tight crevices exposed to hot (>70 oC)
chloride, bromide, iodide, fluoride, or sulfate-containing solutions. Crevices can stem from
adhering process stream deposits or scales, metal-to-metal joints (for example, poor weld joint
design or tube-to-tubesheet joints), and gasket-to-metal flange and other seal joints.

Pitting is defined as localized corrosion attack occurring on openly exposed metal surfaces in the
absence of any apparent crevices. This pitting occurs when the potential of the metal exceeds the
anodic breakdown potential of the metal oxide film in a given environment. When the anodic
breakdown potential of the metal is equal to or less than the corrosion potential under a given set
of conditions, spontaneous pitting can be expected.

Titanium alloys are widely used in hydrogen containing environments and under conditions in
which galvanic couples or cathodic charging causes hydrogen to be evolved on metal surfaces.
Although excellent performance is revealed for these alloys in most cases, hydrogen
embrittlement has been observed.

The surface oxide film of titanium is a highly effective barrier to hydrogen penetration. Traces of
moisture or oxygen in hydrogen-containing environments very effectively maintain this protective
film, thus avoiding or limiting hydrogen uptake. On the other hand, anhydrous hydrogen gas
atmospheres may lead to absorption, particularly as temperatures and pressures increase.

Stress-corrosion cracking (SCC) is a fracture, or cracking, phenomenon caused by the
combined action of tensile stress, a susceptible alloy, and a corrosive environment. The metal
normally shows no evidence of general corrosion attack, although slight localized attack in the
form of pitting may be visible. Usually, only specific combinations of metallurgical and
                                                                                      BACK     INDEX




environmental conditions cause SCC. This is important because it is often possible to eliminate or
reduce SCC sensitivity by modifying either the metallurgical characteristics of the metal or the
makeup of the environment.

Another important characteristic of SCC is the requirement that tensile stress is present. These
stresses may be provided by cold work, residual stresses from fabrication, or externally applied
loads.

The key to understanding SCC of titanium alloys is the observation that no apparent corrosion,
either uniform or localized, usually precedes the cracking process. As a result, it can sometimes be
difficult to initiate cracking in laboratory tests by using conventional test techniques.

It is also important to distinguish between the two classes of titanium alloys. The first class, which
includes ASTM grades 1, 2, 7, 11 and 12, is immune to SCC except in a few specific environments.
These specific environments include anhydrous methanol/halide solutions, nitrogen tetroxide
(N2O4), and liquid or solid cadmium. The second class of titanium alloys, including the aerospace
titanium alloys, has been found to be susceptible to several additional environments, most notably
aqueous chloride solutions.

The coupling of titanium with dissimilar metals usually does not accelerate the corrosion of
titanium. The exception is in strongly reducing environments in which titanium is severely
corroding and not readily passivated. In this uncommon situation, accelerated corrosion may
occur when titanium is coupled to more noble metals. In its normal passive condition, materials
that exhibit more noble corrosion potentials beneficially influence titanium.

The general corrosion resistance of titanium can be improved or expanded by one or a
combination of the following strategies:

   •     Alloying
   •     Inhibitor additions to the environment
   •     Precious metal surface treatments
   •     Thermal oxidation
   •     Anodic protection.

Alloying. Perhaps the most effective and preferred means of extending resistance to general
corrosion in reducing environments has been by alloying titanium with certain elements. Beneficial
alloying elements include precious metals (>0.05 wt% Pd), nickel ( >= 0.5 wt%), and/or
molybdenum (>= 4 wt%). These additions facilitate cathodic depolarization by providing sites of
low hydrogen overvoltage, which shifts alloy potential in the noble direction where oxide film
passivation is possible. Relatively small concentrations of certain precious metals (of the order of
0.1 wt%) are sufficient to expand significantly the corrosion resistance of titanium in reducing acid
media.

These beneficial alloying additions have been incorporated into several commercially available
titanium alloys, including the titanium-palladium alloys (grades 7 and 11), Ti-0.3Mo-0.8Ni (grade
                                                                                  BACK     INDEX




12), Ti-3Al-8V-6Cr-4Zr-4Mo, Ti-15Mo-5Zr, and Ti-6Al-2Sn-4Zr-6Mo. These alloys all offer
expanded application into hotter and/or stronger HCl, H2SO4, H3PO4, and other reducing acids as
compared to unalloyed titanium. The high-molybdenum alloys offer a unique combination of high
strength, low density, and superior corrosion resistance.




Fig 1. Corrosion of dissimilar metals coupled to titanium in flowing ambient-temperature seawater
                                                                                                   BACK         INDEX




Corrosion Resistance
The corrosion resistance of titanium is well documented. A stable, substantially inert oxide film provides the
material with outstanding resistance to corrosion in a wide range of aggressive media. Whenever fresh
titanium is exposed to the atmosphere or to any environment containing oxygen, it immediately acquires a thin
tenacious film of oxide. It is the presence of this surface film that confers on the material its excellent corrosion
resistance. Provided that sufficient oxygen is present, the film is self healing and re-forms almost at once if
mechanically damaged.

Oxidising and Non-Oxidising Environments

Since titanium depends for its passivity on the presence of an oxide film, it follows that it is significantly more
resistant to corrosion in oxidising solutions than in non-oxidising media where high rates of attack can occur.
Thus the material can be used in all strengths of aqueous nitric acid at temperatures up to the boiling point.
Similarly, it is not attacked by wet chlorine gas and by solutions of chlorine compounds such as sodium chlorite
and hypochlorite.

There is no evidence of pitting or stress corrosion cracking in aqueous solutions of inorganic metal chlorides.
Titanium also has exceptional resistance to sea water even under high velocity conditions or in polluted water.
While the material normally has a significant corrosion rate in media such as sulphuric or hydrochloric acids
which produce hydrogen on reaction with the metal, the presence of a small amount of oxidising agent in the
acid results in the formation of a passive film. Hence, titanium is resistant to attack in mixtures of strong
sulphuric and nitric acids, hydrochloric and nitric acids and even in strong hydrochloric acid containing free
chlorine. The presence in solution of cupric or ferric ions also reduces the corrosion rate, as does alloying with
noble metals or the use of an anodic protection technique.

Formation of Protective Oxide Films

Protective oxide films on titanium are usually formed when the metal has access to water, even though this
may only be present in trace quantities or in vapour form. Thus, if titanium is exposed to highly oxidising
environments in the complete absence of water, rapid oxidation can occur and a violent, often pyrophoric,
reaction results. Examples of this type of behaviour are found in reactions between titanium and dry nitric acid
and between titanium and dry chlorine. However, the amount of moisture necessary to prevent attack under
these conditions is small and can be as little as 50 ppm.

Summary of Corrosion Resistance

The corrosion resistance of commercially pure titanium to simple chemical environments is summarised in
Table 1.
                                                                                           BACK   INDEX




                       Table 1. Resistance of pure titanium to simple chemical reagents.

Reagent                                              Concentration          Temperature       Rating
                                                     (% by wt.)             (°C)
Acetic Acid                                          5,25,50,75,99.5        Boiling           A
Acetic Anhydride                                     99                     Boiling           A
Aluminium Chloride                                   5,10                   100               A
                                                     25                     100               C
Ammonia, Anhydrous                                   100                    40                A
Ammonium Chloride                                    1,10,saturated         100               A
Ammonium Hydroxide                                   28                     Room,60,100       A
Aqua Regia (1 HNO3:3 HCl)                            -                      Room,60           A
Barium Chloride                                      5,20                   100               A
Benzene                                              -                      Room              A
Benzoic Acid                                         Saturated              Room,60           A
Boric Acid                                           10                     Boiling           A
Bromine                                              Liquid                 Room              C
Bromine-saturated Water                              -                      Room,60           A
Calcium Chloride                                     5,10,25,28             100               A
                                                     73                     177               C
Calcium Hypochlorite                                 2,6                    100               A
Chlorine Gas, Dry                                    -                      30                C
Chlorine Gas, Wet                                    -                      75                A
Chromic Acid                                         10,50                  Boiling           A
Citric Acid, Aerated                                 10,25,50               100               A
Cupric Chloride                                      55                     118               A
Ethyl Alcohol                                        95                     Boiling           A
Ethylene Dichloride                                  100                    Boiling           A
Ferric Chloride                                      50                     113,150           A
Formic Acid, Aerated                                 10,25,50,90            100               A
Formic Acid, Non-Aerated                             10                     Boiling           A
                                                     25,50                  Boiling           C
Hydrobromic Acid                                     30                     Room              A
Hydrochloric Acid                                    1,3                    60                A
                                                     2,3                    100               C
                                                     15,37                  35                C
Hydrofluoric Acid                                    1                      Room              C
Hydrogen Sulfide                                     -                      70                A
Iodine                                               -                      130               C
Lactic Acid                                          100                    Boiling           A
Magnesium Chloride                                   5,20,42                Boiling           A
Magnesium Sulfate                                    Saturated              Room              A
Manganous Chloride                                   5,20                   100               A
Mercuric Chloride                                    1,5,10,Saturated       100               A
                                                                                                BACK      INDEX




 Methyl Alcohol                                       99                      60                     C
 Nickel Chloride                                      5,20                    100                    A
 Nitric Acid                                          All                     Boiling                A
                                                      Red Fuming              Room,50,70             C
 Oxalic Acid                                          0.5,1,5,10              35                     A
                                                      0.5,1,5,10              60,100                 C
 Phosphoric Acid                                      5,10,20,30              35                     A
                                                      35-80                   35                     B
                                                      10                      80                     C
 Potassium Chloride                                   36                      111                    A
 Potassium Hydroxide                                  10                      Boiling                A
 Sodium Chloride                                      Saturated               Room, 111              A
 Sodium Dichromate                                    Saturated               Room                   A
 Sodium Hydroxide                                     10                      Boiling                A
                                                      73                      113-129                B
 Sodium Hypochloride                                  10 g/l Cl2              Boiling                A
 Sodium Nitrate                                       Saturated               Room                   A
 Sodium Phosphate                                     Saturated               Room                   A
 Sodium Sulphide                                      Saturated               Room                   A
 Sodium Sulphite                                      Saturated               Room                   A
 Stearic Acid                                         100                     180                    A
 Sulphur, Molten                                      100                     240                    A
 Sulphur Dioxide, Dry                                 100                     Room,60                A
 Sulphur Dioxide + Water                              -                       Room,70                A
 Sulphuric Acid                                       1,3,5                   35                     AB
                                                      10                      35                     B
                                                      20-50                   35                     C
                                                      1,5                     Boiling                C
 Tataric Acid                                         10,25,50                100                    A
 Trichloroethylene                                    -                       Boiling                A
 Zinc Chloride                                        20,50,75                150                    A
                                                      75                      200                    B


Effect of Alloying Elements

Generally, titanium alloys that have been developed for high strength and good creep resistant properties have
inferior corrosion resistance to the commercially pure material, but there are some alloying additions that can
improve corrosion properties. By comparison with alloys for aerospace, there has only been a restricted
amount of work carried out to develop titanium alloys for corrosion resistant applications. One of the most
successful of these involves the addition of small amounts of palladium to the commercially pure material. This
not only improves its resistance to reducing acids such as sulphuric, hydrochloric, and phosphoric but also
raises the critical temperature at which crevice corrosion in sea water can occur. This principle of palladium
additions is now being extended to some of the higher strength alloys in order to combine corrosion resistance
with good tensile properties. Other corrosion resistant alloys that have been developed over the years include
Ti-0.8%Ni-0.3%Mo as a possible substitute for Ti/Pd alloys, and Ti-6%Al-7%Nb which is used as a surgical
                                                                                                   BACK       INDEX




implant material.

Galvanic Corrosion

When designing equipment for the chemical or oil industries or for some general engineering applications it is
essential to consider the deleterious galvanic effects that may result from contact between dissimilar metals. If
two metals are coupled together in an electrolyte, the less noble or anodic member of the couple will normally
tend to corrode, the extent of the attack depending upon the difference in electrode potential between the two
materials and also on the relative anode to cathode area ratios. Titanium differs from most materials in that, if
coupled to a more noble metal in an aggressive solution, the electrode potential of the titanium tends to be
raised and the corrosion rate is reduced rather than increased.

As a practical example, consider the case of pipework systems handling seawater (see Figure 1). Ideally these
would be fabricated entirely from titanium but where this is not possible, alloys which are galvanically near
compatible with titanium such as Inconel 625, Hastelloy C, 254 SMO, Xeron 100 or composite materials may
be selected to be in direct contact with titanium at joints. Although several of the highly alloyed stainless steels
and nickel based alloys are only marginally less noble than titanium in their passive state, once they become
active the rate of localised attack can be dramatic, leading to rapid failure.




    Figure 1. Galvanic corrosion of titanium-dissimilar metal couples at different area ratios in static sea water.

In situations where it is not possible to avoid galvanic contact between titanium and a less noble metal, there
are a number of possible techniques to reduce the risk of corrosion:

·        Coating of the titanium in the vicinity of the joint to reduce the effective cathode to anode surface area
ratio;
                                                                                                     BACK      INDEX




·       Application of cathodic protection;

·       Electrical insulation of the titanium by the use of non-conducting gaskets and sleeved bolts;

·       Installation of short easily replaced flanged sacrificial heavier wall sections of the less noble metal;

·       Chemical dosing.

Crevice Corrosion

Most metals are subject to increased corrosion in crevices formed between themselves and other metals or
non-metals. The reason for this preferential corrosion is that, because of restriction in circulation of the
solution, there is either a differential concentration effect or differential aeration within the crevice. This can
lead to a difference in electrode potential between the metal in the crevice and that outside it, where free
circulation of solution is possible. A galvanic reaction can then be set up between the two areas.

Titanium is particularly resistant to this form of attack and is only subject to it in certain specific instances. For
example, corrosion has been reported in an application involving wet chlorine but attempts to reproduce it in
the laboratory have been largely unsuccessful. This attack has been attributed to the fact that slow dehydration
of the wet chlorine can occur in crevices where there is a large ratio of metal area to gas volume. Crevice
corrosion under heat transfer conditions is possible in sodium chloride solutions at temperatures down to 70°C
but the pH of the solution is important. This is illustrated in Figure 2.




    Figure 2. Influence of temperature, concentration and pH on crevice and pitting corrosion of commercially
                              pure titanium in sea water and sodium chloride brines.
                                                                                                   BACK      INDEX




Effect of Crevice Size and Shape

With titanium, the shape and size of crevice appear to have a critical influence on corrosion behaviour. When
the two surfaces are close together they are either not wetted by the corrodent or, if they are wetted initially,
the flow of solution is restricted and corrosion is stifled before the titanium oxide film is disrupted. When the
surfaces are too far apart, diffusion of oxygen is sufficiently rapid to passivate the material.

Crevice Corrosion Resistant Alloys

The use of titanium/palladium alloys virtually eliminates the risk of crevice corrosion in sea water. This is
illustrated in Figure 3.




          Figure 3. Influence of temperature, concentration and pH on crevice and pitting corrosion of
                           titanium/palladium in sea water and sodium chloride brines.

Stress Corrosion

Although titanium and its alloys are resistant to corrosion in many media, including aqueous solutions of
chlorides, stress corrosion of commercially pure titanium and of titanium alloys can take place in a limited
number of highly specific environments.

Red Fuming Nitric Acid Environments

The first reported instance of stress corrosion cracking of titanium was in red fuming nitric acid. Here, cracking
                                                                                                   BACK       INDEX




was mainly intergranular but the phenomenon only occurred under anhydrous conditions, the presence of as
little as 1.5 to 2% water completely inhibiting the reaction. All titanium alloys are susceptible to stress corrosion
in this environment but for some the presence of excess nitrogen dioxide is necessary while others can crack
in the absence of this component.

Methanol Environments

The only other environment that has been shown to cause stress corrosion of commercially pure titanium as
well as titanium alloys is methanol. Failure again is by intergranular cracking and the mechanism is more likely
if bromine, chlorine, or iodine ions are present in the alcohol. Again the presence of a small amount of water
will completely prevent attack, 4% giving immunity to all grades and all alloys.

Chlorinated Hydrocarbon Atmospheres

While commercially pure titanium is not affected, stress corrosion of some titanium alloys can take place in
chlorinated hydrocarbons. It is known, for example, that on prolonged exposure at elevated temperatures in
the presence of some metals, the vapours of trichlorethylene can partially decompose to form hydrochloric
acid. This causes stress corrosion of certain titanium alloys, particularly those containing aluminium and care
must be taken when degreasing these materials. However, even with these alloys the operation is perfectly
safe if attention is paid to working conditions. The correct degreasants containing additions to prevent
decomposition should be used and the time of contact between the titanium and the degreasant should not be
excessively long.

Hot Salt Stress Corrosion Cracking

Although it has been demonstrated in laboratory tests that titanium alloys are susceptible to hot salt stress
corrosion cracking, no service failures have ever been reported, even though titanium alloys have been used in
aerospace applications at temperatures as high as 600°C. When cracking does take place it can either be
intergranular or transgranular in form and all the commercially available alloys except the commercially pure
grades are susceptible to some degree.

Pitting

Titanium and its alloys are extremely resistant to pitting attack in seawater and other chloride containing
solutions at ambient and moderately elevated temperatures. However, if a titanium alloy sample containing an
existing fatigue crack is loaded under plane strain conditions, the presence of seawater will reduce the
resistance of the material to crack propagation. The susceptibility of titanium alloys to this form of cracking
appears to be adversely affected by aluminium, tin and oxygen contents, whereas the presence of certain beta
stabilisers such as niobium and tantalum reduces the risk of attack. Commercially pure grades are not affected
at oxygen levels below 0.32%.

Erosion Resistance

Erosion is an accelerated form of attack usually associated with high water velocities and with local turbulence
which removes the oxide from the surface of film forming metals thus exposing bare metal to the corrodent. As
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a result of its ability to repair its protective oxide film quickly, titanium has an extremely high resistance to this
form of attack. In pure sea water, for example, erosion is negligible at flow rates as high as 18 m s-1. It is even
resistant to seawater containing sand and carborundum grit flowing at 2 m s-1. The erosion rate under these
conditions corresponds to a penetration of only 1 mm in nearly eight years. It is notable, however, that with
very coarse carborundum at higher speeds the erosion rate of titanium is higher than that of materials such as
cupro-nickel. This is because, under these conditions, there is not sufficient time for the oxide film to reform
and the underlying titanium is of lower hardness than cupro-nickel. These test conditions are very much more
severe than those normally encountered in service, however, and it has been amply demonstrated that
titanium is completely unaffected in condensers and coolers handling waters having a high sand content,
whereas under the same conditions cupro-nickels can fail within 2 to 3 years.

Under those conditions where tubes have become blocked by extraneous matter, impingement attack causing
rapid failure of copper base materials has not affected titanium. This has been substantiated in service and in
experimental heat exchangers running under laboratory conditions at flow rates of at least 4 m s-1.




CORROSION PROPERTIES


GENERAL


Titanium and its alloys provide excellent resistance to general and localized attack under most oxidizing,
neutral and inhibited reducing conditions in aqueous environments. They also remain passive under mildly
reducing conditions, although they may be attacked by strongly reducing or complexing media. Titanium is
especially known for its outstanding resistance to chlorides and other halides generally present in most
process streams.


Titanium's corrosion resistance is due to a stable, protective, strongly adherent oxide film which forms instantly
when a fresh surface is exposed to air or moisture. This passive film is typically less than 250 A. (A, an
angstrom, is 4 x 10^-9 in.) Film growth is accelerated under strongly oxidizing conditions such as in HNO3 and
CrO3 (nitric acid, chromic acid), etc. media.


The composition of this film varies from TiO2 at the surface to Ti2O3 to TiO at the metal interface. Oxidizing
conditions promote the formation of TiO2. This film is transparent in its normal thin configuration and not
detectable by visual means.


A study of the corrosion resistance of titanium is basically a study of the properties of the oxide film. The oxide
film on titanium is very stable and is attacked only by a few substances including hot concentrated reducing
acids, most notably, hydrofluoric acid. Titanium is capable of healing this film almost instantaneously in every
environment where a trace of moisture or oxygen is present because of titanium's strong affinity for oxygen.


Anhydrous conditions in the absence of a source of oxygen should be avoided since the protective film may
not be regenerated if damaged.
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RESISTANCE TO WATERS



FRESH WATER - STEAM
Titanium resists all forms of corrosive attack by fresh water and steam to temperatures as high as 600 degrees
F (316 degrees C). The corrosion rate is very low and a slight weight gain is generally experienced. Titanium
surfaces are likely to acquire a tarnished appearance in hot water or steam but will be free of corrosion.


Some natural river waters contain manganese which deposits as manganese dioxide on heat exchanger
surfaces. This is harmful and promotes pitting in both austenitic stainless steels and copper alloys.
Chlorination treatments used to control sliming result in severe pitting and crevice corrosion on stainless steel
surfaces. Titanium is immune to these forms of corrosion and is an ideal material for handling all natural waters.


SEAWATER - GENERAL CORROSION
Titanium resists corrosion by seawater to temperatures as high as 500 degrees F (260 degrees C). Titanium
tubing which has been exposed to seawater for many years at depths of over a mile shows no measurable
corrosion. It has provided over twenty five years of trouble-free seawater service for the chemical, oil refining
and desalination industries. Pitting and crevice corrosion are totally absent, even when marine deposits form.
The presence of sulfides in seawater does not affect the resistance of titanium to corrosion. Exposure of
titanium to marine atmospheres or splash or tidal zones does not cause corrosion.


EROSION
Titanium has the ability to resist erosion by high velocity seawater. Velocities as high as 120 ft./sec. cause only
minimal rise in the erosion rate. The presence of abrasive particles, such as sand, has only a small effect on
the corrosion resistance of titanium under conditions that are extremely detrimental to copper and aluminum
base alloys. Titanium is considered one of the best cavitation-resistant materials available for seawater service.


STRESS-CORROSION CRACKING
TIMETAL 35A and TIMETAL 50A are essentially immune to stress- corrosion cracking (SCC) in seawater. This
has been confirmed many times. Other unalloyed titanium grades with an oxygen content greater than 0.25
wt.% may be susceptible to SCC under some conditions.


CORROSION FATIGUE
Titanium, unlike many other materials, does not suffer a significant loss of fatigue properties in seawater. In
fatigue- limited applications, Boiler Code criteria or actual in situ fatigue testing should be considered.


CREVICE CORROSION
Crevice corrosion of unalloyed titanium may occur in seawater at temperatures above the boiling point.
TIMETAL Code-12 (Grade 12) and TIMETAL 50A Pd (Grades 7 and 16) and 35A Pd (Grades 11 and 17) offer
resistance to crevice corrosion in seawater at temperatures up to 500 degrees F (260 degrees C).
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GALVANIC CORROSION
The Coupling of titanium with dissimilar metals does not usually accelerate the corrosion of the titanium. The
exception is in highly reducing acidic environments where titanium may not passivate. Under these conditions,
it has a potential similar to aluminum and will undergo accelerated corrosion when coupled to other more noble
metals.
Table 1




 gives the galvanic series in seawater. In this environment titanium is passive and exhibits a potential of about
0.0 V versus a saturated calomel reference cell (SCE) which places it high on the passive or noble end of the
series.

For most environments, titanium will be the cathodic member of any galvanic couple. It may accelerate the
corrosion of the other member of the couple, but in most cases, the titanium will generally remain unaffected.
Figure 2
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shows the accelerating effect that titanium has on the corrosion rate of various metals when they are
galvanically coupled in seawater. If the area of the titanium exposed is small in relation to the area of the other
metal, the effect on the corrosion rate is negligible. However, if the area of the titanium (cathode) greatly
exceeds the area of the other metal (anode), severe corrosion of the other metal may result.


Because titanium is the cathodic member, hydrogen may be evolved on its surface proportional to the galvanic
current flow. This may result in the formation of surface hydride films that are generally stable and cause no
problems, If the temperature is above 176 degrees F (80 degrees C), however, hydrogen may diffuse into the
metal and cause hydride-related embrittlement.
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In order to avoid problems with galvanic corrosion, it is best to construct equipment of a single metal. If this is
not practical, use two metals that are close together in the galvanic series, insulate the joint or cathodically
protect the less noble metal. If dissimilar metals are necessary, and since titanium is usually not attacked,
construct the critical parts from titanium, and use large areas of the less noble metal and heavy sections to
allow for increased corrosion.




More Reading:


                              Title: Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of
                                                                 Metals

                                       Division: General Chemical Engineering / CRC Press / 英文版

                                          Author/Editor: Philip A. Schweitzer, P.E. Star:

                                                             ISBN: 0849382432

                           Introduce Date: 2007 年 05 月 05 日 19:48 , Release Date: 2007 年 05 月 05 日 19:51

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                           Format: pdf (editorial)


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    Cathodic                                                       Contents                                     page




    Protection                                                     Introduction
                                                                   History
                                                                                                                  1
                                                                                                                  1
                                                                   The Principles of Cathodic Protection          1
                                                                       Sacrificial anodes                         2
                                                                       Impressed current                          2
                                                                   Advantages and Uses of Cathodic Protection     2
                                                                       Pipelines
                                                                       Storage tanks
                                                                       Steel pilings
                                                                       Reinforced concrete
                                                                       Ships
                                                                       Offshore structures
                                                                   Basic Requirements for Cathodic Protection     3
                                                                   Design Factors                                 4
                                                                   Monitoring and Maintenance                     6
                                                                   Sources of advice                              7
                                                                   Further Information                            7




This is an update of a DTI publication first issued in 1981. The
new version has been prepared by Eur Ing R. L. Kean of ARK
Corrosion Services and Mr K. G. Davies, Corrosion Engineer,
under contract from NPL for the Department of Trade and
Industry.
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                                                                                                                              Cathodic Protection


1.0 Introduction                                                        Corrosion is an electro-chemical process that involves the
                                                                        passage of electrical currents on a micro or macro scale. The
This Guide describes the basic principles of cathodic protection,
                                                                        change from the metallic to the combined form occurs by an
the areas of use, and the general factors to be considered in the
                                                                        “anodic” reaction:
choice and design of a system. It gives a basic introduction and
                                                                                                                     +
                                                                                 M                  →            M            +            e-
simple technical data on cathodic protection. Further assistance
and information may be gained from organisations listed in                       (metal)                  (soluble salt)                (electron)

Section 10, various independent or commercial consultants, and          A common example is:
                                                                                                                     ++
product suppliers.                                                               Fe                 →            Fe           +            2e-
                                                                        This reaction produces free electrons, which pass within the
                                                                        metal to another site on the metal surface (the cathode), where
2.0 History
                                                                        it is consumed by the cathodic reaction. In acid solutions the
The first reported practical use of cathodic protection is
                                                                        cathodic reaction is:
generally credited to Sir Humphrey Davy in the 1820s. Davy’s                          +
                                                                                 2H       +                      2e-          →            H2
advice was sought by the Royal Navy in investigating the                     (hydrogen ions                                                (gas)
corrosion of copper sheeting used for cladding the hulls of naval               in solution)

vessels. Davy found that he could preserve copper in seawater
                                                                        In neutral solutions the cathodic reaction involves the
by the attachment of small quantities of iron, zinc or tin. The
                                                                        consumption of oxygen dissolved in the solution:
copper became, as Davy put it, “cathodically protected”. It was
                                                                                O2              +         2H2O            +       4e-      →             4OH-
quickly abandoned because by protecting the copper its anti-                                                                                             (alkali)
fouling properties became retarded, hence reducing the
                                                                        Corrosion thus occurs at the anode but not at the cathode
streamline of the ships, as they began to collect marine growths.
                                                                        (unless the metal of the cathode is attacked by alkali).

The most rapid development of cathodic-protection was made in
the United States of America and by 1945, the method was well
established to meet the requirements of the rapidly expanding                                                                              -         -
                                                                            2M → 2M
                                                                                          ++
                                                                                               + 4e
                                                                                                      -
                                                                                                                         O2 + 2H2O + 4e → 4OH
oil and natural gas industry, which wanted to benefit from the
advantages of using thin-walled steel pipes for underground                    (corrosion)

transmission.
                                                                                                               -
                                                                                                 Electron (e ) flow in metal

In the United Kingdom, where low-pressure, thicker-walled cast-
iron pipes were used extensively, very little cathodic protection
was applied until the early 1950s. The increasing use of
cathodic protection in modern times has arisen, in part, from the       Figure 1. Corrosion cell / Bimetallic corrosion

initial success of the method as used from 1952 onwards to
protect about 1000 miles of wartime fuel-line network. The              The anode and cathode in a corrosion process may be on two

method is now well established and is used on a wide variety of         different metals connected together forming a bimetallic couple,

immersed and buried facilities and infrastructure, as well as           or, as with rusting of steel, they may be close together on the

reinforced concrete structures, to provide corrosion control.           same metal surface.
                                                                        This corrosion process is initially caused by:
                                                                        Differerence in natural potential in galvanic (bimetallic) couples.
3.0 The Principles of Cathodic Protection
                                                                        Metallurgical variations in the state of the metal at different
Metal that has been extracted from its primary ore (metal oxides
                                                                        points on the surface.
or other free radicals) has a natural tendency to revert to that
                                                                        Local differences in the environment, such as variations in the
state under the action of oxygen and water. This action is called
                                                                        supply of oxygen at the surface (oxygen rich areas become the
corrosion and the most common example is the rusting of steel.
                                                                        cathode and oxygen depleted areas become the anode).




                                                                    1                                                                                               1
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                                                                                                                Cathodic Protection


                                                                         4.0 Advantages and Uses of Cathodic
The principle of cathodic protection is in connecting an external
                                                                               Protection
anode to the metal to be protected and the passing of an
                                                                         The main advantage of cathodic protection over other forms of
electrical dc current so that all areas of the metal surface
                                                                         anti-corrosion treatment is that it is applied simply by
become cathodic and therefore do not corrode. The external
                                                                         maintaining a dc circuit and its effectiveness may be monitored
anode may be a galvanic anode, where the current is a result of
                                                                         continuously. Cathodic protection is commonly applied to a
the potential difference between the two metals, or it may be an
                                                                         coated structure to provide corrosion control to areas where the
impressed current anode, where the current is impressed from
                                                                         coating may be damaged. It may be applied to existing
an external dc power source. In electro-chemical terms, the
                                                                         structures to prolong their life.
electrical potential between the metal and the electrolyte
solution with which it is in contact is made more negative, by the
                                                                         Specifying the use of cathodic protection initially will avoid the
supply of negative charged electrons, to a value at which the
                                                                         need to provide a “corrosion allowance” to thin sections of
corroding (anodic) reactions are stifled and only cathodic
                                                                         structures that may be costly to fabricate. It may be used to
reactions can take place. In the discussion that follows it is
                                                                         afford security where even a small leak cannot be tolerated for
assumed that the metal to be protected is carbon steel, which is
                                                                         reasons of safety or environment. Cathodic protection can, in
the most common material used in construction. The cathodic
                                                                         principle, be applied to any metallic structure in contact with a
protection of reinforcing carbon steel in reinforced concrete
                                                                         bulk electrolyte (including concrete). In practice, its main use is
structures can be applied in a similar manner.
                                                                         to protect steel structures buried in soil or immersed in water. It
                                                                         cannot be used to prevent atmospheric corrosion on metals.
Cathodic protection can be achieved in two ways:
                                                                         However, it can be used to protect atmospherically exposed
    - by the use of galvanic (sacrificial) anodes, or
                                                                         and buried reinforced concrete from corrosion, as the concrete
    - by “impressed” current.
                                                                         itself contains sufficient moisture to act as the electrolyte.

Galvanic anode systems employ reactive metals as auxiliary
                                                                         Structures that are commonly protected by cathodic protection
anodes that are directly electrically connected to the steel to be
                                                                         are the exterior surfaces of:
protected. The difference in natural potentials between the
                                                                               Pipelines
anode and the steel, as indicated by their relative positions in
                                                                               Ships’ hulls
the electro-chemical series, causes a positive current to flow in
                                                                               Storage tank bases
the electrolyte, from the anode to the steel. Thus, the whole
                                                                               Jetties and harbour structures
surface of the steel becomes more negatively charged and
                                                                               Steel sheet, tubular and foundation pilings
becomes the cathode. The metals commonly used, as
                                                                               Offshore platforms, floating and sub sea structures
sacrificial anodes are aluminium, zinc and magnesium. These
metals are alloyed to improve the long-term performance and
                                                                         Cathodic protection is also used to protect the internal surfaces
dissolution characteristics.
                                                                         of:
                                                                               Large diameter pipelines
Impressed-current systems employ inert (zero or low
                                                                               Ship’s tanks (product and ballast)
dissolution) anodes and use an external source of dc power
                                                                               Storage tanks (oil and water)
(rectified ac) to impress a current from an external anode onto
                                                                               Water-circulating systems.
the cathode surface.
                                                                         However, since an internal anode will seldom spread the
                                                                         protection for a distance of more than two to five pipe-
The connections are similar for the application of cathodic
                                                                         diameters, the method is not usually practical, or suitable, for
protection to metallic storage tanks, jetties, offshore structures
                                                                         the protection of small-bore pipework.
and reinforced concrete structures.




                                                                     2                                                                        2
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                                                                                                                Cathodic Protection


Cathodic protection is applied to control the corrosion of steel        a) Electrical continuity. The resistance of the conductor and
embedded in reinforced concrete structures (bridges, buildings,               structure should be such as to minimise the potential drop
port and harbour structures, etc.) – See Guide in Corrosion                   of the return protective currents through the structure.
Control, Corrosion and Protection of Steel in Concrete and it’s
Monitoring.                                                             b) Coatings. The provision of a protective/insulating coating
Cathodic protection can be applied to copper-based alloys in                  to the structure will greatly reduce the current demanded
water systems, and, exceptionally, to lead-sheathed cables and                for cathodic protection of the metallic surface. The use of a
to aluminium alloys, where cathodic potentials have to be very                well-applied and suitable coating, increases the effective
carefully controlled.                                                         spread of cathodic protection current. A combination of
                                                                              applying both a coating and cathodic protection will

5.0 Basic Requirements for                                                    normally result in the most practical and economic overall
    Cathodic Protection                                                       protection system. Ideal coatings are those that have a
                                                                              high electrical resistance, are continuous and will adhere
The essential features of cathodic protection to metals that are              strongly to the surface to be protected. Other desirable
surrounded by a conducting electrolyte, in each of the two types              coating characteristics include; stability in the environment,
of system are as follows:                                                     abrasion resistance, and compatibility with the alkaline
                                                                              environment created or enhanced by cathodic protection.
a)       A galvanic system requires:
         i)   Sacrificial anodes                                        c) Structure isolation. It is often desirable to limit the spread
         ii) Direct welding to the structure or a conductor                   of cathodic protection. For pipelines and tanks, this may be
              connecting the anode to the structure                           achieved by the insertion of monolithic electrical isolation
         iii) Secure and minimum resistance connections                       joints in the structure. Insulating flange kits are sometimes
              between conductor and structure, and between                    used though they often require regular maintenance.
              conductor and anode.                                            Polarisation cells that restrict low voltage cathodic
                                                                              protection dc currents, but allow passage of high voltage ac
b)       An impressed-current system requires:                                currents, may be used to isolate low-resistance earthing
         i)   Inert anodes (clusters of which, connected together             systems from a well-coated protected structure.
              often in a backfill, are called the “groundbed”).
         ii) A dc power source.                                         d) Test facilities. It is important to consider the location of test
         iii) Electrically well insulated, minimum resistance and             facilities, test stations, corrosion monitoring coupons,
              secure conductors between anodes and power                      permanent half cells (reference electrodes), and the
              source.                                                         manner that data can be routinely collected or viewed.
         iv) Secure and minimum resistance connections
              between power source and structure.
                                                                        6.0 Design Factors
                                                                        6.1        Initial considerations
In both cases, fundamental design decisions must be made to
                                                                        Modifications to the structure to incorporate requirements, such
select the type of system and the most suitable type of anode
                                                                        as those discussed in section 5, are best made at the early
appropriate to that system. Also required, is the determination
                                                                        design and pre-construction phase of the structure. For
of the size and number of the power sources, or sacrificial
                                                                        underground structures it may be necessary to visit the
anodes, and their distribution on the structure.
                                                                        proposed site, or for pipelines the proposed route, to obtain
                                                                        additional information on low-resistivity areas, availability of
Other requirements that must be met to ensure that cathodic
                                                                        electric power, and the existence of stray dc current or other
protection is applied in the most economic and reliable manner
                                                                        possible interaction.
are:




                                                                    3                                                                        3
                                                                                                                  BACK          INDEX

                                                                                                                  Cathodic Protection


It is common practice for a survey to be made before design.              The potential values measured on a cathodically protected
This survey is often combined with a study to establish                   structure will be dependent on the anodic and cathodic
economic justification for the recommended anti-corrosion                 reactions, structural geometry, and internal electrical
proposal while the principal data necessary for design (chemical          resistance. However, the provision of a protective coating will
and physical) are also collected.                                         have by far the greatest effect on the potential for a given
                                                                          applied current. The potentials will generally be most negative
If the structure already exists, measurement of existing                  at a point nearest to the anode or groundbed and, for pipelines,
structure-to-soil potentials is essential to give valuable                will attenuate towards the natural corrosion potential as the
information as to which areas are anodic and which are                    distance from the anode or groundbed increases.
cathodic. In addition, with the application to the structure of
temporary cathodic-protection current, using any convenient dc            An example of potential attenuation is that, in the case of a
source and a temporary anode system (groundbed), a more                   power-impressed system, a single cathodic-protection
accurate assessment of current demand and the likely spread of            installation may supply cathodic protection to as much as
protection to the structure may be assessed.                              150 km of extremely well coated pipeline, whereas for similar-
                                                                          sizes of bare (uncoated) pipelines it may be necessary to have
Design of a cathodic-protection system for a new structure                installations at only 2 km intervals.
should include the calculation of:
Current demand                                                            6.3       Economics of decisions
Resistance to earth of the anodes                                         At the design stage of a cathodic-protection scheme, a decision
Quantity and location of anodes or anode systems                          must be made as to whether the scheme will be a galvanic or
Electrical supply requirements                                            impressed-current system. In specific circumstances, the use
Test and monitoring facilities.                                           of both types of systems may be appropriate, but care is
                                                                          required to avoid interaction between them.
Project specifications and European or national guideline
documents should be consulted.                                            Galvanic systems have the advantage of being –
                                                                             a)      simple to install
In the case of onshore pipelines and other structures,                       b)      independent of a source of external electric power
negotiation with landowners, public authorities, or other                    c)      suitable for localised protection
interested parties, for easements and wayleaves for                          d)      less liable to cause interaction on neighbouring
groundbeds, cable routes, transformer-rectifier sites, and                           structures.
electricity supplies should also be undertaken at the design
stage.                                                                    However, the current output available from the practical size
                                                                          and weight of galvanic anodes is relatively small and depends
6.2      Potential level and distribution                                 principally on the electrical resistivity of the electrolyte (local
In practice, the structure-to-electrolyte potentials are measured         environment if buried / submerged / concrete). Thus, galvanic
using a standard half-cell (reference electrode). For example, a          anodes of aluminium and zinc, which have similar driving emfs
common protection criterion used for steel in an aerobic                  to steel of approximately 0.5V, are limited to use in electrolytes
electrolyte of nearly neutral pH is a negative value of minus 850         of less than 5 Ohm.m resistivity. The anodes are usually self-
mV. When exposed to sulphate-reducing bacteria, steel would               regulating because their current output is usually less than their
require a more negative potential of minus 950 mV. Both values            maximum output capability and is controlled by the difference in
are with respect to a copper/copper sulphate half-cell. Ideally, to       potential between the two metals. The current from the anodes
attain a high degree of accuracy and in order to minimise                 is not normally controllable; thus changes in the structure, such
measurement errors, the half-cell should be very close to the             as the deterioration of a coating, that causes an increase in
surface at which the potential is being measured.                         protection current demand, may necessitate the installation of
                                                                          further sacrificial anodes to maintain protection.



                                                                      4                                                                         4
                                                                                                                 BACK        INDEX

                                                                                                                 Cathodic Protection


                                                                           however, be inspected at periodic intervals to ensure they are
Impressed-current installations have the advantage of being –              capable of supplying continued protection.
   a)     able to supply a relatively large current
   b)   able to provide of high dc driving voltages (up to 50V).           Any secondary structure residing in the same electrolyte may
        Enables it to be used in most types of electrolytes                receive and discharge the cathodic protection direct current by
   c)   able to provide a flexible output that may accommodate             acting as an alternative low-resistance path (interaction).
        changes in, and additions to, the structure being                  Corrosion will be accelerated on the secondary structure at any
        protected                                                          point where current is discharged to the electrolyte. This
Generally, however, care must be taken in the design to                    phenomenon is called "stray current corrosion".
minimise interaction on other structures and, if no ac supply is           Interaction may occur, for example, on a ship that is moored
available, an alternative power source (solar, diesel, etc.) is            alongside a cathodically protected jetty, or on a pipeline or
required. Impressed current systems require regular                        metal-sheathed cable that crosses a cathodically protected
maintenance and monitoring.                                                pipeline.
                                                                           Interaction may be minimized by careful design of the cathodic
Generally, galvanic systems have found favour for small well-              protection system. In particular, by design of a scheme to
coated, low current demand, structures or for localised                    operate at the lowest possible current density and by
protection. Impressed current schemes are utilised for large               maintaining good separation between the protected structure
complex structures, which may be of bare metal or poorly                   and the secondary structure, and between the groundbeds or
coated. However, in North Sea offshore work, it has been found             anodes and the secondary structure.
cost effective to provide galvanic protection to large uncoated
platforms, and similar structures, where the initial cost of coating       It is an advantage of sacrificial-anode schemes that they are
and the cost of maintenance are very high. In addition, the                not prone to creating severe interaction problems and therefore
galvanic anodes offer easy to install robust systems, which                they are popular for protection in congested and complex
being independent of a power source, provide protection                    locations.
immediately on “float-out” of the structure.                               Methods and procedures are available for overcoming
                                                                           interaction, and testing should be carried out in the presence of
6.3      Problems to be avoided                                            interested parties, so that the choice of remedial measures may
There are certain limitations to the use of cathodic protection.           be agreed, if and when the acceptable limit of interaction is
Excessive negative potentials can cause accelerated corrosion              exceeded.
of lead and aluminium structures because of the alkaline
environments created at the cathode. These alkaline conditions             6.4         Types of equipment
may also be detrimental to certain coating systems, and may                Various galvanic anode alloys of magnesium, aluminium or zinc
cause loss of adhesion of the coating. Hydrogen evolution at               are available in a variety of block, rod or wire forms. These
the cathode surface may, on high-strength steels, result in                alloys are cast around steel inserts to enable fixing of the
hydrogen embrittlement of the steel, with subsequent loss of               anode and to maintain electrical continuity and mechanical
strength. On some high strength steels, this may lead to                   strength towards the end of the anode life. The insert may be
catastrophic failures. It may also cause disbondment of                    directly welded or bolted to the structure to be protected, or
coatings; the coating would then act as an insulating shield to            anodes may be connected to the structure by means of an
the cathodic-protection currents.                                          insulated lead, usually of copper, as for onshore and offshore
                                                                           pipelines.
Consideration must also be given to spark hazards created by
the introduction of electric currents into a structure situated in a       Impressed-current groundbeds in soils have traditionally
hazardous area. Generally sacrificial anode systems do not                 consisted of high-silicon cast iron. However, mixed metal oxide
cause problems, as they are self-regulating and are often                  (MMO) anodes are becoming increasingly popular for all
regarded as systems that can be ‘fit and forget’. They must,               environments because of their good mechanical and electrical



                                                                       5                                                                    5
                                                                                                                BACK         INDEX

                                                                                                                 Cathodic Protection


characteristics and compact size. For seawater applications                Galvanic-anode outputs may also be monitored, as can
and areas where chlorides are present, MMO anodes work well                currents in electrical bonds between structures. Tests to
as do high-silicon cast iron alloyed with chromium. Other                  measure interaction are usually conducted annually where
anodes consist of lead alloy and platinum formed in a thin layer           areas are at risk or after adjustments to cathodic-protection
on a titanium or niobium base                                              current output.


There are many possible sources of dc power; the most popular              Maintenance includes the mechanical maintenance of power-
is the selenium plate or silicon-diode rectifier with transformer          supply equipment and the maintenance of painted surfaces of
unit in conjunction with an existing ac supply or diesel- or gas-          equipment.
engine-driven alternator. For most applications, a constant dc
voltage or constant current systems are used.                              It is good practice to inform all owners of cathodic protection
                                                                           systems and infrastructure in the area of influence of any new
In remote areas, power sources include thermo- electric                    cathodic protection systems, or of significant changes to
generators, closed-cycle vapour turbines, and solar or wind                existing systems, so that the effect on these facilities may be
generators. The latter two are used in conjunction with lead-              assessed.
acid or similar storage batteries. The choice is dependent on
power requirements, maintenance capabilities, and                          8.0 Sources of Advice
environmental conditions.
                                                                           Corrosion/Cathodic Protection Consultants – Various listings.

There are also automatic control units available that will adjust
                                                                           Institute of Corrosion
current output in accordance with potential changes at a half
                                                                           Corrosion House, Vimy Court, Leighton Buzzard
cell.
                                                                           Bedfordshire. LU7 1FG


7.0 Monitoring and Maintenance                                             National Association of Corrosion Engineers (NACE)
Cathodic-protection systems may be monitored effectively by                International
the measurement of structure-to-electrolyte potentials, using a            Houston, Texas, USA
high input impedance voltmeter and suitable half-cell. The
standard practical half-cells are copper/copper sulphate,                  Institute of Materials, Minerals and Mining
silver/silver chloride/seawater, silver/silver chloride/ potassium         1 Carlton House Terrace, London. SW1Y 5DB
chloride and zinc.
                                                                           The Institution of Civil Engineers
Adjustments are made to the cathodic-protection current output             One Great George Street, Westminster, London SW1P 3AA
to ensure that protective potentials are maintained at a
sufficiently negative level as defined by the project specification.       Corrosion Protection Association (Reinforced Concrete)
The level of protection in soils and water is accepted at steel            Association House, 99 West Street, Farnham, Surrey GU9 7EN
potentials of minus 850 mV (wrt Cu/CuSO4) or minus 800 mV
(wrt Ag/AgCl/seawater).                                                    The Society of Operations Engineers
                                                                           22 Greencoat Place, London. SW1P 1PR
Transformer rectifier outputs may be displayed by telemetry at
central control stations. Many cathodic protection systems are             Galvanisers Association
increasingly being controlled and monitored by remote                      6 Wren’s Court, 56 Victoria Road, Sutton Coldfield
computers and modem links. Other communication systems                     West Midlands B72 1SY
that enable, for example, pipe-to- soil potentials to be monitored
from a helicopter or light aeroplane, are available.                       Paint Research Association
                                                                           8 Waldegrave Road, Teddington, Middlesex, TW11 8LD


                                                                       6                                                                     6
                                                                                                              BACK        INDEX

                                                                                                              Cathodic Protection



Pipeline Industries Guild                                                BS EN 12696       Cathodic protection of steel in concrete
14/15 Belgrave Square, London SW1X 8PS                                   Part 1 : Atmospherically exposed concrete


                                                                         BS EN 12954 Cathodic protection of buried or immersed
                                                                         metallic structures – General principles and application for

9.0 Further Information                                                  pipelines.

The following references provide further information on cathodic
                                                                         BS EN 13173 Cathodic protection for steel offshore floating
protection. Potential users are recommended to employ
                                                                         structures.
qualified and experienced specialists to design and undertake
the work. The following handbook provides listings of various
                                                                         BS EN 13174 Cathodic protection for harbour installations.
manufacturers, suppliers, consultants, and contractors.


The Corrosion Handbook, 1999, (incorporating Corrosion
Prevention Directory), MPI Group, (Inst. of Materials, Inst. of
Corrosion)


Other useful Publications:
J.H. Morgan 'Cathodic Protection' National Association of
Corrosion Engineers (NACE) 1987 2nd Edition.

                                             nd
Peabody’s Control of Pipeline Corrosion. (2       edition, Ed by R
Bianchetti), NACE, Houston, 2000.


Corrosion and corrosion control. H H Uhlig, Wiley, New York,
1985 (3rd edition).


Corrosion. L L Shreir (2 vols), Newnes-Butterworth, 19 (3rd
edition).


Cathodic Protection Criteria - A Literature Survey' National
Association of Corrosion Engineers (NACE) 1989.


W.V. Baeckmann 'Handbook of Cathodic Corrosion Protection',
  rd
(3 edition) Gulf Pub., 1997.


Standards
BS 7361 Part 1 1991 'Cathodic Protection Part 1 - Code of
Practice for Land and Marine Applications' British Standards
Institution, U.K.


BS EN 12473 General principles of cathodic protection in sea
water.


BS EN 12474 Cathodic protection for submarine pipelines.


                                                                     7                                                                  7
                                                                                              BACK        INDEX




Corrosion Prevention by Cathodic Protection
Billions of dollars are spent worldwide each year replacing industrial structures, equipment and municipal
infrastructure that have prematurely failed or reached the end of their life cycle. Cathodic protection is a
cost-effective means of extending the life of underground or submerged steel structures to ensure that the
design life is attained or surpassed.

Levelton's professionals are engineers and technologists with advanced training from NACE International. As
professionals, they have extensive experience and educational training for designing cathodic protection
systems to best meet the specific needs of each unique situation they encounter. Levelton's engineers keep
abreast of the latest technological advancements.

Cathodic protection applications are varied and diverse:


                              •   Underground piping and tanks for water, petroleum, natural gas, sewage,
                                   steam, chemical and petroleum products.

                              •   Marine structures including docks, ships, piling, buoys, log lifts, barges and
                                   sewage outfalls.

                              •   Internal protection of tanks and piping.

                              •   Concrete bridge decks, parkades, and piling.

                              •   Hydraulic elevator cylinders.

                              •   Above-ground tank bottoms.


                                                  Pipe for Corrosion Prevention by Cathodic
                                                                    Protection




              Promo Rectifier
                                                                                            BACK       INDEX




Providers of complete, comprehensive and cathodic protection consulting services:


                              •   Design.

                              •   System supply and installation.

                              •   Cathodic protection surveys and system monitoring.

                              •   System inspection and troubleshooting.

                              •   Quality assurance and laboratory testing.

                              •   Material sales.


Providers of complete project management services:


                              •   Preparation of drawings.

                              •   Preparation of specifications and tender documents.

                              •   Evaluation of tenders and selection of a contractor.

                              •   Quality assurance supervision.

                              •   Cost management.

                              •   System energization.

                              •   Final acceptance testing.


Levelton Consultants Ltd. has successfully provided cathodic protection services to industrial, municipal, marine,
and petroleum sectors in Canada and internationally for over 30 years. We participate in projects ranging from
the simplest galvanic anode systems to the most complex impressed current installations.
                                                                                              BACK       INDEX




Introduction to Cathodic Protection

Foreword

Corrosion or deterioration of metals has posed a problem to industry for many years. Of all the various
anti-corrosion systems used, Cathodic Protection is one of the most efficient, being a positive and economical
solution to the multiple corrosion problems encountered either on shore or offshore (marine environments).




When dissimilar metals are in electrical or physical contact (the former through an electrolyte), galvanic corrosion
can take place. The process is akin to a simple DC cell in which the more active metal becomes the anode and
corrodes, where as the less active metal becomes the cathode and is protected. The galvanic series shown
below in Table 1 can be used to predict the metal which will corrode in contact with another metal, based on
whether it is cathodic or anodic with respect to another. On top of the table are the "Noble" or cathodic (protected)
metals and at the bottom, the more active or Anodic metals.

Table 1
Standard electromotive force series for selected metals


             Metal-metal ion equilibrium (unit activity)   Potential at 25 oC (77 oF), V
             Ag/Ag+                                        +0.80
             Cu/Cu2+                                       +0.34
                    +
             H2/H                                          (reference) 0
                     2+
             Fe/Fe                                         -0.44
             Zn/Zn2+                                       -0.76
             Al/Al3+                                       -1.66
             Mg/Mg2+                                       -2.36


Cathodic Protection is an electrochemical means of corrosion control in which the oxidation reaction in a galvanic
cell is concentrated at the anode and suppresses corrosion of the cathode in the same cell. Figure 1 shows a
simple cathodic protection system. The steel pipeline is cathodically protected by its connection to a sacrificial
magnesium anode buried in the same soil electrolyte.
                                                                                             BACK       INDEX




Figure1

Cathodic protection was first developed by Sir Humphrey Davy in 1824 as a means of controlling corrosion on
British naval ships. Virtually all modern pipelines are coated with an organic protective coating that is
supplemented by cathodic protection systems sized to prevent corrosion at holidays (defects) in the protective
coating. This combination of protective coating and cathodic protection is used on virtually all immersed or buried
carbon steel structures, with the exception of offshore petroleum production platforms and reinforced concrete
structures.

Fundamentals of Cathodic Protection
Table1, shows the theoretical electrochemical potentials obtained by pure metals in 1 N solutions of their own
ions. Figure2, shows two of these metals, iron and zinc, separately immersed in a weak mineral acid (or sea
water). The chemical reactions that occur in Figure2 are:


                             Fe --> Fe2+ + 2e-               Oxidation reaction
                             2H+ + 2e-                       Reduction reaction
                             2H+ + Fe --> Fe2+ + H2          Net reaction
                                       2+      -
                             Zn --> Zn      + 2e             Oxidation reaction
                               +      -
                             2H + 2e --> H2                  Reduction reaction
                             2H+ + Zn --> Zn2+ + H2          Net reaction
                                                                                        BACK      INDEX




Figure2

Both metals corrode, and both corrosion (oxidation) reactions are balanced by an equal reduction reaction,
which in both cases involves the liberation of hydrogen gas from the acid environments. The two corrosion
reactions are independent of each other and are determined by the corrosivity of hydrochloric acid on the two
metals in question.

If the two metals were immersed in the same acid and electrically connected (Figure3), the reactions for zinc
would then become:


                           Zn --> Zn2+ + 2e-                    Oxidation
                           2H+ + 2e- --> H2                     Reduction
                                                                                             BACK       INDEX




Figure3

Almost all of the oxidation reaction (corrosion of zinc) has been concentrated at the zinc electrode (anode) in
Figure3, and almost all of the reduction reaction (hydrogen liberation) has been concentrated at the iron
electrode (cathode). The oxidation of the zinc anode in Figure3, is much faster than that in Figure2. At the same
time, most of the corrosion of iron in Figure2, has stopped in Figure3. As shown schematically, the zinc anode in
Figure2, has been used to cathodically protect the iron cathode in Figure3.
Of course, some corrosion of the iron may still occur; whether or not this happens depends on the relative sizes
of the zinc and iron electrodes. Some reduction of hydrogen may still occur on the zinc anode. The anode is the
electrode at which a net oxidation reaction occurs, whereas cathodes are electrodes at which net reduction
reactions occur. All cathodic protection systems require an anode, a cathode, an electric circuit between the
anode and cathode, and an electrolyte. Thus, cathodic protection will not work on structures exposed to air
environments. The air is a poor electrolyte, and it prevents current from flowing from the anode to the cathode.
Cathodic Protection can be accomplished by two widely used methods:

1. By coupling a given structure (say Fe) with a more active metal such as zinc or magnesium. This produces a
galvanic cell in which the active metal works as an anode and provides a flux of electrons to the structure, which
then becomes the cathode. The cathode is protected and the anode progressively gets destroyed, and is hence,
called a sacrificial anode.

2. The second method involves impressing a direct current between an inert anode and the structure to be
protected. Since electrons flow to the structure, it is protected from becoming the source of electrons (anode). In
                                                                                              BACK       INDEX




impressed current systems, the anode is buried and a low voltage DC current is impressed between the anode
and the cathode.

Sacrificial anode systems are simpler. They require only a material anodic to the protected steel in the
environment of interest. Figure4, shows an impressed-current system used to protect a pipeline. The buried
anodes and the pipeline are both connected to an electrical rectifier, which supplies direct current to the buried
electrodes (anodes and protected cathode) of the system. Unlike sacrificial anodes, impressed-current anodes
need not be naturally anodic to steel, and in fact, they seldom are. Most impressed-current anodes are made
from non-consumable electrode materials that are naturally cathodic to steel. If these electrodes were wired
directly to a structure, they would act as cathodes and would cause accelerated corrosion of the structure they
are intended to protect. The direct current source reverses the natural polarity and allows the materials to act like
anodes. Instead of corrosion of the anodes, some other oxidation reaction, that is, oxygen or chlorine evolution,
occurs at the anodes, and the anodes are not consumed.




Figure 4

Impressed-current systems are more complex than sacrificial anode systems. The capital expenses necessary
to supply direct current to the system are higher than for a simple connection between an anode and a cathode.
The voltage differences between anode and cathode are limited in sacrificial anode systems to approximately 1
V or even less, depending on the anode material and the specific environment. Impressed-current systems can
use larger voltage differences. The larger voltages available with impressed-currents allow remote anode
locations, which produce more efficient current distribution patterns along the protected cathode. These larger
voltages are also useful in low-conductivity environments, such as freshwater and concrete, in which sacrificial
anodes would have insufficient throwing power.
                                                                                                 BACK       INDEX




Cathodic Protection Monitoring of Offshore Pipelines
and Structures in Alaskan Waters


J. P. LaFontaine, J Britton


Pipelines and structures located offshore of Alaska face unique challenges to monitoring cathodic protection.
Advances in Cathodic Protection Monitoring technology are discussed. New portable ROV instrumentation as
well as fixed monitoring of parameters affecting cathodic protection system performance are reviewed. Case
histories from the southern coast of Alaska as well as Arctic waters are detailed.
Introduction
It is common knowledge that cathodic protection (CP) is necessary to limit corrosion on metallic structures in
marine environments. Monitoring CP can provide valuable data to owners and operators regarding:


1. The level of protection.
2. The remaining service life of the system.
3. Improvements for future designs.


In the environmentally sensitive coastal waters of Alaska, it is critical that the performance of the CP (cathodic
protection) system on a structure or pipeline can be monitored. From the fast currents of Cook Inlet to the Frozen
Beafort Sea the marine environment of Alaska presents many unique challenges from a cathodic protection
standpoint. The current density required to achieve polarization on steel in Cook Inlet is over 6.5 times higher
than that required in the Gulf of Mexico. In addition the cold temperatures of these waters are as much as 30%
less conductive that ambient waters. The nearly year-round ice cover and permafrost make the Arctic one of the
most challenging environments yet encountered by corrosion engineers.
Monitoring - General
The basic criteria for cathodic protection of steel in sea-water is that it is polarized to at least (-) 0.800 Volts vs.
Ag/AgCl (silver / silver chloride) or (-) 0.850 Volts vs. Cu/CuSO4 (copper / copper sulfate). This value can be
determined by employing either reference cell, but typically in sea-water silver / silver chloride is used.
Measuring the potential will tell you if are currently protected. However measurement of other parameters is
necessary to determine the remaining service life of your system. Among these are the current density pick-up
on the steel and the anode current output. These values can be compared to design values to determine if the
system is operating as expected. On coated structures i.e. pipelines, anode current output can be used to
determine the efficiency of the coating.
Cook Inlet
Several factors make Cook Inlet one of the most corrosive marine environments for steel structures in the world:


1. Extreme tidal ranges create tidal currents as high as 8.7 knots (1). The high velocity water provides constant
oxygen replenishment to the steel surface. In addition sand and other particulates are churned into the water
column, in effect "blasting" the steel surface, preventing it from forming carbonate layers, which would otherwise
decrease current demand.


2. The water temperatures are cold, ranging from 50 °F (10 °C) to 29 °F (-2 °C). The cold water has a high
                                                                                                  BACK         INDEX




dissolved O2 concentration, which further increases current density demand on the steel (1).


3. The resistivity of Cook Inlet water is as much as twice that of ambient 77 °F (25 °C) sea-water. This effect is a
                                                                        result of the low temperature as well as
                                                                        fresh water input.


                                                                        It is imperative in such conditions that a
                                                                        comprehensive         cathodic         protection
                                                                        monitoring program is followed.


                                                                        Pipeline Surveys


                                                                        There are a number of critical aging
                                                                        pipelines in the Cook inlet that have only
                                                                        ever been surveyed using trailing wire
                                                                        type remote electrode techniques and
                                                                        some riser drop cell readings. It is now
                                                                        well accepted that these surveys give no
                                                                        detailed information regarding the true
                                                                        pipe   potential     unless      the    electrode
                                                                        position with relation to the pipeline is well
                                                                        known, and the system is corrected for the
                                                                        IR errors caused by the impressed current
                                                                        system.    Many      of   the    pipelines     are
                                                                        installed using pull tubes so that even the
                                                                        drop cell readings are meaningless.


                                                                        Two or three electrode techniques would
                                                                        provide better data validity providing that
                                                                        periodic pipeline contacts can be made to
                                                                        re-calibrate the true remote pipeline
                                                                        potential (Figure 1). This can be difficult
                                                                        because    most      of   the    pipelines     are
                                                                        concrete weight coated for stability and
                                                                        mechanical protection, so if the pipe
                                                                        doesn't have anodes (bracelets), there is
no way to calibrate unless concrete is removed.



Figure 1. Offshore pipeline cathodic protection survey method.




Potential Attenuation Modeling
                                                                                              BACK       INDEX




Understanding and recent improvement in techniques has made modeling much more accurate, and if a few
parameters can be measured on the line, predictive models can be used to estimate the worst case scenario of
potential versus coating efficiency. Fixed or retrofitted permanent monitors can provide these reference points.
This approach is particularly effective on pipelines that use impressed current. An example of a recent survey
illustrates this approach (Figure 2). A predictive potential profile was determined before the pipeline was installed
for the purposes of designing the CP system. After installation and start up the pipeline potential was measured
at 5 locations. The original model was recalculated using the field measured endpoint potentials. The close
agreement between the field measurements and the model confirm the validity of this approach.




Figure 2. Modeled potential profiles compared to actual field data on a marine pipeline.



Production Facilities


Economic and logistic drivers make fixed instrumentation preferable to surveys with portable instrumentation.
The same extreme marine conditions that make cathodic protection a challenge in Cook Inlet also make diver
and ROV work very difficult. This is particularly true on jackets. Impressed current CP (ICCP) systems or
ICCP/galvanic anode hybrid systems are required to achieve the high current demands in Cook Inlet. Fixed
reference electrodes distributed across the structure are critical to evaluating system performance. Such an
approach was used on the Marathon Steelhead platform. This four-pile structure was set in 183-ft. (55.5 M) of
sea-water. An array of Ag/AgCl and Zinc reference electrodes were installed down both sides of one leg at 20-ft.
intervals. By monitoring the steel potential with such an array, the current output from the ICCP system can be
optimized. On Steelhead the initial current output was 2100 A. After 30 days however it was determined that to
                                                                                              BACK       INDEX




achieve the proper potentials, output could be lowered to 960 A. After two years of service output was lowered to
628 A. The monitoring system allowed frequent potential measurements to be made simultaneously at many
locations.
The Arctic Ocean
A project to develop the Northstar oil field, located in the Beafort Sea, marked the first time in the Alaskan Arctic
that a warm oil production pipeline, buried in the sea floor, has been used to transport oil and gas from a
manmade offshore island (Figure 3). Fixed cathodic protection monitoring on this pipeline is a necessity due to
environmental concerns and logistics.




Figure 3. General layout of the subsea portion of the Northstar pipeline.



The overall strategy was to measure the effectiveness of the CP system with a combination of fixed monitors
supplemented by a survey program. A sacrificial anode system was used on this line instead of impressed
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current ground beds due to the high resistance of the permafrost. Near the shore crossing (Point Storkersen), the
following instruments were installed:


1. Anode Monitor (Figure 4)
2. Current Density Monitors (Figure 5)
3. Permanent Reference Cell (Figure 6)
4. Monitoring Panel (Figure 7)




Figure 4. Schematic of monitored anode.




Figure 5. Schematic of the current density monitor.
                                                                                             BACK        INDEX




Figure 6. Permanent reference cell.




Figure 7. Monitoring panel at the shore crossing.



Each one of these monitors had Ag/AgCl (silver/silver chloride) reference cell, a Zinc reference cell, and a
temperature transducer. Initial readings from the system after installation indicates the cathodic protection
system to be working optimally. The current output from the test anode was below 0.00001 amps. The pipeline
potential was measured at (-) 1.068 V vs. Ag/AgCl. The very low anode current output combined with the near
anode potential of the pipe indicates that the CP system is working very well. The effectiveness of the coating
system was confirmed as data from the coated CD monitor indicates ~100% coating efficiency. The measured
sea-mud temperatures of 26 to 28 °F (-3.3 to -2.2 °C) were in agreement with geotechnical survey data.
Future Developments
It is probable that the development of new thermally applied metallic coatings will be a part of future deepwater or
                                                                                          BACK      INDEX




high temperature CP systems. Large capacity mid-depth systems will certainly shift more toward impressed
current, as cost and flexibility become more important factors. The future success of these systems will depend
largely on information gathered from monitoring systems installed on the early deployments of the technology.
References
1. C.E. Hedborg, "Cathodic Protection in Cook Inlet Arctic Waters", Materials Performance, February 1991
                                                                                         BACK       INDEX




Offshore Cathodic Protection 101
what it is, and how it works.

Richard Baxter, Jim Britton


How Does Steel Corrode in Water?

To understand cathodic protection one must first understand the corrosion mechanism. For corrosion to
occur, three conditions must be present.


1. Two dissimilar metals
2. An electrolyte (water with any type of salt or salts dissolved in it)
3. A metal (conducting) path between the dissimilar metals


The two dissimilar metals may be totally different alloys, such as steel and aluminum, but are more usually
microscopic or macroscopic metallurgical differences on the surface of a single piece of steel.


If the above conditions exist, at the more active metal surface (in this case we will consider freely corroding
steel which is non uniform), the following reaction takes place at the more active sites: (two iron ions plus
four free electrons)
                                             2Fe => 2Fe++ + 4e-


The free electrons travel through the metal path to the less active sites where the following reaction takes
place: (oxygen gas converted to oxygen ion - by combining with the four free electrons - which combines
with water to form hydroxyl ions)
                                           O2 + 4e- + 2H20 => 4 OH-


Recombinations of these ions at the active surface produce the following reaction, which yields the iron
corrosion product ferrous hydroxide: (iron combining with oxygen and water to form ferrous hydroxide)


                                      2Fe + O2 + 2H2O => 2Fe (OH)2


This reaction is more commonly described as 'current flow through the water from the anode (more active
site) to the cathode (less active site).



How Does Cathodic Protection Stop Corrosion?

Cathodic protection prevents corrosion by converting all of the anodic (active) sites on the metal surface to
cathodic (passive) sites by supplying electrical current (or free electrons) from an alternate source.


Usually this takes the form of galvanic anodes which are more active than steel. This practice is also referred
to as a sacrificial system, since the galvanic anodes sacrifice themselves to protect the structural steel or
pipeline from corrosion.
                                                                                        BACK       INDEX




In the case of aluminum anodes, the reaction at the aluminum surface is: (four aluminum ions plus twelve
free electrons)
                                          4Al => 4AL+++ + 12 e-


and at the steel surface, (oxygen gas converted to oxygen ions which combine with water to form hydroxyl
ions)


                                      3O2 + 12e- + 6H20 => 12OH-


As long as the current (free electrons) is arriving at the cathode (steel) faster than oxygen is arriving, no
corrosion will occur.


Figure 1: Sacrificial anode system in seawater




Basic Considerations When Designing Sacrificial Anode Systems
                                                                                         BACK       INDEX




The electrical current which an anode discharges is controlled by Ohm's law; that is:


                                                   I=E/R


I= Current flow in amps
E= Difference in potential between the anode and cathode in volts
R= Total circuit resistance in ohms


Initially current will be high because the difference in potential between the anode and cathode are high, but
as the potential difference decreases due to the effect of the current flow onto the cathode, current gradually
decreases due to the polarization of the cathode. The circuit resistance includes both the water path and the
metal path, including any cable in the circuit. The dominant value here is the resistance of the anode to the
seawater.


For most applications the metal resistance is so small compared to the water resistance that it can be ignored.
(Not true for sleds, or long pipelines protected from both ends). In general, long thin anodes have lower
resistance than short fat anodes. They will discharge more current, but will not last as long.


Therefore a cathodic protection designer must size the anodes so that they have the right shape and surface
area to discharge enough current to protect the structure and enough weight to last the desired lifetime
when discharging this current. As a general rule of thumb:


Length of the anode determines how much current the anode can produce, and consequently
how many square feet of steel can be protected.


Cross Section (Weight) determines how long the anode can sustain this level of protection.



Impressed Current Cathodic Protection Systems

Due to the high currents involved in many seawater systems it is not uncommon to use impressed current
systems. Impressed current systems use anodes of a type that are not easily dissolved into metallic ions, but
rather sustain an alternative reaction, oxidization of the dissolved chloride ions.
                                             2Cl- => Cl2 + 2e-
Power is supplied by an external DC power unit..
                                                                                              BACK       INDEX




  Figure 2: Impressed current cathodic protection system in seawater




  How Do We Know When We Have Enough Cathodic Protection?

We know whether or not we have enough current by measuring the potential of the steel against a standard
reference electrode, usually silver silver/chloride (Ag/AgCl sw.), but sometimes zinc (sw.).


Current flow onto any metal shifts its normal potential in the negative direction. History has shown that if steel
receives enough current to shift the potential to (-) 0.800 V vs. silver / silver chloride (Ag / AgCl), the corrosion
is essentially stopped.


Due to the nature of the films which form, the minimum (-0.800 V) potential is rarely the optimum potential,
                                                                                             BACK       INDEX




and designers try to achieve a potential between (-) 0.950 V and (-) 1.000 V vs. Ag/AgCl sw.


Figure 3: Protected vs Unprotected structures as verified by cathodic protection potential




More reading: http://www.cathodicprotectionpapers.com/




Cathodic Protection:
More reading: http://corrosiontest.its.manchester.ac.uk/lecturenotes/JDS_Notes/cpindex.htm



Case Study:
Cathodic protection in concrete: http://www.concrete.cv.ic.ac.uk/research/Case/cathodic-protection/cp-main.htm


Standard:
Norsok Standard Cathodic Protection: http://www.standard.no/pronorm-3/data/f/0/01/36/8_10704_0/M-503.pdf
                                                                                                          BACK        INDEX




                                                                         orscan
                                                  T
                                                  H
                                                  ENREPORT                                     AUGUST 2008    Volume 2   Issue 2




Cathodic
                                                                                            ...IN SHORT
                                                                                            News Update from



Protection pg                                                             2
                                                                                            Government Security News
                                                                                            The Federal Communications Commission
                                                                                            has been given approval to require
                                                                                            providers of wireline, wireless, paging,



Theory:
                                                                                            satellite and cable communications to
                                                                                            submit explanatory reports to the FCC
                                                                                            whenever their communications services



                                                                                                                      pg6
                                                                                            have been seriously disrupted...

With bandwidth demand increasing and the rising installation costs to
install additional fiber cables to satisfy this need, more communication
service providers are looking to extend the life expectancy of their
existing outside plant. Selection of the fiber optic cable type plays an
important role in how effective you will be in this endeavor...




pg4
                                                                                                  Welcome to the Norscan

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                                                                                                  address current issues that

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                                                                                                  today, and provide value and
                                                                                                  solutions. We appreciate any
In previous Norscan newsletters we have mentioned a lot about                                     comments or suggestions that
conditioning the outside plant to accommodate central office monitoring                           our readership may have which
                                                                                                  can be forwarded to:
and cable locate equipment. There seems to be some confusion as to
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locate applications – in particular when central office tone transmitters
                                                                                                  please use the above address.
are installed....




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                                                                                                                        BACK          INDEX                2



CATHODIC PROTECTION THEORY:
    First Line of defense for fiber optic cable!

      With bandwidth demand increasing along with rising installation costs to install
THE FIRST LINE OF DEFENSE FOR FIBER OPTIC CABLE
      additional fiber cables to satisfy this need, more communication service provider are
      looking to extend the life expectancy of their existing outside plant. Selection of the fiber

W       ith bandwidth demand increasing an important role to will be effective negatively charged exposed cable area (Figure
      optic cable type playsand the rising installation costs in how attracted to theyou will be in this endeavor. 1).
        install additional fiber cables to ‘armored’ fiber optic               These elements will the over time, forming a protective layer on
      Deployment of an satisfy this need, more communicationcable provides build upmost effective means of the
      provides protection to the fiber optic strands within the cable. Fiber negative DC potential cable
service providers are looking to extend the life expectancy of their existing exposed cable armor as long as the optic armored is energizing
outside plant. Selection of the fiber optic cable type plays an important role the cable, thus slowing the corosion process of the exposed armor area.
      can also be used as a sensing device to gain valuable insight in determining if the cable is
in how effective you will be in this endeavor. Deployment of an ‘armored’
      being affected by construction, rodent and lightning strike damage that could eventually
fiber optic cable provides the most effective means of protection for the The severity of the faulted area can be determined by measuring the amount
       optic strands within the optic strands. With can goal to extend the life of a flowing optic cable armor
fiber affect the fiber cable. Fiber optic armored cable the also of negative DC current (fault current) that is fiber through thecable
      installation, maintaining the integrity of the exposed area. Low fault current ensures 2), indicate good cable
be used as a sensing device to gain valuable insight in determining ifthe protective cable armorvalues (Figurethat the fiber
      optic affected by construction, rodent, or lightning the specified cable manufactures life expectancy of exposed
cable is beingstrands are protected beyond strike damage conditions; higher fault current values (Figure 3) indicate more the
that could eventually affect the fiber optic strands. With the goal to extend cable armor, which could lead to communication outages. Knowing how
      cable.
the life of a fiber optic cable installation, maintaining the integrity of the limited access is to below grade and aerial cable installations (eliminating
protective cable armor ensures that the fiber optic strands are protected the need for continuous on-site visual inspections), this methodology is,
                                                                                   far, the best solution for determining plant is to install
     A very economical way to extend the life of abyfiber optic armored cableyour current outside plant cable
beyond the specified cable manufactures life expectancy of the cable.
                                                                               conditions.
     a cable monitoring unit that utilizes a cathodic protection process. Similarly used in the
     pipeline way to extend the life of a fiber the erosion process of the pipeline the same effect can be
A very economicalindustry to eliminate optic armored cable
       is to install cable monitoring unit that utilizes a cathodic protection basic principal is to apply a negative you
plantrealizeda on fiber optic armored cable. The Knowledge of the Ground Fault activity is significant because DCgain
process. Similarly used in the pipeline industry to eliminate the corosion visibility into the physical condition of the cable armor that is there to protect
     Voltage (Direct Current Voltage) to the cable armor whether that be a below grade or
process of the pipeline, the same effect can be realized on fiber optic the fiber optic stands within your outside plant. You get a direct indication
     above grade installation. When exposed armor areas of the cable come in contact with
armored cable. The basic principal is to apply a negative DC Voltage whether the outside plant is in good condition with relative low ground fault
     Local Ground, elements in that surrounding installation area will be ground fault activity the
(Direct Current Voltage) to the cable armor (whetherthe be a below grade activity (between 0 and 5.00 mA), moderate attracted to (between
     negatively charged exposed areas of area (Figure 1). These elements ground fault activity over time
or above grade installation). When exposed armorcable the cable come 5.00 and 10.00 ma), and severe will build up that is above 10.00
     forming a protective the surrounding installation area cable armor activity that between negative mA could
in contact with Local Ground, elements inlayer on the exposed mA. Any Ground Faultas long isas the 20.00 and 27.00 DC
    potential is energizing the cable, thus slowing the erosion process of the exposed armor
    area.
 Figure 1: Cathodic Protection Theory

       Central Office
                        Negative
                                                               Below grade Fiber
                                                              Optic Armored Cable
           Battery                                                                            Exposed Cable
                                                                                                 Armor


                        Positive


                                                                                                   Elements in the soil are
                                                                                                   attracted towards the exposed
                                                                                                   Cable Armor which slows
                                                                                                   down the erosion process.
                                                                                                   (Cathodic Protection)
                                    Current Flow from
                                      Local Ground




                                                        Figure 1: Cathodic Protection Theory

          7 Terracon Place Winnipeg, MB Canada R2J 4B3             Tel: (204) 233-9138      Fax: (204) 233-9188      Email: newsletter@norscan.com


    The severity of the faulted area can be determined by measuring the amount of negative
                                                                                                                              BACK       INDEX             3



  Innovative Partnership cont’d...
 Figure 2: Cathodic Protection Theory




be indicating that something is seriously affecting the cable armor, which could eventually make its way to the fiber optic strands and cause an outage.
Preemptive measures that are initiated to locate and repair the source of the exposed cable armor occurrences can avoid communication outages. Again,
emphasizing the fact that if the cable armor is in good condition, so are the fiber strands within the cable.



 Figure 3: Cathodic Protection Theory

                                                               Large area of exposed Cable
                                                              Armor, High ground fault activity.
                                      L          H
            Central Office
                           Negative
                                                                      Below grade Fiber
                                                                     Optic Armored Cable
                Battery                                                                          Exposed Cable
                                                                                                    Armor


                           Positive


                                                                                                      Elements in the soil are
                                                                                                      attracted towards the exposed
                                                                                                      Cable Armor which slows
                                                                                                      down the erosion process.
                                                                                                      (Cathodic Protection)
                                          Current Flow from
                                            Local Ground




            7 Terracon Place Winnipeg, MB Canada R2J 4B3                   Tel: (204) 233-9138     Fax: (204) 233-9188     Email: newsletter@norscan.com
                                                                                                                                                            INDEX                4



Outside Plant
                                                                                                                                             BACK




           Tone Conditioning
I  n previous Norscan newsletters we have mentioned a lot about This means the LTU is a self balancing ground return path device for all
   conditioning the outside plant to accommodate central office monitoring industrial standard cable locate transmitters manufactured today. There
and cable locate equipment. There seems to be some confusion as to the
best practice available when conditioning the outside plant for cable locate
                                                                                             FIGURE 2
applications – in particular when central office tone transmitters are installed.
   Outside Plant Tone Conditioning
When the cable conductor (cable armor, trace wire or copper pair) is not
   In previous Norscan newsletters we have mentioned a lot can conditioning the outside
properly conditioned for tone locate applications, the resultsaboutseriously
   plant the success of central office monitoring and cable locate equipment. There seems
hamperto accommodateperforming an accurate cable locate to all areas of
   to be some confusion as to the best practice available with providing conditioning the
the outside plant. The main areas of concern are a means of regards to an
   outside ground return path for the cable locate in particular when signal
adequate plant for cable locate applications andsignal and achievingcentral office tone
   transmitters are installed. of the the cable conductor (cable One method
balance throughout all areasWhen outside plant cable network. armor, trace wire or copper
    to connect properly conductor for tone Local Ground, often referred as
is pair) is not the cable conditionsdirectly tolocate applications, the results can seriously
   hamper the success of performing an accurate cable locate to all areas of the outside
                             1) the cable at each termination location. Although
Hard Grounding (Figure of concern are a means of providing an adequate ground return path
   plant. The main areas
             easy to do, this methodology does not bode well for achieving
relatively cable locate signal and achieving signal balance throughout all areas of the outside
   for the
   plant cable network. One method is to connect the plant. Typically, directly
optimum cable locate signals to all areas of the outsidecable conductor what to Local
   Ground or most of the cable locate signal will be absorbed by the each
occurs is thatoften referred to Hard Grounding (Figure 1) the cable atHard termination
   location. Although relatively easy to do, the methodology does leaving
Ground termination that is located closest tothis transmitter location,not bode well for
little, if any, tone signal for the rest of the cable network. the outside plant. Typically what
   achieving optimum cable locate signals to all areas of
    occurs is that most of the cable locate signal will be absorbed by the Hard Ground
 termination that is located closest outside plant for tone signal leaving little
Some users have tried to balance theirto the transmitter location, distribution if any tone
    signal for the rest of the cable network.
          FIGURE 1
                                                                                                                    are two model types: one is the 0K LTU used for single
                                                                                                                    ended long haul cable installations and the other is the
                                                                                                                    FC LTU or Fixed Current type used in branch cable
                                                                                                                    networks. Both types can be incorporated into the same
                                                                                                                    cable network to achieve the optimum in tone signal
                                                                                                                    balance and distribution.

                                                                                                                    For single end long haul cable installations that are
                                                                                                                    between 25 and 100 kms (15 and 60 miles), use the 0K
                                                                                                                    LTU device. This LTU type will draw the optimum amount
                                                                                                                    of tone signal to the termination location (Figure 3).

using different values of resistance at termination locations,
                              Figure 1: Hard Grounded Tone Signal Return
but to no avail. There are far too many variables that play                                                                                                  FIGURE 3
   Some method, such to balance there outside plant for
into this users have triedas the cable resistive loss of the tone signal distribution using
                                                                                     (CO)
                                                                                                                     Maximum Tone Signal is drawn to the
   different values of is being used within the locations but
cable conductor that resistance at terminationnetwork and to no avail. There Frequency) too
                                                                                  Transmitter
                                                                          (Cable Locate
                                                                                         are far                      termination location via the 0K LTU
cable capacitance values, which vary if the cable is directly
   many variables that play into this method such as the cable resistive loss of the cable
                                                                              4200 CMS
                                                                              CABLE MANAGEMENT SYSTEM




   conductor a conduit installation. Fortunately, Norscan
buried or in that is being used within the network and cable capacitance values which vary                                                                       (Termination)
                                                                                                                                                                0K LTU Ground
   if the cable is direct buried or in a conduit installation. Fortunately Norscan has been
has been studying cables and cable installations for many                                                                                                         Return Path
   studied cables and cable installations for many years and has come up with a very                                                                                Device

   practical and efficient with a for practical tone signal
years and has come up means veryproviding and efficientbalance and distribution to all
                                                                              Metallic Conductor                                                                    LTU


   areas of providing tone signal balance and distribution to
means for the outside plant. The Norscan Line Termination Unit (LTU) is specifically
                                                                       (Armor, Tracewire or Copper Pair)                         100 kms - 60 mi.

    areas of to outside the required impedance path to ground for cable locate tones above
alldesigned theprovide plant. The Norscan Line Termination
   250 Hz is specifically designed to provide the required
Unit (LTU)(Figure 2).                                                                                   Tone Signal Ground Return Path

impedance path to ground for cable locate tones above
250 Hz (Figure 2).


               7 Terracon Place Winnipeg, MB Canada R2J 4B3                         Tel: (204) 233-9138     Fax: (204) 233-9188          Email: newsletter@norscan.com
                                                                                                                                                        BACK        INDEX              5


Cont’d...Outside Plant Tone Conditioning
Branch cable networks require even distribution of tone signals to all areaslies within how to energize the metallic conductor along with ensuring
of the outside plant. The Fix Current LTU devices will ensure each branch   full distribution and balance of the cable locate frequency throughout the
receives an equal amount of locate current, which then allows for full tone entire network. Signal distribution and balancing has more to do with how
signal distribution (Figure 4). Placement of the FC-LTU devices is at the   the cable is terminated (providing a ground return path for the cable locate
                                                                                                                          signal), rather than using
            FIGURE 4                                                                                                      hard grounding methods to
                                                                                                                          provide ground return paths
                             Tone Signal is distributed and balanced evenly                                               (which are the main cause
                 (CO)
              Transmitter
                               throughout cable network via FC-LTU device                                                 of poor distribution and
                                                                                                                          balancing). Line Termination
                                                                                    FC- LTU
       (Cable Locate Frequency)
4200 CMS
 CABLE MANAGEMENT SYSTEM
                                                                                                                          Unit devices are an efficient,
                                                                                                    (Termination)         and flexible product that can
                                                                                                   FC-LTU Ground          provide optimum tone signal
                                                                                                                          distribution and balance to
                                                                                                     Return Path
                                                                                                       Device
                                                                                                                          your cable network.
             Metallic Conductor                                                                                                              FC-LTU
      (Armor, Tracewire or Copper Pair)

                                                                                                             FC-LTU
                                             Tone Signal Ground Return Path




end of each branch line,
which can be in a splice                                                                                                                                            FIGURE 5
enclosure or remote                                                       (CO)                  Tone Signal is distributed/balanced on Long
building location. It is                                               Transmitter                Haul lines with Short Spur lines using a
                                                                (Cable Locate Frequency)
important to remember                                                                                combination of 0K and FC LTUs
that the entire branch
                                          4200 CMS
                                          CABLE MANAGEMENT SYSTEM




                                                                                                             Long Haul Line – 100 kms/60 miles
network      accumulated                                                                                                                                               (Termination)
distance     (all  cable                                                                                                                                              Ok-LTU Ground
                                                                                                                                                                        Return Path
segments          added                                                                                                                                                   Device
together) cannot exceed                               Metallic Conductor                                                  Short Spur Lines
                                                                                                                                                                          Ok-LTU

linear distances of 100                        (Armor, Tracewire or Copper Pair)                                      FC- LTU


kms or 60 miles.                                                                                                                                        FC-LTU




If there is a need on long                                                     Tone Signal Return Path
haul installations to locate
short spur lines that are
connected to the main
trunk line, a combination
of Ok and FC LTU devices are used. An Ok-LTU device would be installed at
the end of the main feed line to draw tone locate signal to that location. The
short spur lines connected to the main feed line would be terminated with
the FC LTU, allowing limited amounts of tone signal to pass to these branch
cable locations (Figure 5).

Cable locating is a relatively straight forward application: energize a metallic
conductor that is within a fiber optic cable with a cable locate frequency. Once
energized, the cable can be located using a handheld receiver to pick up
this signal, thus revealing the location and depth of the cable. The challenge




                           7 Terracon Place Winnipeg, MB Canada R2J 4B3                    Tel: (204) 233-9138        Fax: (204) 233-9188             Email: newsletter@norscan.com
                                                                                                                        BACK         INDEX               6




     ...IN SHORT


FCC:
By Jacob Goodwin, Editor-in-Chief
                                        COMMUNICATIONS CARRIERS MUST
                                        NOW REPORT THEIR OUTAGES

T  he Federal Communications Commission has been given approval
   to require providers of wireline, wireless, paging, satellite and cable
communications to submit explanatory reports to the FCC whenever their
                                                                                 outage reports and that the federal government would spend an additional
                                                                                 $156,000 per year reviewing the submissions.

                                                                                 The FCC does not plan to publish the results of this new information
communications services have been seriously disrupted.
                                                                                 collection process.
The new reporting requirement, which was approved by the Office of
                                                                                 The reports will be received by the FCC’s Public Safety and Homeland
Management and Budget on February 19 and announced publicly on
                                                                                 Security Bureau, which will use them to help determine the state of network
March 17, will enable the FCC to monitor the reliability and security of these
                                                                                 reliability and security.
communications services and take swift remedial action, if necessary.

“The reporting requirement is also essential to the FCC’s mission of ensuring    Published March 17th, 2008 in Government Security News
that the public is protected from major disruptions to telephone services,”      http://www.gsnmagazine.com/cms/features/news-analysis/603.html
according to an internal FCC document prepared last December.

Under the FCC’s new policies, a communications carrier will be required
to submit to the FCC a “bare-bones notification” of any “outage” (which it
defines as a “significant degradation” of the carrier’s service) within two
hours. The carrier must submit a more detailed “Initial Communications
Outage Report” to the FCC within three days and a “Final Communications
Outage Report” within 30 days.

In these reports, the carrier must identify the reporting entity, the date and
time of the beginning of the outage, a brief description of the problem,
the particular services affected, and the geographic area impacted by the
outage.

The FCC noted in its internal document that the reports could contain what
is called “Critical Infrastructure Information,” which would be shared with
DHS officials “to protect the United States from terrorist activity and to
otherwise protect domestic security.”

The FCC estimated that approximately 79 communications carriers would
file a total of 4,819 such reports each year, consuming about two hours
preparing reports for each separate outage. The commission calculated
that carriers would spend close to $278,000 annually preparing such



          7 Terracon Place Winnipeg, MB Canada R2J 4B3              Tel: (204) 233-9138     Fax: (204) 233-9188      Email: newsletter@norscan.com
                                                                                                 BACK       INDEX




                                           NORSOK STANDARD

                                       COMMON REQUIREMENTS

                              CATHODIC PROTECTION
                                                     M-503

                                           Rev. 2, September 1997



 Please note that whilst every effort has been made to ensure the accuracy of the NORSOK standards neither OLF nor
                         TBL or any of their members will assume liability for any use thereof.




M-503, Rev. 2, September 1997                                                                             page 1
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FOREWORD
INTRODUCTION
1 SCOPE
2 NORMATIVE REFERENCES
3 DEFINITIONS
4 CATHODIC PROTECTION DESIGN
         4.1 General
         4.2 Electrical continuity requirements
         4.3 Mud zone
         4.4 Protection of concrete structures
5 DESIGN PARAMETERS
         5.1 Design life
         5.2 Current density requirements
         5.3 Coated surfaces
         5.4 Mudmats, skirts and piles
         5.5 Current drain to wells
         5.6 Current drain to anchor chains
         5.7 Pipelines
         5.8 Electrolyte resistivities
         5.9 Sacrificial anodes
6 ANODE MANUFACTURING
         6.1 Pre-production test
         6.2 Coating
         6.3 Insert-steel materials
         6.4 Aluminium anode/materials
         6.5 Zinc anode/materials
7 ANODE INSPECTION, TESTING AND TOLERANCES
         7.1 Steel inserts
         7.2 Chemical analysis
         7.3 Anode weight
         7.4 Anode dimensions and straightness
         7.5 Insert dimensions and position
         7.6 Anode surface irregularities
         7.7 Cracks
         7.8 Internal defects, destructive testing
         7.9 Electrochemical quality control testing




M-503, Rev. 2, September 1997                                 page 2
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FOREWORD

NORSOK (The competitive standing of the Norwegian offshore sector) is the industry initiative to
add value, reduce cost and lead time and remove unnecessary activities in offshore field
developments and operations.

The NORSOK standards are developed by the Norwegian petroleum industry as a part of the
NORSOK initiative and are jointly issued by OLF (The Norwegian Oil Industry Association) and
TBL (Federation of Norwegian Engineering Industries). NORSOK standards are administered by
NTS (Norwegian Technology Standards Institution).

The purpose of this industry standard is to replace the individual oil company specifications for use
in existing and future petroleum industry developments, subject to the individual company's review
and application.

The NORSOK standards make extensive references to international standards. Where relevant, the
contents of this standard will be used to provide input to the international standardisation process.
Subject to implementation into international standards, this NORSOK standard will be withdrawn.

INTRODUCTION

Revision 2 of this standard is made to reflect an agreement with the authorities regarding cathodic
protection for large subsea pipeline systems.

1 SCOPE

This Standard gives requirements for cathodic protection design of submerged installations and
seawater containing compartments, and manufacturing of sacrificial anodes.




M-503, Rev. 2, September 1997                                                                  page 3
                                                                                     BACK      INDEX




2 NORMATIVE REFERENCES

The following standards include provisions which, through reference in this text, constitute
provisions of this NORSOK standard. Latest issue of the references shall be used unless otherwise
agreed. Other recognized standards may be used provided it can be shown that they meet or exceed
the requirements of the standards referenced below.

ASTM D 1141         Specification for Substitute Ocean Water.
AWS D1.1            Structural Welding Code - Steel.
DNV RP B401         Cathodic Protection Design.
EN 287              Approval testing of welders - Fusion welding - Part 1.
EN 288              Specification and approval of welding procedures for metallic materials - Part 1,
                    2, 3.
EN 10002            Metallic materials. Tensile testing. Part 1: Method of test (at ambient
                    temperature).
EN 10204            Metallic products - Types of inspection documents.
ISO 1461            Metallic coatings - Hot-dip galvanized coating on fabricated ferrous products -
                    Requirements.
ISO 8501            Preparations of steel substrates before application of paints and related products
                    - Visual assessment of surface cleanliness.
NORSOK M-501        Standard for Surface Preparation and Protective Coating.
NORSOK M-505        Standard for Corrosion Monitoring Design (presently M-CR-505).
NACE RP0387         Metallurgical and Inspection Requirements for Cast Sacrificial Anodes for
                    Offshore Applications.
NACE RP0492         Metallurgical and Inspection Requirements for Offshore Pipeline Bracelet
                    Anodes.
U.S. Mil. Spec.     Military Specification for Anodes, Corrosion preventive, Zinc; slab disc and rod
MIL-A-18001         shaped.

3 DEFINITIONS

Can                  Can requirements are conditional and indicates a possiblilty open to the user
                     of the standard.
May                  May indicates a course of action that is permissible within the limits of the
                     standard (a permission).
Normative references Shall mean normative in the application of NORSOK standards.
Shall                Shall is an absolute requirement which shall be followed strictly in order to
                     conform with the standard.
Should               Should is a recommendation. Alternative solutions having the same
                     functionality and quality are acceptable.




M-503, Rev. 2, September 1997                                                                page 4
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4 CATHODIC PROTECTION DESIGN

4.1 General

The cathodic protection system shall be designed with due regard to environmental conditions,
neighbouring structures and other activities. The cathodic protection system design should be based
on sacrificial anodes. Both stand-off, flush-mounted and bracelet anodes may be used. The exact
location and distribution of the different types of anodes shall be part of the detailed corrosion
protection design. The design shall be subject to verification at the end of the fabrication phase.
When stand-off anodes are used precautions shall be taken in the installation and distribution of
these anodes so they do not impede subsea intervention operations.

The cathodic protection system shall be capable of polarizing all submerged steel of the installations
to a potential between -800 mV and -1050 mV vs the Ag/AgCl/seawater reference electrode, and to
maintain the potential in this interval throughout the design life of the installations.

Recommendation:
The use of impressed current cathodic protection systems can be considered for floating production
units.

The CP system shall be designed for the lifetime of the installation using the calculation procedure
described in DNV RP B401. Retrofitting can be planned for if this is documented to be cost
effective. Computer models can be used in the detailed design to verify the protection of parts with
complicated geometry e.g. in the pile area for jackets, conductor guide frames and J-tube
bellmouths and to evaluate any interference effects between anodes and/or between structures.

In the design calculation, data given in clause 6 of this document shall be used. For calculation of
surface areas, the latest revisions of drawings shall be used, and all areas below the mean water
level shall be included. Reference to drawings and revision numbers shall be given.

Items covered in the design shall be listed, with description of surface treatment (bare, painted,
rubber coated etc.). Items not covered in the design shall also be listed, i.e. temporary items to be
removed. Items to which current drain is allowed shall be listed.

For high strength steel materials (minimum specified yield strength >700 MPa, maximum actual
yield strength 950 Mpa) a special evaluation is required, with respect to hydrogen impact. The
impact can be documented according to EN 10002.

Monitoring of cathodically protected structures shall be according to NORSOK Standard M-505, if
used.

4.2 Electrical continuity requirements

All items to be protected shall be electrically connected and should have a welded or brazed
connection to an anode. All bolted/clamped components with surface area exceeding 1 m2 shall
have an all welded/brazed connection to an anode. For all bolted/clamped assemblies without an all
welded/brazed electrical grounding, it shall be verified that the electric resistance is less than 0.10
ohm. Coating on contact surfaces shall be removed prior to assembly.




M-503, Rev. 2, September 1997                                                                   page 5
                                                                                       BACK       INDEX




If the contact is made by using the copper cables welded/brazed at each end, these shall be stranded
and have a minimum cross section of 16 mm2. The copper cable shall be brazed to the cable shoe.

4.3 Mud zone

Steel parts exposed to seabed mud shall be cathodically protected by sacrificial anodes, if possible
installed in the submerged zone. Rock-dumping of pipelines shall be considered equivalent with
mud zone exposure.

4.4 Protection of concrete structures

In order to obtain cathodic protection of embedded steel in contact with exposed items, all steel
(embedded steel and exposed steel) shall be polarized. This polarization shall be achieved by
sacrificial anodes.

The sacrificial anodes supplying current to the rebar system shall be mounted on permanent steel
items or special embedment plates exposed to sea water and which are in electrical contact with the
rebar system through a welded connection. It shall be verified by measurements that electrical
continuity is achieved throughout the rebar system.

5 DESIGN PARAMETERS

5.1 Design life

The design life shall be as specified in the contract documents. Due regard shall be taken to the
fabrication, outfitting and installation phase before normal production starts.

5.2 Current density requirements

The current densities to be used in the design are given in table 5.1. The current densities shall be
used for steel, stainless steel, aluminium and other metallic materials.

Table 5.1 - Current densities in mA/m2 for cathodic protection design, valid for bare steel
surface temperatures up to 25 °C.

                                                          Current Densities, mA/m²
                                                   Initial          Mean             Final
Southern North Sea (Up to 57 °N)                    160              80              105
Northern North Sea (57 - 62 °N)                     180              90              120
Norwegian Sea (62 °N - 68 °N)                       200              100             130
Internally in flooded compartments                  160              80               95
Pipelines if burial is specified                     50              40               40
Sediments (mud)                                      25              20               20

For the first 20 meters below mean water level, the values in table 5.1 shall be increased by 10%.


M-503, Rev. 2, September 1997                                                                   page 6
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On surfaces with operating temperatures exceeding 25°C, the current density shall be increased
with 1 mA/m2 per °C difference between operating temperature and 25°C. This addition shall be
made before any effect of coating is included.

For embedded steel in concrete structures the following current densities shall be used for the
surface area of embedded steel. The values are applicable for initial, final and mean current
densities.

Concrete seawater exposed on one side below -10 m: 2 mA/m2 embedded steel
Concrete seawater exposed on both sides below -10 m: 1 mA/m2 embedded steel

For surfaces at elevation -10m to +5 m these values shall be increased by 50%.

For light weight aggregate concrete or other concrete grades with equivalent pore structure, the
design current densities can be reduced by 30%.

When the actual embedded steel surface area (m2) to reinforced concrete volume (m3) ratio, B,
exceeds 6, an adjustment factor 6/B may be applied to the design current densities.

5.3 Coated surfaces

For coated structures where the coatings are selected and applied according to NORSOK Standard
M-501 Surface Preparation and Protective Coating, the current densities given in clause 5.1 may be
multiplied by a factor given in table 5.2.

For design according to table 5.2 the initial current density ratio shall be assumed equal to 0.02.

Table 5.2 - Current density ratio for thin-film coated structures.

   Design life, years              Mean                       Final
          10                        0.05                       0.10
          20                        0.10                       0.20
          30                        0.18                       0.40
          40                        0.28                       0.65
          50                        0.40                       1.00

For conductors and other components subjected to wear, the initial current density ratio should be
given special consideration.




M-503, Rev. 2, September 1997                                                                   page 7
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Table 5.3 - Current density ratio for pipeline coatings and pipeline heat insulation coatings.

Design life,      Asphalt + concrete                 Rubber                 Polypropylene
  years
                  Mean          Final        Mean             Final       Mean          Final
       10         0.023        0.026         0.012            0.014       0.018        0.021
       20         0.033        0.052         0.017            0.029       0.030        0.048
       30         0.052        0.095         0.026            0.060       0.048        0.088
       40         0.070        0.140         0.039            0.099       0.067        0.132
       50         0.090        0.170         0.056            0.150       0.085        0.160

The values in table 5.3 shall be used for pipelines and when these coatings are used on items other
than pipelines. The coating quality should be according to commonly applied industry standards.

5.4 Mudmats, skirts and piles

In addition to current supply to the sea water exposed surfaces, extra anode capacity shall be
included to supply current drain as follows:

   •    Surfaces of mudmats, skirts and piles exposed to sediments: 20 mA/m2 based on outer
        surface area.
   •    If the top end of the piles cannot be closed, the internal surface to be included in the design
        shall be calculated for the top 5 times the internal diameter. The current drain shall be based
        on sea water current density criteria.

5.5 Current drain to wells

In the design of the cathodic protection system 5 Amps per well shall be included for platform
wells. For subsea wells the current addition shall be 8 Amps per well. The anodes for this current
drain shall be installed on the structure (for platform completed wells) or the subsea equipment for
subsea wells. Permanent electrical contact from the anodes to the wells must be secured.

5.6 Current drain to anchor chains

For anchor systems with mooring topside only, 30 m of each chain shall be accounted for in the
cathodic protection design. For anchor system with mooring point below sea level, the seawater
exposed chain section from sea level to mooring point and 30 m from mooring point shall be
accounted for in the cathodic protection design for each chain.

5.7 Pipelines

Anode spacing should not exceed 200 m. Amount of anodes shall be increased by a factor of 2 for
the first 500 m from platforms and subsea installations.

The current drain to the armour steel of flexible pipelines shall be included by 0.5 mA/m², related to
outer surface area.



M-503, Rev. 2, September 1997                                                                    page 8
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5.8 Electrolyte resistivities

Actually measured resistivities for seawater and bottom sediments/mud shall be used as far as
possible. If such measured values are not available for the installation site, the seawater resistivity
shall be set to 0.30 ohm m at all depths, and the seabed mud resistivity shall be taken as 1.30 ohm
m.

5.9 Sacrificial anodes

5.9.1 Electrochemical properties

The sacrificial anodes shall comply with the requirements given in clause 6 and 7. For design
purposes the data given in table 5.4 shall be used unless otherwise documented. If higher values for
current capacity of aluminium anodes are documented, a lower amount of anode material can be
used.

Table 5.4 - Design values for sacrificial anodes.

                             Seawater                           Sediments
                                    Current                                              Temperature
Anode type       Potential/mV                        Potential/ mV      Current
                                    Capacity                                             limits °C
                 Ag/AgCl/                            Ag/AgCl/           Capacity
                 Seawater                            Seawater           Ah/kg
                                    Ah/kg
Aluminium        -1050              2000             -1000              1730 1)          Max. 30
                                                                        750              Max. 30
Zinc, U.S. Mil
               -1030                780              - 980
Spec 18001
                                                                        580              30-50
NOTE

1) At temperatures above 30°C, the design values given in DNV RP B401, shall be used.

5.9.2 Anode Shape and Utilization Factor

Stand-off anodes shall be used as far as possible with a minimum distance to the steel surface of
300 mm. The insert steel should protrude through the end faces. The utilization factor shall be 0.90.

Flush-mounted anodes except bracelets shall have a utilization factor of 0.90.

Bracelet anodes shall be designed in such a way that a utilization factor of minimum 0.80 can be
achieved.

Bracelet anodes used on steel jackets to reduce wave loads shall be designed in such a way that the
same utilization factor as for stand-off anodes (i.e. 0.90) can be achieved.




M-503, Rev. 2, September 1997                                                                    page 9
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The dimensions and shape of insert steel and attachments shall be designed to withstand mechanical
loads that may act on the anodes, for instance wave loads, loads by water currents or vibration
caused by piling operations, or loads that will act on the anodes when penetrating into the sea
bottom sediments.

When protecting a coated structure, the anode legs shall also be coated.

6 ANODE MANUFACTURING

6.1 Pre-production test

Prior to the commencement of the works, a preproduction test shall be carried out to ascertain that
all moulds inserts, casting equipment and other components are in accordance with applicable codes
of practice, governing drawings and data sheets. Test casting shall be carried out to demonstrate that
all the specified requirements can be met. At least one test anode shall be inspected destructively as
described in 7.8. For deliveries below 15 ton net alloy and/or a limited number of anodes, the extent
of testing is subject to special agreement.

6.2 Coating

The exposed (external) surface of the anode shall be free from coating.

Flush mounted anodes shall be coated on the side facing the mounting surface. Bracelet anodes
shall also be coated on the sides facing cement or lining. The coating shall be minimum 100
microns epoxy mastic.

6.3 Insert-steel materials

Inserts shall be fabricated from weldable structural steel plate/sections according to a recognized
standard. Rimming steels shall not be used.

The carbon equivalent of insert materials shall be compatible with the structural elements to which
it is attached, and not exceed a value of 0.41. The carbon equivalent value shall be calculated using
the formula:


CE = C +        +                +


The following carbon equivalent formula may be used as an alternative if all elements are not
known.


CE = C +        + 0.04


Certificate shall be according to EN 10204, 3.1B.




M-503, Rev. 2, September 1997                                                                page 10
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All fabrication welding of steel inserts shall be in accordance with relevant requirements of AWS
D1.1 or an equivalent standard, and performed by welders qualified according to EN 287/AWS
D1.1. Qualification of welding procedures shall be in accordance with the requirements of EN
288/AWS D1.1, or equivalent.

Insert steel for aluminium sacrificial anodes shall be blast cleaned to Sa 2½ ISO 8501-1 prior to
casting. The cleanliness of the surface shall be maintained to casting commences.

Insert steel for zinc anodes shall be blast cleaned to minimum standard Sa 2 ½ ISO 8501-1 or
galvanized according to ISO 1461 or equivalent. Rust discolouration and/or visual surface
contamination of zinc coated surface shall not be permitted. The finish shall be maintained until
casting.

6.4 Aluminium anode/materials

6.4.1 Chemical composition

The aluminium anode material shall be of the AlZnIn type conforming to table 6.1.

Table 6.1 - Chemical composition of aluminium anode materials.

   ELEMENT          MAX %      MIN %

Zinc      (Zn)     5.5         2.5
Indium    (In)     0.040       0.015
Iron      (Fe)     0.09        -
Silicon   (Si)     0.10        -
Copper    (Cu)     0.005       -
Others    (Each)   0.02
Aluminium (Al)     Remainder


6.4.2 Electrochemical characteristics

The electrochemical properties shall be qualified according to DNV RP B401, Appendix B, Free
running test. Closed circuit resistance shall be adjusted to give a nominal anodic current density of
1.0 ± 0.1 A/m2. Minimum 16 samples from full scale anodes shall be used.

The electrochemical characteristics shall be documented for seawater at 5 - 12°C. For the alloy
specified in 6.4.1 the requirements in table 5.4 shall apply.

6.5 Zinc anode/materials

6.5.1 Chemical composition

The chemical composition of the material shall be in accordance with US Military Specification Mil
- 18001. Other alloys can be used if properly documented.




M-503, Rev. 2, September 1997                                                                page 11
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6.5.2 Electrochemical characteristics

The electrochemical characteristics shall be documented for seawater and conform with the
requirements in table 5.4.

7 ANODE INSPECTION, TESTING AND TOLERANCES

7.1 Steel inserts

All welds shall be visually inspected.

Required surface finish shall be verified by visual inspection immediately prior to casting.

7.2 Chemical analysis

Two samples from each batch shall be taken for chemical analysis.

The samples shall be taken in the beginning and at the end of casting from the pouring stream.

For smaller alloying furnaces (max 500 kg) it is acceptable to take one sample per batch. The
sample shall be taken in the beginning of the first batch and at the end of the second batch, then in
the beginning of the third batch and so on.

The samples shall be analyzed to verify required chemical composition.

All anodes from batches whose chemical composition do not meet the requirements stated in 6.4.1
and 6.5.1, respectively, shall be rejected.

7.3 Anode weight

Individual anodes of each type shall have a weight within +/- 3% of the nominal weight for anodes
with total weight above 50 kg. Minimum 10% of the number of anodes shall be weighed, either
individually or in small batches, to confirm general compliance with this requirement.

The total contract weight shall be no more than 2% above and not below the nominal contract
weight.

7.4 Anode dimensions and straightness

7.4.1 Stand-off and flush

Dimensional tolerances shall conform to NACE RP0387.

7.4.2 Bracelet

Dimensional tolerances shall conform to NACE RP0492.




M-503, Rev. 2, September 1997                                                                  page 12
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7.5 Insert dimensions and position

Tolerances on insert position within the anode shall be prepared by the anode manufacturer and
comply with utilization factor requirements. Anode insert protrusions, fixing centers, and any other
critical dimensions shall be measured.

7.6 Anode surface irregularities

Anode surface irregularities shall be according to NACE RP0387 and RP0492 with the following
additional requirements.

   •   Shrinkage depressions which exposes the insert are not acceptable.
   •   Cold shuts or surface laps shall not extend over a total length of more than 150 mm.

All anodes shall be inspected visually to confirm compliance with the above requirements.

7.7 Cracks

Zinc anodes shall be free from cracking.

Cracks can be accepted in aluminium anodes provided the cracks will not cause any mechanical
failure during installation, transportation or service of the anode. The combination of cracks and
lack of bond to the anode core is not accepted.

Cracks in the area where the anodes are not fully supported by the anode core are not acceptable.

1. Stand-off and flush anodes

   •   Cracks within the section of an anode supported by the insert are not acceptable if the length
       is more than 100 mm and/or the width more than 2 mm.
   •   Cracks penetrating to the steel inserts or through the anode are not permitted.
   •   Maximum 10 cracks pr. anode.

2. Bracelet anodes

   •   For sections of anodic material not wholly supported by the anode insert, no visible cracks
       shall be permitted.
   •   Cracks penetrating to the steel inserts or through the anode are not permitted
   •   Cracks with a length of more than 200 mm and/or width greater than 5 mm are not
       acceptable

Provided the above is satisfied, the following cracks are acceptable in transverse direction:

   •   Cracks with a length of less than 50 mm and width less than 5 mm.
   •   Cracks with a length between 50 mm and 200 mm and width less than 1 mm.
   •   Cracks with a length of 50-200 mm shall be limited to 2 per half bracelet or 4 per anode.
   •   Cracks which follow the longitudinal direction of the anodes shall not exceed 100 mm in
       length or/and 1 mm in width.




M-503, Rev. 2, September 1997                                                                   page 13
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3. Others

     •   Acceptance criteria for other anodes not defined above shall be established by the anode
         manufacturer.

7.8 Internal defects, destructive testing

At least two anodes of each size shall be subject to close inspection by destructive testing
(sectioning) for lack of bond between the steel inserts and the anode material and to verify that the
requirements of NACE RP0387 and RP0492 to internal defects are met. For smaller anode
deliveries the extent of testing within each anode type/size shall take account of anode design and
number of anodes. If one or both anodes fails, two additional anodes shall be subject to destructive
testing. If these do not satisfy specified requirements, the whole anode lot shall be rejected.

For non-tubular cores (e.g. bracelet anodes) where prevention of voids may be particularly difficult,
the limits shall be prepared by anode manufacturer and agreed with Purchaser prior to manufacture.

The insert position within the anode shall be confirmed by measurement on the cut faces.

7.9 Electrochemical quality control testing

The following shall be tested:

     •   Closed circuit potential.
     •   Consumption rate.
     •   Visual examination of corrosion pattern (uneven consumption, intergranular attack, etc.)

The tests are to be carried out for each 15 tonnes of anodes produced. The electrochemical test data
shall be included in the material certificate.

The closed circuit potentials and the capacity shall comply with the criteria stated in table 7.1. or an
agreed deviation based on the test method. For capacity of aluminium anodes single values down to
2500 Ah/Kg are acceptable, while average for each batch shall be minimum 2600 Ah/Kg.

The test procedure shall be according to DNV RP B 401, Appendix A. The test shall be carried out
in natural seawater or artificial seawater according to ASTM D1141.

Table 7.1 - Requirements to electrochemical performance (production testing) at all current
densities

                            Electrochemical Capacity       Closed circuit potential,
                                    Average                          mV
                                     (Ah/kg)                (Ag/AgCl Seawater)
AlZnIn                                2600*                           -1070
Zn                                      780                           -1030

NOTE - * Single values of min. 2500 Ah/kg are acceptable.



M-503, Rev. 2, September 1997                                                                  page 14
                                                                                                    INDEX




              ASTM - American Society for Testing and Materials

•   B 117 - 97 Practice for Operating Salt Spray (Fog) Apparatus
•   C 876 - 91 Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete
•   G 1 - 90 Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
•   G 2 - 88 Test Method for Corrosion Testing of Products of Zirconium, Hafnium, and Their Alloys in
    Water at 680oF or in Steam at 750oF
•   G 2M - 88 Test Method for Corrosion Testing of Products of Zirconium, Hafnium, and Their Alloys in
    Water at 633oK or in Steam at 673oK [Metric]
•   G 3 - 89 Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
•   G 4 - 95 Guide for Conducting Corrosion Coupon Tests in Field Applications
•   G 5 - 94 Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization
    Measurements
•   G 15 - 97a Terminology Relating to Corrosion and Corrosion Testing
•   G 16 - 95 Guide for Applying Statistics to Analysis of Corrosion Data
•   G 28 - 97 Test Methods of Detecting Susceptibility to Intergranular Attack in Wrought, Nickel-Rich,
    Chromium-Bearing Alloys
•   G 30 - 97 Practice for Making and Using U-Bend Stress-Corrosion Test Specimens
•   G 31 - 72 Practice for Laboratory Immersion Corrosion Testing of Metals
•   G 32 - 92 Test Method for Cavitation Erosion Using Vibratory Apparatus
•   G 33 - 88 Practice for Recording Data from Atmospheric Corrosion Tests of Metallic-Coated Steel
    Specimens
•   G 34 - 97 Test Method for Exfoliation Corrosion Susceptibility in 2XXX and 7XXX Series Aluminum
    Alloys (EXCO Test)
•   G 35 - 88 Practice for Determining the Susceptibility of Stainless Steels and Related
    Nickel-Chromium-Iron Alloys to Stress-Corrosion Cracking in Polythionic Acids
•   G 36 - 94 Practice for Evaluating Stress-Corrosion-Cracking Resistance of Metals and Alloys in a
    Boiling Magnesium Chloride Solution
•   G 37 - 90 Practice for Use of Mattsson's Solution of pH 7.2 to Evaluate the Stress-Corrosion Cracking
    Susceptibility of Copper-Zinc Alloys
•   G 38 - 73 Practice for Making and Using C-Ring Stress-Corrosion Test Specimens
•   G 39 - 90 Practice for Preparation and Use of Bent-Beam Stress-Corrosion Test Specimens
•   G 40 - 98 Terminology Relating to Wear and Erosion
•   G 41 - 90 Practice for Determining Cracking Susceptibility of Metals Exposed Under Stress to a Hot
    Salt Environment
•   G 44 - 94 Practice for Evaluating Stress Corrosion Cracking Resistance of Metals and Alloys by
    Alternate Immersion in 3.5% Sodium Chloride Solution
•   G 46 - 94 Guide for Examination and Evaluation of Pitting Corrosion
•   G 47 - 90 Test Method for Determining Susceptibility to Stress-Corrosion Cracking of High-Strength
    Aluminum Alloy Products
•   G 48 - 97 Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related
    Alloys by Use of Ferric Chloride Solution
•   G 49 - 85 Practice for Preparation and Use of Direct Tension Stress-Corrosion Test Specimens
                                                                                                     INDEX




•   G 50 - 76 Practice for Conducting Atmospheric Corrosion Tests on Metals
•   G 51 - 95 Test Method for Measuring pH of Soil for Use in Corrosion Testing
•   G 52 - 88 Practice for Exposing and Evaluating Metals and Alloys in Surface Seawater
•   G 54 - 84 Practice for Simple Static Oxidation Testing
•   G 56 - 82 Test Method for Abrasiveness of Ink-Impregnated Fabric Printer Ribbons
•   G 57 - 95a Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode
    Method
•   G 58 - 85 Practice for Preparation of Stress-Corrosion Test Specimens for Weldments
•   G 59 - 97 Practice for Conducting Potentiodynamic Polarization Resistance Measurements
•   G 60 - 95 Test Method for Conducting Cyclic Humidity Tests
•   G 61 - 86 Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for
    Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys
•   G 64 - 91 Classification of Resistance to Stress-Corrosion Cracking of Heat-Treatable Aluminum
    Alloys
•   G 65 - 94 Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus
•   G 66 - 95 Test Method for Visual Assessment of Exfoliation Corrosion Susceptibility of 5XXX Series
    Aluminum Alloys (ASSET Test)
•   G 67 - 93 Test Method for Determining the Susceptibility to Intergranular Corrosion of 5XXX Series
    Aluminum Alloys by Mass Loss After Exposure to Nitric Acid (NAMLT Test)
•   G 69 - 97 Practice for Measurement of Corrosion Potentials of Aluminum Alloys
•   G 71 - 81 Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes
•   G 73 - 93 Practice for Liquid Impingement Erosion Testing
•   G 75 - 95 Test Method for Determination of Slurry Abrasivity (Miller Number) and Slurry Abrasion
    Response of Materials (SAR Number)
•   G 76 - 95 Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets
•   G 77 - 97 Test Method for Ranking Resistance of Materials to Sliding Wear Using Block-on-Ring Wear
    Test
•   G 78 - 95 Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in
    Seawater and Other Chloride-Containing Aqueous Environments
•   G 79 - 83 Practice for Evaluation of Metals Exposed to Carburization Environments
•   G 81 - 97a Test Method for Jaw Crusher Gouging Abrasion Test
•   G 82 - 83 Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion
    Performance
•   G 83 - 96 Test Method for Wear Testing with a Crossed-Cylinder Apparatus
•   G 84 - 89 Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions
    as in Atmospheric Corrosion Testing
•   G 85 - 94 Practice for Modified Salt Spray (Fog) Testing
•   G 87 - 97 Practice for Conducting Moist S02 Tests
•   G 91 - 97 Practice for Monitoring Atmospheric S02 Using the Suffation. Plate Technique
•   G 92 - 86 Practice for Characterization of Atmospheric Test Sites
•   G 96 - 90 Guide for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and
    Electrochemical Methods)
•   G 97 - 97 Test Method for Laboratory Evaluation of Magnesium Sacrificial Anode Test Specimens for
    Underground Applications
                                                                                                     INDEX




•   G 98 - 91 Test Method for Galling Resistance of Materials
•   G 99 - 95a Test Method for Wear Testing with a Pin-on-Disk Apparatus
•   G 100 - 89 Test Method for Conducting Cyclic Galvanostaircase Polarization
•   G 101 -97 Guide for Estimating the Atmospheric Corrosion Resistance of Low-Alloy Steels
•   G 102 - 89 (1994)` Practice for Calculation of Corrosion Rates and Related Information from
    Electrochemical Measurements
•   G 103-97 Test Method for Performing a Stress-Corrosion Cracking Test of Low Copper Containing
    AI-Zn-Mg Alloys in Boiling 6% Sodium Chloride Solution
•   G 104 -89 Test Method for Assessing Galvanic Corrosion Caused by the Atmosphere
•   G 105 - 89 Test Method for Conducting Wet Sand/Rubber Wheel Abrasion Tests
•   G 106 - 89 Practice for Verification of Algorithm and Equipment for Electrochemical Impedance
    Measurements
•   G 107 -95 Guide for Formats for Collection and Compilation of Corrosion Data for Metals for
    Computerized Database Input
•   G 108 -94 Test Method for Electrochemical Reactivation (EPR) for Detecting Sensitization of AISI
    Type 304 and 304L Stainless Steels
•   G 109-92 Test Method for Determining the Effects of Chemical Admixtures on the Corrosion of
    Embedded Steel Reinforcement in Concrete Exposed to Chloride Environments
•   G 110 - 92 Practice for Evaluating Intergranular Corrosion Resistance of Heat-Treatabie Aluminum
    Alloys by Immersion in Sodium Chloride + Hydrogen Peroxide Solution
•   G 111 -97 Guide for Corrosion Tests in High-Temperature or High-Pressure Environment, or Both
•   G 112 - 92 (1997) Guide for Conducting Exfoliation Corrosion Tests in Aluminum Alloys
•   G 115 - 93f' Guide for Measuring and Reporting Friction Coefficients
•   G 116-93 Practice for Conducting Wire-on-Bolt Test for Atmospheric Galvanic Corrosion
•   G 117-93 Guide for Calculating and Reporting Measures of Precision Using Data From Interlaboratory
    Wear or Erosion Tests
•   G 118-96 Guide for Recommended Format of Wear Test Data Suitable for Databases
•   G 119-93 Guide for Determining Synergism Between Wear and Corrosion
•   G 123-96 Test Method for Evaluating Stress-Corrosion Cracking of Stainless Alloys with Different
    Nickel Content in Boiling Acidified Sodium Chloride Solution
•   G 129-95 Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to
    Environmentally Assisted Cracking
•   G 132-96 Test Method for Pin Abrasion Testing
•   G 133-95 Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear
•   G 134-95 Test Method for Erosion of Solid Materials by a Cavitating Liquid Jet
•   G 135-95 Guide for Computerized Exchange of Corrosion Data for Metals
•   G 137-97 Test Method for Ranking Resistance of Plastic Materials to Sliding Wear Using a
    Block-on-Ring Configuration
•   G 139-96 Test Method for Determining Stress-Corrosion Cracking Resistance of Heat-Treatable
    Aluminum Alloy Products Using Breaking Load Method
•   G 140-96 Test Method for Determining Atmospheric Chloride Deposition Rate by Wet Candle Method
•   G 142-96 Test Method for Determination of Susceptibility of Metals to Embrittlement in Hydrogen
    Containing Environments at High Pressure, High Temperature, or Both
•   G 143-96 Test Method for Measurement of Web/Roller Friction Characteristics
                                                                                                     INDEX




•   G 146-96 Practice for Evaluation of Disbonding of Bimetallic Stainless Alloy/Steel Plate for Use in
    High-Pressure, High-Temperature Refinery Hydrogen Service
•   G 148-97 Practice for Evaluation of Hydrogen Uptake, Permeation, and Transport in Metals by an
    Electrochemical Technique
•   G 149-97 Practice for Conducting the Washer Test for Atmospheric Galvanic Corrosion
•   G 150-97 Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels


           NACE - National Association of Corrosion Engineers

•   TM0170 Visual Standard for Surfaces of New Steel Airblast Cleaned with Sand Abrasive
•   TM0174 Laboratory Methods for the Evaluation of Protective Coatings Used as Lining Materials in
    Immersion Service
•   TM0175 Visual Standard for Surfaces of New Steel Centrifugally Blast Cleaned with Steel Grit and
    Shot
•   TM0183 Evaluation of Internal Plastic Coatings for Corrosion Control
•   TM0184 Accelerated Test Procedures for Screening Atmospheric Surface Coating Systems for
    Offshore Platforms and Equipment
•   TM0185 Evaluation of Internal Plastic Coatings for Corrosion Control of Tubular Goods by Autoclave
    Testing
•   TM0186 Holiday Detection of Internal Tubular Coatings of 10 to 30 mils (0.25 to 0.76 mm) Dry Film
    Thickness
•   TM0375 Abrasion Resistance Testing of Thin Film Baked Coatings and Linings Using the Falling Sand
    Method
•   TM0384 Holiday Detection of Internal Tubular Coatings of Less Than 10 mils (0.25 mm) Dry Film
    Thickness
•   RP0172 Surface Preparation of Steel and Other Hard Materials by Water Blasting Prior to Coating or
    Recoating
•   RP0178 Design, Fabrication, and Surface Finish of Metal Tanks and Vessels to be Lined for Chemical
    Immersion Service
•   RP0184 Repair of Lining Systems
•   RP0188 Discontinuity (Holiday) Testing of Protective Coatings
•   RP0281 Method for Conducting Coating (Paint) Panel Evaluation Testing in Atmospheric Exposure
•   RP0287 Field Measurement of Surface Profile of Abrasive Blast Cleaned Steel Surfaces Using a
    Replica Tape
•   RP0288 Inspection of Linings on Steel and Concrete
•   RP0372 Method for Lining Lease Production Tanks with Coal Tar Epoxy
•   RP0376 Monolithic Organic Corrosion Resistant Floor Surfacings
•   RP0386 Applications of a Coating System to Interior Surfaces of Covered Railroad Hopper Cars in
    Plastic, Food and Chemical Service
•   RP0487 Considerations in the Selection and Evaluation of Interim Petroleum-Based Coatings
                                                                                                 INDEX




                    SSPC - Steel Structures Painting Council

•   PA 1 Shop, Field, & Maintenance Painting
•   PA 2 Measurement of Dry Paint Thickness with Magnetic Gages
•   PA Guide 3 A Guide To Safety in Paint Application
•   PA Guide 4 A Guide to Maintenance Repainting with Oil Base or Alkyd Painting System
•   Guide to Vis 1 Pictorial Surface Preparation Standards for Painting Steel Surfaces
•   Guide to Vis 2 Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces
•   SP 1 Solvent Cleaning
•   SP 2 Hand Tool Cleaning
•   SP 3 Power Tool Cleaning
•   SP 5 White Metal Blast Cleaning
•   SP 6 Commercial Blast Cleaning
•   SP 7 Brush-Off Blast Cleaning
•   SP 8 Pickling
•   SP 10 Near-White Blast Cleaning
•   PS Guide 1.00 Guide for Selecting Oil Base Painting Systems
•   PS 1.04 Three-Coat Oil-Alkyd (Lead and Chromate Free) Painting System for Galvanized or
    Non-Galvanized Steel (With Zinc Dust-Zinc Oxide Linseed Oil Primer)
•   PS 1.07 Three-Coat Oil Base Red Lead Painting System
•   PS 1.08 Four-Coat Oil Base Red Lead Painting System
•   PS 1.09 Three-Coat Oil Base Zinc Oxide Painting System (Without Lead or Chromate Pigment)
•   PS 1.10 Four-Coat Oil Base Zinc Oxide Painting System (Without Lead or Chromate Pigment)
•   PS 1.11 Three-Coat Oil Base Red Lead Painting System
•   PS 1.12 Three-Coat Oil Base Zinc Chromate Painting System
•   PS 1.13 One-Coat Oil Base Slow Drying Maintenance Painting System (Without Lead or Chromate
    Pigment)
•   PS Guide 2.00 Guide for Selecting Alkyd Painting Systems
•   PS 2.03 Three-Coat Alkyd Painting System with Red Lead Iron Oxide Primer (For Weather Exposure)
•   PS 2.05 Three-Coat Alkyd Painting System for Unrusted Galvanized Steel (For Weather Exposure)
•   PS Guide 3.00 Guide for Selecting Phenolic Painting Systems
•   PS Guide 4.00 Guide for Selecting Vinyl Painting Systems
•   PS 4.01 Four-Coat Vinyl Painting System with Red Lead Primer (For Salt Water or Chemical Use)
•   PS 4.02 Four-Coat Vinyl Painting System (For Fresh Water, Chemical, and Corrosive Atmospheres)
•   PS 4.03 Three-Coat Vinyl Painting System with Wash Primer (For Salt Water and Weather Exposure)
•   PS 4.04 Four-Coat White or Colored Vinyl Painting System (For Fresh Water, Chemical, and
    Corrosive Atmospheres)
•   PS 4.05 Three-Coat Vinyl Painting System with Wash Primer and Vinyl Alkyd Finish Coat (For
    Atmospheric Exposure)
•   PS Guide 7.00 Guide for Selecting One-Coat Shop Painting System
•   PS 8.01 One-Coat Rust Preventive Painting System with Thick-Film Compounds
•   PS 9.01 Cold Applied Asphalt Mastic Painting Ssytem with Extra-Thick Film
•   PS 10.01 Hot Applied Coal Tar Enamel Painting System
                                                                                                   INDEX




    •   PS 10.02 Cold Applied Coal Tar Mastic Painting System
    •   PS 11.01 Black (or Dark Red) Coal Tar Epoxy-Polyamide Painting System
    •   PS Guide 12.00 Guide for Selecting Zinc-Rich Painting System
    •   PS 12.01 One-Coat Zinc-Rich Painting System
    •   PS 13.01 Epoxy-Polyamide Painting System
    •   PS 14.01 Steel Joist Shop Painting System
    •   PS Guide 15.00 Guide for Selecting Chlorinated Rubber Painting Systems
    •   PS 16.01 Silicone Alkyd Painting System for New Steel
    •   PS Guide 17.00 Guide for Selecting Urethane Painting Systems
    •   PS 18.01 Three-Coat Latex Painting System
    •   PS Guide 19.00 Guide for Selecting Painting Systems for Ship Bottoms
    •   PS Guide 20.00 Guide for Selecting Painting Systems for Boottoppings
    •   PS Guide 21.00 Guide for Selecting Painting Systems for Topsides
    •   PS Guide 22.00 Guide for Selecting One-Coat Preconstruction or Prefabrication Painting Systems


                              API - American Petroleum Institute

    •   Publ 941 Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum
        Refineries and Petrochemical Plants
    •   Publ 942 Controlling Weld Hardness of Carbon Steel Refinery Equipment to Prevent Environmental
        Cracking



ASME - American Society of Mechanical Engineers

AWS - American Welding Society

BSI - British Standards Institution

CSA - Canadian Standards Association

DIN - Deutsches Institute for Normung

ISO - International Organization for Standardization

NIST - National Institute of Standards and Technology
                                                                BACK   INDEX




Piper Alpha Videos: Production vs. Safety




                  http://www.youtube.com/v/XahGcezm3qM

                    http://v.blog.sohu.com/fo/v4/1263696

           http://www.youtube.com/v/BdRcALtA8CE&hl=zh_CN&fs=1
                                                                                             BACK   INDEX




Corrosion Slide Shows




                                     Combat corrosion costs and wins:
http://www.authorstream.com/player.swf?p=Barbara-36416-Frank-Garber-Presentation-Combat-Corrosion-Costsa
          nd-win-Background-Summary-really-cost-Motor-Vehicles-as-Entertainment-ppt-powerpoint

                                         Kinetic of reaction:
                http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/activa2.swf

                               Corrosive damage in metals and its prevention:
         http://www.slideshare.net/tkgn/corrosive-damage-in-metals-its-prevention?type=powerpoint

                                           Synthesis of reactions:
                         http://www7.tltc.ttu.edu/kechambe/flash/reactionsv15.swf

                                         Extraction of Aluminum:
                           http://www.sciencelessons.co.uk/flash/aluminium.swf
                                                                                            BACK       INDEX




                                               Redox reactions:
                   http://faculty.ksu.edu.sa/ALKHULAIWI/DocLib2/Electrochemistry.swf
                          http://www.youtube.com/v/a6RR4kPsnlE&hl=zh_CN&fs=1

                                               Electrolysis:
                               http://www.khayma.com/chim/electrolysis.swf
                              http://www.edukate.net/ed1_files/electrolysis.swf

                                 Iowa State University’s Science Animation
         [ http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/animationsindex.htm ]

                                            Electrolysis videos:
                        http://www.youtube.com/v/yMMrJTE3pyM&hl=zh_CN&fs=1
                         http://www.youtube.com/v/zhm0ozrpHJ8&hl=zh_CN&fs=1
                        http://www.youtube.com/v/lVK8RxkmOec&hl=zh_CN&fs=1

                                               Galvanic or voltaic cells:
                           http://www.kentchemistry.com/links/Redox/flash/halfcells.swf
                  http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf
http://education.uoit.ca/assets/Research~and~Teaching/Learning_objects/~voltaic_Zinc_Copper/zoltaic_zinc_lo.s
                                                         wf
                             http://www.wainet.ne.jp/~yuasa/flash/EngVoltaic_Cell.swf
                                  http://preparatorychemistry.com/Section_6_4.swf
                     http://demo.ydp.com.pl/raw/malezja/t15/media/td_chem_t15_03_a02.swf
                                                                                               BACK         INDEX




                                                        AP for H2S Services
               Recommended Reading:
                                                           Electrocoating

Anodic Protection Study Materials:

An impressed current technique can be applied if the material passivates in the particular environment. in this
case the structure is made more anodic by drawing electrons out of it until it enters the passive region.
There are advantages to this such as the cost of running the system is cheaper. However, the disadvantages
are high such as more complicated control system, and a non safe system if the power fails or becomes
uncontrolled. If the environment changes then the system may not passivate the same way.
As a result, anodic protection is not very popular.


In circumstances where cathodic protection is not practical, such as in strongly alkaline or acidic
environments, anodic protection is a useful corrosion control technique. Specifically, in metal-environment
conditions where active-passive behaviour is demonstrated, anodic protection is usually effective. In practise,
the metal-environment potential is held in the passive region by polarizing the structure in the electropositive
direction. Historically, anodic protection has the widest application in the process industries and in particular
on mild or stainless steel equipment used for concentrated sulfuric acid storage. Equipment, such as pulp mill
digesters and recausticizing (white, green & black) liquor clarifiers and storage tanks have also been
effectively protected.




    Application:


    •   Batch and continuous digesters
    •   Clarifiers and liquor storage tanks (white, green & black)
    •   Sulfuric acid storage tanks and piping
    •   Sulphuric acid cooler




Piracy Kill Creativity
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                                                                         BACK   INDEX




              Recommended Reading:




http://electrochem.cwru.edu/ed/encycl/art-a02-anodizing.htm

http://en.wikipedia.org/wiki/Anodizing



Corrosion Control by Anodic Protection

http://www.platinummetalsreview.com/pdf/pmr-v4-i3-086-091.pdf




http://materials.globalspec.com/Industrial-Directory/anodic_protection




Piracy Kill Creativity
If you like the books please purchase genuine!
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                                                         BACK   INDEX




                ANODIC
                PROTECTION
                Feasibility of anodic protection
                is firstly demonstrated and
                tested by Edeleanu in 1954




Corrosion control of metal structure by impressed
 anodic current.

Interface potential of the structure is increased into
  passive corrosion domain.

Protective film is formed on the surface of metal
  structure which decrease the corrosion rate down
  to its passive current.

Can be applied for active-passive metals/alloys
 only.




                                                                        1
                                                               BACK   INDEX




Anodic protection can decrease corrosion rate
substantially.
 Anodic protection of 304SS exposed to an aerated
          H2SO4 at 300C at 0.500 vs. SCE
     Acid          NaCl, M   Cor. Rate μm/y   Cor. Rate μm/y
concentration, M             (Unprotected)     (Protected)

      0.5           10-5         360              0.64
      0.5           10-3          74               1.1
      05
      0.5              1
                    10-1          81               51
                                                   5.1
       5            10-5        49000             0.41
       5            10-3        29000              1.0
       5            10-1         2000              5.3




Metals which can be passivated and de-
activated
 The metals which can be passivated by oxidation
 and activated by reduction are those which have a
 higher oxide less soluble than a lower oxide and
 will thus each corrosion domain forms an angle.
 The lower the apex of this angle in the diagram
           titanium                       etc )
 (such as titanium, chromium and tin etc.), the
 easier it will be to passivate the metal by oxidation
 and it will be difficult to reactivate the passivated
 metals by reduction.




                                                                              2
                                                       BACK   INDEX




                                      Titanium and
                                      chromium can be
                                      passivated very
                                      easily and their
                                      passivation
                                      process will occur
                                      more often than
                                      not,
                                      spontaneously,
                                      even in the
                                      absence of
                                      oxidizing agent.




Experimental potential - pH diagram for chromium




                                                                      3
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   Anodic polarization curve of AISI
   304 SS in 0.5 M H2SO4




Anodic protection parameters :
   (can be obtained from anodic polarization
     measurement)

     Range of potential in which metal is in
     passivation state (protection range)
     Critical current density
     Flade potential

   Optimum potential for anodic protection is
    midway in the passive region




                                                               4
                                                       BACK   INDEX




  Flade potential (EF)
           E F = E O − n 0,059pH
                   F

In which EFO : Flade potential at pH = 0
n : a constant (between 1 and 2) depends of metal
  composition and environment conditions
  Metals having EF < equilibrium potential of
  hydrogen evolution reaction (HER) can be passivated
                          (i e
  by non oxidizing acid (i.e. titanium)
  Increasing temperature will reduce the protection
  potential range and increase the critical current
  density and therefore anodic protection will be more
  difficult to be applied.




              Parameters that should be
           considered for anodic protection
             design (Flade potential is not
                included in the figure)           10




                                                                      5
                                                                 BACK   INDEX




    Influences of temperature and chloride concentration on
           anodic polarization curve of stainless steels
                        (schematic figure)




Anodic polarization curves of a mild steel in 10% sulfuric acid at
                         22 and 600C




                                                                                6
                                                                   BACK   INDEX




    For metals exposed in aggressive ions
    containing - environment
             g

    Interface potential of metal should be :
     Eprot>Elogam>Eflade

          ll                  l     l hl l         h
    Basically : Eflade is equal or slightly lower than
    Epp.




Schematic figure of potential range for anodic protection of a
 stainless steel which is susceptible to pitting corrosion in an
            environment containing aggressive ions




                                                                                  7
                                                     BACK   INDEX




Increasing of chloride ions concentration
results in a significant decrease of protection
               g                     p
potential range.
Consequently, in aggressive ions containing-
environment anodic protection is applied
only for metals which have relatively high
protection potential and high pitting potential.
Increasing temperature leading to a decrease
of Eprot




  Schematic figure of anodic protection system for
      protecting inner surface of storage tank




                                                                    8
                                                        BACK   INDEX




CATHODES FOR ANODIC PROTECTION
 Should be permanent and can be used as current
    ll t    ith t       i ifi t d        d ti
 collector without any significant degradation.
 Having large surface area in order to suppress
 cathodic overpotential.
 Low cost.
Platinum clad brass can be used for anodic protection
 cathodes because this cathode has low overpotential
 and its degradation rate is very low, however it is
 very expensive.




 Cathodes used in recent anodic protection
 systems




                                                                       9
                                                            BACK   INDEX




 Comparison of anodic and cathodic protection :
                  Anodic            Cathodic
                  protection        protection
Applicability     Active-passive
                  Active passive    All metals
                  metals only
Corrosives        Weak to           Weak to
                  aggressive        moderate
Relative          High              Low
investment cost
i    t    t    t
Relative         Very low           Mediums to
operation cost                      high
Equipment        Potentiostat +     Sacrificial anodes or
                                    DC power supply +
                 cathode/s          ICCP anode/s




Throwing          Very high         Low to high
power

Significant of
  g               Often a direct         p
                                    Complex
applied current   measure of        Does not
                  protected         indicate
                  corrosion rate    corrosion rate
Operating         Can be            Must usually be
conditions        accurately and
                           y        determined by y
                  rapidly           empirical
                  determined by     testing
                  electrochemical
                  measurement




                                                                           10
                                                               BACK   INDEX




      Typical applications of anodic protection




Anodic protection has been applied to protect storage tanks,
reactors, heat exchangers and transportation vessels for
corrosive solutions.

Heat exchangers (tubes, spirals and plates types) including
their anodic protection systems can be easily to purchase in
the market.

i.e. AISI 316 SS HE is used to handle 96-98% sulfuric acid
solution at 1100C. Anodic protection decreases corrosion
rate of the stainless steel, initially from 5mm/year down to
0.025mm/year and therefore less contaminated sulfuric
acid can be obtained.




                                                                              11
                                                           BACK   INDEX




                         DATA
 Effect of chromium content on critical current density
and Flade potential of iron exposed in 10% sulfuric acid.




 Effects of nickel and chromium contents on critical current
density passivation potential in 1N and 10 N H2SO4 containing
                         0.5 N K2SO4




                                                                          12
                                                              BACK   INDEX




    Requirement of critical protection current densities
   for several austenitic stainless steels (18-20 Cr , 8-12
            Ni) exposed in different electrolytes
    Protection current density : current density required
                    to maintain passivity




Effect of sulfuric acid concentration at 240C on the corrosion
      rate and critical current density of stainless steel




                                                                             13
                                                               BACK   INDEX




 Effect of stirring of electrolyte on the corrosion rate and
     i       t f         td     it t     i t i     i it
requirement of current density to maintain passivity on a
                    stainless steel at 27 0C




  Current density requirements for anodic
  protection




                                                                              14
                                                   BACK   INDEX




Anodic Protection Using a Galvanic Cathode
 A cylindrical tank of 304 stainless steel for
   storing deaerated sulfuric acid (pH=0) is
   found to corrode rapidly. To provide anodic
   protection, a galvanic cathode of platinum
   will be installed. The tank has a diameter of
   5 m and the depth of acid is 5 m.
a. Draw a labeled sketch of the polarization
   diagram for the tank and calculate the
   passivation potential versus SHE.
b. What is the area of platinum required to
   ensure stable passivity?
c. What will the corrosion potential be when
   the tank achieves passivity?




    Data:
        304 stainless steel:
        Ecor = -0.44 V vs SCE
        icor = 10-3 A/cm2
        Tafel slope anodic = 0.07 V/decade
        icrit = 1.4 x 10-2 A/cm2
        ipas = 4 x 10-7 A/cm2

      H+ reduction on platinum
       i0 = 10-3 A/cm2
       Tafel slope cathodic = 0.03 V/decade

      SCE = +0.2416 V vs.SHE




                                                                  15
                                                                                          BACK      INDEX




            CORROSION PREVENTION BY ELECTROCHEMICAL METHODS

Introduction
         Corrosion can be prevented by application of electrochemistry principles. This basically
falls into two distinct areas, sacrificial anodes and cathodic protection by impressed currents.

Sacrificial Anodes.
        In this preventative technique, corrosion is allowed to occur on a piece of metal that is
extraneous to the structure, for example, a zinc attached to a steel boat hull. The zinc corrodes in
place of the steel hull. The principles behind this process were discussed previously and will not be
repeated except to show the relevant Evans diagram.

       E
      (V)
                                Cathodic Reaction 1

                                                         Anode Reaction 3



      Ecorr 1+2
                                                                             Total Cathode 1+3
                         Cathode Reaction 3



                                        Anodic Reaction 2




                                    log Current Density
                                                  2                   icorr 1+2
                                            μA/cm

        In this case the anode#3 was protected from corrosion by anode reaction #2. One initial
principle is that the sacrificial material must have a potential lower than the material it is trying to
protect.
        Simple examples of the application of this protection technique include:-
Galvanized bolts, automobile steel and mail boxes, zincs placed outboard engines and steel boat
hulls, aluminum blocks on oil rigs, etc.
Typical coatings which work on this principle are:-
zinc on steel, aluminum on steel, cadmium on steel.
        It should be remembered that cadmium is only slightly below steel in the galvanic series.
As such it does not have much "Throwing power" which is the ability to protect over large
distances.
                                                                                     BACK        INDEX




        The consumption rate of the anodes was measured at Key West. The table below lists some
these rates.:-
        Material             Rate(lb/amp.yr)
            Zn                 24
            Mg                 17.5
            Al-Zn-Sn           20-7
            Al-Zn-In           8

           Note that the rate of consumption depends on the material and the current flowing.
           To design for protection the approximate current per square foot required for protection
should be known. Tables exist for this data. For example:-

Environment                                   Current Density for Protection
                                                     mA/m2(mA/ft2)

Immersed in Seawater                Well Coated        Poor Coating     Uncoated
Stationary 1-2 (0.1-0.2)             2-20(0.2-2)        20-30(2-3)
Low Vel 1-3 f/s(0.3-1m/s)            2-5                5-20                50-100
Med Vel 3-7 f/s(1-2 m/s)             5-7                10-30               150-300
High Vel Turb flow                   250-1000           250-1000            250-1000

Buried Underground.
Soil resistivity Ω.m                  0.5-5              5-15               15-40
                                       1-2                0.5-1              0.1-0.05

Hot sulfuric acid tank                500,000 (50,000)
Fresh water flowing pipes             50-100 (5-10)
Water heaters slow flow               10-30 (1-3)
Pilings in tidal seawater             60-80 (6-8)
Reinforcing steel                     1-5 (0.1-0.5)

           From this type of data the amps.yr data can be calculated to determine the size,
separation and replacement time for sacrificial anodes.
           Testing indicated that the corrosion rate of buried galvanized pipe varied from 0.6 to
19.5oz/in2 depending on the soil type. In buried conditions the type of soil must be known so that
accurate predictions can be made.

Cathodic Protection by Impressed Current.

       The objective here is to ensure the component requiring protection is maintained in its
cathodic region by the application of a voltage or cathodic current. The system is shown
schematically below:-
                                                                                     BACK      INDEX




                                      DC rectifier
                              - ve                       +ve


                        electrons




               Structure to protect
                                                             Anode in impressed
                                                             current system

An anode is involved. In some cases the anode can be a consumable anode and manufactured from
a cheap material such as scrap metal. In other cases, the anode should not be consumed if possible.
Such a case is for cathodic protection of steel in reinforced concrete. Anodes cannot easily be
replaced. The table above provides the required currents for protection. Typical anodes with
consumption rates are shown below:-
                                                                                    BACK       INDEX




Environment         Anode Material          Curr Den A/m2        Loss(lb/amp.yr)

Seawater            Pb-6%Sb-1%Ag             160-220               0.03-0.2
                    Pb-6%Sn-2%Ag             160-220               0.03-0.06
                    Pt on Ti,Nb or Ta        540-3200              0.008-0.016
                    Graphite                 10-40                 0.5-1.0
                    Fe-14.5Si-4.5%Cr         10-40                 0.5-1.0
                    Lead                                           0.1-0.25
                    Scrap steel                                    20
                    Aluminum                                       10-12

       The steel and aluminum are consumable anodes and so must be replaced at intervals. The
platinum anode is usually a coating on another metal in the form of a mesh. An example will be
shown in class. This mesh can be placed below the surface of concrete above the reinforcing steel.
Usually experts are called in to design cathodic protection systems as a phenomena called stray
currents can occur.

                                     DC rectifier
                             - ve                       +ve


                       electrons




         This happens when a short circuit path is available between the anode and the cathode so
that is carries current. An example would be the close proximity of another metal conductor to
both the anode and cathode. The short circuiting component then corrodes instead of the anode.
                                                                                     BACK       INDEX




Anodic protection.
        An impressed current technique can be applied if the material passivates in the particular
environment. in this case the structure is made more anodic by drawing electrons out of it until it
enters the passive region.
        There are advantages to this such as the cost of running the system is cheaper. However,
the disadvantages are high such as more complicated control system, and a non safe system if the
power fails or becomes uncontrolled. If the environment changes then the system may not
passivate the same way.
        As a result, anodic protection is not very popular.

Inhibitors.

      Inhibitors are used to reduce and block corrosion. They work by several different
mechanisms, some of which will be presented here.

Adsorption inhibitors.

        Adsorption inhibitors protect by adsorption on to the metal or metal oxide film exposed to
electrolyte. Organic inhibitors are aliphatic and aromatic amines (N compounds), thiourea( S
compounds) and aldehydes (O compounds). All these have a charged state, for example aliphatic
amines have ammonium cations present, R3NH+. The S and O compounds have a negative charge
on them. Thiourea bonds strongly to a metal by sharing its electrons with the metal surface. This
blocks solvating water molecules and also stops hydrogen gas molecule formation.
        N and O compounds are less adsorbed on the metal surface than the S type compounds.
They tend to select active anodic sites. The larger the molecule the greater the inhibition as they
displace solvating water molecules.

Poisons.

       These type of inhibitors block either of the hydrogen ion reduction or formation of
hydroxyl ions cathodic reduction reactions. The hydrogen ion reduction reaction is inhibited by
the group V metals or metalloids such as P, As or Sb. As2O3 is added at about 0.25M. The
combination of hydrogen atoms to hydrogen molecules is blocked in a reaction of the form:-

                                AsO+ + 2Hads + e- -> As + H2O
Alternatively:-
                                 As2O3 + 6Hads -> 2As + 3H2O
Scavengers

        Scavengers act to remove the oxygen preferentially before it can be used in the cathodic
reactions. Two popular examples are hydrazine and the sulfite ion.
                             N2H4 + 5/2 O2 -> 2NO2- + 2H+ + H2O

                                    SO32- + 1/2 O2 -> SO42-
                                                                                        BACK      INDEX




Filming Inhibitors.

        The addition of specific ions with high redox reaction potentials will produce local
reactions to form protective films. Two ions of this type are the chromate and nitrite ions.
        They have redox reactions:-
                         NO2- + 8H+ + 6e -> NH4+ +2H2O Eo = +0.9V

                      2CrO42- + 10H+ + 6e -> Cr2O3 + 5H2O Eo = +1.31V

        Both these reactions induce iron to dissolve in the ferric state with 3+ rather than in the
ferrous state as 2+. The ferric oxides are stable on the surface and block further corrosion.

                                  Fe3+ + 3H2O -> Fe2O3 + 6H+

Vapor phase.

        These tend to be nitrites, carbonate and benzoate filming inhibitors attached to parachutes
of an organic cation. An example is dicyclohexyl ammonium nitrite. The inhibitor evaporates onto
the metal surface.
INDEX




                                                                                                                                              Cathodic protection
BACK




                                                                                                           Introduction
                                                                                                           Electrochemical
                                                                                                                                     •   one of the most widely used methods
                                                                                                                                     •   works almost all the time on all
                         Cathodic and Anodic
                                                                                                             thermodynamics
                                                                                                             Electrochemical
                                                                                                             kinetics                    metals and environments
                                                                                                           Corrosion rate
                                                                                                                                     •   first used in 1820s to combat marine
                             Protection                                                                      measurements
                                                                                                           Various forms of
                                                                                                             corrosion
                                                                                                                                         corrosion
                                                                                                           Corrosion mitigation      •   now used primarily to coated protect
                                                                                                                cathodic and
                                                                                                                anodic protection
                                                                                                                                         carbon steel in neutral environments
                                                                                                                coatings and
                                                                                                                inhibitors           •   examples: pipelines, oil and gas
                                                                                                                material selection
                                                                                                                and design
                                                                                                                                         wells, offshore structures, seagoing
                                                                                                                                         ship hulls, marine pilings, water
                                                                                                                                         tanks, some chemical equipment
                                                                                                       1                                                                                      2
                           Principles of cathodic protection:
                               impressed current method                                                                                  Impressed current method
        Introduction
        Electrochemical
                                  Eapplied = Erev − ηc                  iapplied = ic − ia ≈ ic            Introduction
                                                                                                           Electrochemical
                                                                                                                                     • example:
          thermodynamics                                                                                     thermodynamics              – mild steel in strong acid
                                                                                                  2e
          Electrochemical                                                                                    Electrochemical
                                                                                                                                         – corrosion current: icorr ≈ 10 A/m2
                                   +

          kinetics
                                                                                     2+
                                                                                          +                  kinetics
        Corrosion rate
                                                                            Fe                             Corrosion rate                – corrosion rate: CR ≈ 11.5 mm/y
          measurements                                                                                       measurements
        Various forms of           potential / V
                                                   Ecorr
                                                                       Fe                                  Various forms of              –   apply cathodic polarization: ηc=120 mV
          corrosion                                                                                          corrosion
        Corrosion mitigation                                                                               Corrosion mitigation          –   reduces corrosion current to: icorr ≈ 0.1 A/m2
                                                                            H
             cathodic and
             anodic protection
                                                                                 +
                                                                                     +
                                                                                                                cathodic and
                                                                                                                anodic protection
                                                                                                                                         –   reduces corrosion rate to: CR ≈ 0.1 mm/y
                                                                                          e
             coatings and                                                                     -                 coatings and
                                                                                                                                         –   impressed current density: iapplied ≈ 150 A/m2
             inhibitors
                                                                                                   H            inhibitors
             material selection
                                                    Eapplied                                                    material selection       –   impressed current per m2: Iapplied ≈ 150 A
             and design                                                                                         and design
                                                                                                                                         –   not practical
                                                       ia≈ 0         icorr ic≈ iapplied
                                                                                                                                         –   need coating




                                      |
                                                               log i / (A m-2)
                                                                                                                                         –   hard to find one for strong acids
                                                                                                       3                                                                                      4
                                                                                                                                                                                                  1
INDEX




                                   Impressed current method:
                                  steel in neutral aerated water                                                                                       Impressed current method
BACK




        Introduction
        Electrochemical
                                    Eapplied = Erev − ηc                           iapplied = ic − ia ≈ ic                  Introduction
                                                                                                                            Electrochemical
                                                                                                                                                      • example:
          thermodynamics                                                                                                      thermodynamics           – mild steel in aerated neutral seawater
          Electrochemical                                         O                                                           Electrochemical
                                                                   2 +
                                                                       2H                                                                              – corrosion current: icorr ≈ 1 A/m2
                                     +




          kinetics                                                                                                            kinetics
                                                                         2O
        Corrosion rate                                                         +4                                           Corrosion rate             – corrosion rate: CR ≈ 1.1 mm/y
                                                                                 e     -
          measurements                                                                                                        measurements
                                                                                                                                                           apply cathodic polarization: ηc=120 mV
                                                                                            4O
        Various forms of                                                                      H-                            Various forms of           –
                                     potential / V




                                                                                                         e
          corrosion
                                                                                                 2+    +2                     corrosion
                                                                                                                                                       –   reduces corrosion current to: icorr ≈ 0.001 A/m2
        Corrosion mitigation                                                                                                Corrosion mitigation
                                                     Ecorr                                  Fe
             cathodic and
             anodic protection                                                     Fe
                                                                                                                                 cathodic and
                                                                                                                                 anodic protection
                                                                                                                                                       –   reduces corrosion rate to: CR ≈ 0.001 mm/y
             coatings and
             inhibitors
                                                                                                                                 coatings and
                                                                                                                                 inhibitors
                                                                                                                                                       –   impressed current density: iapplied ≈ 1 A/m2
             material selection
                                                      Eapplied                    2H
                                                                                                                                 material selection    –   impressed current per m2: Iapplied ≈ 1 A
             and design                                                                                                          and design
                                                                                    2O                                                                 –   practical
                                                                                            +2
                                                     ia≈ 0       ic≈ iapplied icorr           e    -
                                                                                                                                                       –   works even better with coating
                                        |




                                                                                                        H
                                                                                                         2   +   2O
                                                                        log i / (A m-2)                            H-                                  –   alkaline conditions lead to scale precipitation
                                                                                                                        5                                                                                                                                       6
                                    Impressed current method:                                                                                  Principles of cathodic protection:
                                   steel in neutral aerated water                                                                                  sacrificial anode method
        Introduction                                                                                                        Introduction
        Electrochemical
                                                             can one overdo it ?                                            Electrochemical                                                                                                                -
                                                                                                                                                                                                                                                       e
          thermodynamics                                                                                                      thermodynamics                                                                                                    2+   +2




                                                                                                                                                           +
          Electrochemical                                         O                                                           Electrochemical                                                                                              Cu
                                                                       +2
                                                                                                                                                                                                                                      Cu
                                                                   2
                                     +

          kinetics                                                       H                                                    kinetics
                                                                          2O
        Corrosion rate                                                         +4                                           Corrosion rate                                   Cu
                                                                                 e     -                                                                                    Ecorr
          measurements
                                                                                            4O
                                                                                                                              measurements                                                       H    +                     tota
                                                                                                                                                                                                          +e                    l ca




                                                                                                                                                           potential / V
        Various forms of                                                                      H-                            Various forms of                                    couple                             -                tho
                                                                                                                                                                            E                                                          dic
                                     potential / V
                                                                                                         e                                                                                                             H
          corrosion
                                                                                                 2+    +2                     corrosion                                         corr         H+       +e       -           on C             =
                                                                                                                                                                                                                                                   nod
                                                                                                                                                                                                                                                      ic
        Corrosion mitigation
                                                     Ecorr                                  Fe
                                                                                                                            Corrosion mitigation                             Zn
                                                                                                                                                                            Ecorr                                      H o
                                                                                                                                                                                                                                  u
                                                                                                                                                                                                                                              tal a
             cathodic and                                                                                                        cathodic and                                                                              n Zn           = to
             anodic protection                                                     Fe                                            anodic protection
                                                                                                                                                                                                      + 2e
             coatings and                                                                                                        coatings and
                                                                                                                                                                                                 2+
                                                                                                                                                                                            Zn
             inhibitors                                                                                                          inhibitors
                                                                                                                                                                                Zn
             material selection                                                                                                  material selection
             and design                                                           2H                                             and design
                                                                                       2O
                                                                                            +2
                                                      Eapplied                 icorr          e                                                                              Cu
                                                                                                                                                                           I corr,coupled               Cu
                                                                                                                                                                                                      I corr                  Zn
                                                                                                                                                                                                                            I corr              I corrcoupled
                                                                                                                                                                                                                                                  Zn ,




                                                                                                                                                              |
                                                                                                   -




                                        |
                                                                                                        H
                                                                                                         2 +
                                                                                                             2O                                                                                                    log I / A
                                                     ia≈ 0         log i / (A m-2)          ic≈ iapplied       H-
                                                                                                                        7                                                                                                                                       8
                                                                                                                                                                                                                                                                    2
INDEX




                                    Sacrificial anode method                                                                             Solution resistance problem
BACK




        Introduction
        Electrochemical
                                  • sacrificial anode continuously “consumed”                                  Introduction
                                                                                                               Electrochemical                                                                                                           -
                                                                                                                                                                                                                                    e
          thermodynamics            by corrosion and needs replacement                                           thermodynamics                                                                                              2+   +2




                                                                                                                                              +
          Electrochemical                                                                                        Electrochemical                                                                                        Cu
          kinetics                • good candidates:                                                             kinetics                                                                                     Cu
        Corrosion rate                                                                                         Corrosion rate
          measurements              – zinc: used broadly,e.g. galvanized zinc                                    measurements                                 Ecorr,Ω
                                                                                                                                                               Cu
                                                                                                                                                                                                          tota
                                                                                                                                                                                                              l   catho




                                                                                                                                              potential / V
        Various forms of
                                      coating is a common distributed sacrificial                              Various forms of                                 couple
                                                                                                                                                              Ecorr                        I applied RΩ                dic =
          corrosion                                                                                              corrosion                                                                                                            ic
                                      anode for steel                                                                                                                                                                              nod
        Corrosion mitigation                                                                                   Corrosion mitigation
                                                                                                                                                                                                                              tal a
             cathodic and                                                                                           cathodic and                                                                                          = to
             anodic protection      – magnesium: used for underground pipeline                                      anodic protection
                                      protection, i.e. in soil and other low                                                                                  EcorrΩ                     + 2e
             coatings and                                                                                           coatings and                               Zn ,
                                                                                                                                                                                    2+
                                                                                                                                                                               Zn
             inhibitors                                                                                             inhibitors
             material selection       conductivity environments                                                     material selection
                                                                                                                                                                   Zn
             and design                                                                                             and design
                                    – aluminium: improved life in seawater and                                                                                  Cu
                                                                                                                                                              I corr,coupled    I corr,Ωcoupled
                                                                                                                                                                                  Cu
                                                                                                                                                                                                       I corrΩcoupled
                                                                                                                                                                                                         Zn ,                  Zn
                                                                                                                                                                                                                             I corr,coupled




                                                                                                                                                 |
                                      other high conductivity environments because
                                      it polarizes less than zinc and magnesium                                                                                                                 log I / A
                                                                                                           9                                                                                                                                  10
                                      Anodic protection by
                                                                                                                                                               Anodic protection
                                       impressed current
        Introduction
                                                                                e   -
                                                                                                               Introduction
                                                                                                                                         •   suitable for active-passive alloys (e.g.
                                                                         Mn+ + n
        Electrochemical                                                                                        Electrochemical




                                                                                                pitting
                                                                                                                                             stainless steel, nickel alloys, titanium)
                                        +


          thermodynamics                                          M                                              thermodynamics
          Electrochemical                                                                                        Electrochemical
          kinetics                                                                                               kinetics                •   requires a broad potential range for
        Corrosion rate                                    Eapplied                                             Corrosion rate                passivity




                                                                                                passive
          measurements
                                        potential / V                                                            measurements
        Various forms of                                                                                       Various forms of          •   need sizable/expensive electrical
          corrosion
        Corrosion mitigation                             Epp
                                                                                                                 corrosion
                                                                                                               Corrosion mitigation
                                                                                                                                             equipment
             cathodic and
             anodic protection
                                                         Ecorr
                                                                                                                    cathodic and
                                                                                                                    anodic protection
                                                                                                                                         •   risky if potential “slips” into the
                                                                                                                                             active/pitting region




                                                                                                active
             coatings and                                                                                           coatings and
             inhibitors                                                                                             inhibitors
             material selection                                  icorr        icorr                                 material selection
                                                                                                                                         •   used often for very aggressive solutions
             and design                                                                 icrit                       and design




                                           |
                                                        log (current density) / (A m-2)
                                                                                                                                             when other methods fail, e.g. for
                                                                                                                                             protection of tanks storing of strong
                                                                                                                                             acids (e.g. sulphuric, phosphoric, nitric)
                                                                                                          11                                                                                                                                  12
                                                                                                                                                                                                                                                   3
INDEX




                                            Common issues
BACK




        Introduction
        Electrochemical
                                  •   potentiostatic vs. galvanostatic control
          thermodynamics
          Electrochemical
                                  •   reference electrodes
          kinetics
        Corrosion rate
                                  •   current distribution and throwing power
          measurements
        Various forms of
                                  •   complex geometry, crevices
          corrosion
        Corrosion mitigation
                                  •   stray currents
             cathodic and
             anodic protection    •   rectifiers
             coatings and
             inhibitors           •   cost
             material selection
             and design
                                                                                 13
                                                                                      4
                                                                                                 BACK      INDEX



Corrosion Control by Anodic
Protection
By    c. Edeleanu, M.A., Ph.D.
Tube Investments Research Laboratories, Cambridge



   It is well known that corrosion can somc-
times be controlled by cathodic currents and,              The technique of cathodic protection is
even with an elementary knowledge of electro-              well known and has been widely applied
chemistry, it is easy to appreciate why this               to a number of corrosion problems. It
should be so. Corrosion involves the oxida-                is not so well known that corrosion can
tion of the metal and it is reasonable to expect           also be prevented in suitable cases by
that cathodic polarisation, which discourages              anodic protection, using a platinum
oxidation and favours reductions at the metal              electrode system. The author shows that,
surface, should tend to cause protection. In               with adequate laboratory work before-
fact, the position is somewhat more compli-                hand and proper instrumentation, the
cated and, in many cases, other factors                    use of anodic protection can make an
override this apparently simple one.                       efectiue contribution to the life of a
   It is not so well known that corrosion can                            chemical plant.
also be prevented in suitable cases by anodic
polarisation, and it is certainly very much
more difficult to understand why this should
                                                        the driving force available for corrosion to a
be so from the somewhat oversimplified
                                                        minimum, and the other is to ensure that the
theory of corrosion which the non-specialist
                                                        corrosion product itself stifles the reaction
is bound to have. It is probably because of
                                                        by forming a suitably protective film.
this that this method, which is extremely
                                                           Using the terminology devised by Pourbaix
powerful and is often applicable just when
                                                        (I), we say that we make use of immunity in
cathodic protection is not possible, has not
                                                        the first case while in the second we depend
been easily accepted as a practical proposi-
                                                        on passivity.
tion and is still regarded as only a laboratory
                                                           In practice we can achieve immunity by
curiosity. There is, it seems, a feeling, per-
                                                        doing one or more of the following:
haps unconscious, that the method is basically
unsound, and the purpose of the present paper             (I) Using a suitably noble metal
is to explain, in as simple a way as possible,            (2)Removing      unnecessary oxidising
why anodic protection is possible, and when                  agents (e.g. air)
it may be expected to be useful.
                                                          (3) Adding a cathodic inhibitor (lessening
                                                              the effectiveness of the oxidising agents)
General Principles
in Corrosion Control                                      (4) Applying cathodic protection
   If the “brute force” methods of corrosion              In chemical plant it is often not economic
control such as plastic, glass or other coatings        to use noble metals, and if the solutions are
are neglected, there are two basic methods of           highly oxidising the other methods are in-
corrosion control available. One is to reduce           applicable.




Platinum Metals Rev., 1960, 4, (3), 86-91          86
                                                                                                   BACK        INDEX



  Passivity is achieved by:                                     1.6

   (I)   Using a metal having an oxide (or other
                                                                1.2
         similar corrosion product) which is
         virtually insoluble in the medium
                                                               0.8
   (2)   Ensuring that sufficient oxidising agent
         is always present for the oxide to be                 0.4
                                                          J
         formed                                           -
                                                          4
                                                          +
   (3) Applying anodic polarisation to main-              2
                                                          w
                                                                 0
                                                          c
       tain the oxide in constant repair
                                                          2- 0 . 4
   In principle therefore anodic protection
has much in common with the practice of
adding oxidising substances such as chromates
                                                              - 0.8
or nitrites as inhibitors. Cathodic protection

                                                                      i
                                                                          IMMUNITY
on the other hand is, in some ways, related
to practices such as de-aeration.                             -I-2

   The similarity can be taken further. In a                                                               1
metal,/solution system in which corrosion is
low because of immunity, corrosion is gener-
                                                          Fig. 1 Pourbaix diagram .for iron in   aqueous
ally enhanced by either the addition of                                       solutions
oxidising agents or by anodic polarisation,
while in a case depending on passivity it is
dangerous either to de-aerate or to apply                practical way of avoiding corrosion both
cathodic currents.                                       because of the very heavy current require-
                                                         ment and because there is little point in
Protection of Ferrous Materials                          preventing corrosion if to do so we have to
in Acid Solutions                                        decompose the solution.
   Anodic protection will probably prove most               Raising the potential of iron by anodic
useful with iron-based alloys in acid solutions          polarisation or by the addition of a suitable
and for this reason this case has been selected          oxidising agent to sufficiently high values for
as an example. Fig. I shows the Pourbaix                 passivity does, on the other hand, seem to
diagram (I) for iron; the conditions for                 be a more promising way of avoiding corro-
passivity and immunity are indicated. From               sion. This is particularly so since the area
this it will be seen that, in acid solutions,            of passivity for iron, and especially for some
there is a considerable gap of potentials over           of the iron-chromium alloys, is considerably
which neither of these conditions is estab-              larger than indicated by Fig. I which was
lished and which should lead to heavy cor-               obtained by calculation after making certain
rosion.                                                  assumptions.
   Lines A and B in this diagram refer to the               The actual relation between potential and
lower and upper limits of stability of water.            corrosion rate at a given pH is shown dia-
Above A water is oxidised to oxygen and                  grammatically in a somewhat simplified
below A it is reduced to hydrogen.                       manner in Fig. 2 . This is an experimentally
   If we place iron in a strong acid solution            determinable curve for any given solution and
we can in theory protect it cathodically by              alloy by using the potentiostatic techniques
lowering its potential to the region of im-              which are becoming widely used in corrosion
munity. However, since water is not stable               studies (2). From Fig. 2, which is typical
at such low potentials, continuous and rapid             of many cases, it can be seen that once the
hydrogen evolution will occur. This is not a             potential is raised sufficiently to establish




Platinum Metals Rev., 1960, 4, ( 3 )                87
                                                                                                     BACK     INDEX



 passivity the corrosion rate falls to really                From the above it must have become
 negligible values. For example with iron                 obvious that anodic protection is simply a
in normal sulphuric acid the rate falls to                way of ensuring that the potential of the
approximately 0. mgj cm2’day and the cur-
                    I                                     metal is kept sufficiently high for passivity
rent density necessary to maintain passivity              to be stable.
is 5 pA,lcm2. The rate of corrosion of passive
iron in this acid is therefore negligible and iron        Instrumentation
could be a very satisfactory container material.             If the potential of iron is raised appreciably
   It is important to appreciate at this stage            above line A in Fig. I , oxygen evolution takes
that the rate of corrosion of a metal in a given          place (i.e. the solution starts being decom-
acid solution is an accurately determinable               posed and current is wasted) so that this
property provided the potential is specified.             imposes an upper limit to the desirable
The highly scattered and apparently meaning-              potential. With the stainless steels oxygen is
less results often obtainable on conventional             not generally evolved, but the corrosion rate
corrosion “test specimens” are entirely due to            increases above a certain potential so that
the potential wandering in an uncontrolled                again there is an upper limit for the potential,
manner, but once results such as those in                 With titanium (3), and some other metals
Fig. 2 have been obtained for a given metal’              which form non-conductive films, there is
solution system we can fully depend on them               generally much greater latitude and it is
in practice, again provided we also ensure that           often possible to raise the potential by some
the potential of the plant relative to the solu-          tens of volts, but in these cases too the pro-
tion is kept at the correct value. Alternatively          tection can break down if the potential is
we can monitor accurately the rate of corro-              raised sufficiently.
sion by measuring the potential and referring                The important fact is that there is an
to Fig. 2.                                                upper, as well as a lower, limit to the range of
                                                          potentials which give satisfactory results.
                                                          This means that the instrument required for
                                                          anodic protection is a “potentiostat” but the
                                                          exact nature of the instrument depends
                                                          greatly on the system.
                                                             If the range of satisfactory potentials is
                                                          large, as with titanium, a very simple constant
                                                          voltage device such as an accumulator or
                                                          even a dry cell will meet the requirements.
                                                          I n such a case it can safely be assumed that
                                                          the potential of the inert cathode will not
                                                          wander by more than a few hundreds of
                                                          millivolts no matter what the current may be,
                                                          and if the potential between the cathode and
                                                          the plant is kept sufficiently great there will
                                                          be no danger that the potential of the plant
                                                          will fall to the breakdown point. Cotton has
                                                          in point of fact found this system completely
                                                          satisfactory for titanium in hydrochloric acid.
                                                             This simple method should also be applic-
                   CORROSION R A T E
                                                          able in certain cases for ferrous alloys, even
 Fig.2 Relation between potential and corrosion           though the useful potential range is only a
          rate for iron i n sulphuric acid                few hundreds of millivolts but, in general, it



Platinum Metals Rev., 1960, 4, ( 3 )                 88
                                                                                              BACK        INDEX


would be safer to use a true potentiostat.            itself, the rate of corrosion is very high. In
This instrument measures the potential of             some cases this rate can be many orders of
the plant against a standard electrode, and           magnitude greater than that of the passive
maintains it at the desired value by passing a        metal. If a vessel were to go active, in order
polarising current through an inert auxiliary         to re-establish passivity the protective device
electrode.                                            would have to be able to supply a current
   There are numerous potentiostat circuits           equivalent to the highest possible rate of
available and the laboratory types are fully          corrosion. This means that the potentiostat
electronic and can control potentials very            must be able to provide a current many
accurately but have a rather low current out-         orders of magnitude above that necessary for
put. For industrial use output is the main            protection, and if it cannot it may lose control.
requirement, and a servo-operated instrument          This is the reason why monitoring is thought
would be more satisfactory.                           to be advisable. This danger may be one
   The cost of equipment for anodic protec-           reason why the method has not found much
tion should not be high even if a true potentio-      support up to now. Serious as it is, it has
static system is called for but, if the method is     certainly been overstated possibly because, in
to be used to best advantage, it is worth             an effort PO demonstrate the spectacular
installing, at the same time, a monitoring            possibilities of the method, the solution used
system to provide a record of the performance         in the first pilot plant experiments was one
of the plant from the corrosion point of              of the most difficult to handle (6). In that
view ( ) This could also provide a warning
        4.                                            case the potentiostat available was highly in-
should anything unforeseen occur.                     adequate for the purpose (having been con-
   The position is exactly analogous to the           structed for laboratory studies on small
use of a temperature controller on, for in-           specimens) and could supply a current great
stance, a furnace, which will protect the             enough for protection, but there was little in
furnace from overheating, but, without a              hand to allow for even small local accidents.
temperature recorder or at least an indicator,        Nevertheless the plant ran successfully for
the system is incomplete.                             many hundreds of hours. More recent
                                                      American work (7, 8, 9) has shown that the
Dangers and Limitations in the                        risk is not unduly great, and with suitable
                                                      instrumentation it should be possible to
Application of Anodic Protection
                                                      overcome this difficulty entirely.
   The method is particularly suitable for               It is not possible to enumerate all the
application in the heavy chemical field, but          limitations of the method but it is just worth
the solutions handled in chemical plant differ        pointing out that not all metals show an
so greatly that each case has to be studied on        adequate range of passivity, and that with
a laboratory scale before anodic protection           any given metal passivity will not be stable in
can be safely applied.                                all solutions. The method depends on an
   This preliminary work must include a               electrolytic current arriving at the metal so
metallographic study, since there are various         that it is inapplicable above the wash line
types of corrosion such as intercrystalline           in a vessel or in similar places.
corrosion and selective attack that can limit
the use of alloys to a smaller range of poten-        Applications of Anodic Protection
tial than might be appreciated (5).                      Although there have been some reports in
   The greatest danger comes, however, from           the technical press (9, 10)of the use of anodic
the shape of the curve sketched in Fig. 2.            protection, and there have been a few other
In this it can be seen that at potentials just        trials, the method has as yet hardly been
below those at which protection establishes           tried in practice.




Platinum Metals Rev., 1960, 4, ( 3 )             89
                                                                                           BACK          INDEX



   From a corrosion point of view all chemical      the solution is a good conductor. Naturally,
plant tends to be grossly over-designed, since      it is somewhat morc difficult to deal with an
it is like a furnace without a temperature con-     accidental breakdown at the end of a tube
troller or recorder. The scope for the use of       than inside a vessel, but it is relatively easy to
protection and/or monitoring is therefore           assess the risks involved.
enormous. With stainless steel plant, for              It is not possible to protect above the wash
instance, it is usual to maintain acid strength,    line in a vessel where corrosion may be due to
temperatures, pressures or other such vari-         spray. Some parts of valves and pumps are
ables below values which give trouble. Since        also difficult, but there is no reason why
there is generally no means of telling how          materials which are naturally resistant should
near the plant is to losing passivity the           not be used at the danger points in conjunc-
materials are not used to their limit. Another      tion with inferior materials elsewhere. Pro-
way of saying the above is that unnecessarily       vided the materials are suitably selected there
expensive grades of material are usually            should be no complications with stray
selected for chemical plant in order to             currents.
provide some degree of safety.                         In so far as electrodes are concerned the
   It seems that it is possible to make a dis-      standard, if used, could be similar to that
tinction between two uses of anodic protec-         which would be used for pH measurements
tion. In the first instance it should be possible   in the same medium. Bearing in mind how-
to employ it in order to allow existing plant       ever that the accuracy required of the standard
and materials to be used to their limit, with       for this application is not great, very simple
anodic protection and/or monitoring only as         and robust standards could be used instead.
a safety device. With courage however there         For example, a platinum wire responding to
seems no reason why plant should not be             the natural redox potential of the solution
specially designed from inferior materials          would be adequate if this were reasonably
which would depend for survival entirely on         stable.
anodic protection. In this case, of course,            As far as the cathode is concerned there is
the anodic protection system may have to be         again considerable latitude, but it is worth
expensive but the economics could turn out          remembering one point. If a potentiostatic
to be attractive if there were a substantial        system is used there may be short periods
saving on construction material, or if the plant    when the polarity of the current is reversed
could be run under conditions much beyond           so that the cathode becomes an anode. For
anything that could be visualised without           this reason if this electrode is made from, say,
protection.                                         copper or nickel, in the hope that it will be
                                                    protected cathodically, it may well vanish
Plant and Electrode Design                          during these reversals of polarity and, for
   There seems to be only one plant design          this reason, it is felt that noble metals are
feature to take into account. An electrolytic       more convenient. Platinum is a natural choice
current must flow to the plant for protection.      because of its good electrical conductivity,
The current necessary is generally lower than       low hydrogen overvoltage, good sealing to
IopA/cm2, and it is relatively easy to calcu-       glass and not least the ease of cleaning were a
late how far it will “throw” if the conductivity    deposit to be formed as a result of the passage
of the solution is known and if the available       of a current.
voltage range has been established. In
practice it is found that the throwing power        Summary and Conclusions
is enormous, as has been demonstrated by              From a corrosion point of view anodic
recent American work (9), and reasonably            protection is, to a chemical plant, what a
long tubes can be protected easily provided         temperature controller is to a furnace. With-




Platinum Metals Rev., 1960, 4, ( 3 )               90
                                                                                                   BACK        INDEX



out anodic protection chemical plant has to              point of fact there is nothing more strange in
be overdesigned and best use is not made of              protection by an anodic current than there is
materials.                                               in protection by oxidising agents such as
   The method has hardly been used in prac-              chromates, which are universally accepted.
tice although it is simple to apply. This is                There are of course dangers and limitations
probably partly due to an inadequate under-              but, with adequate laboratory work and suit-
standing of how the method works and a                   able instrumentations these do not amount to
feeling that it is a laboratory curiosity. In            a serious objection to the technique.

                                                 References
 I   M. Pourbaix       .                           Thermodynamics of Dilute Aqueous Solutions,
                                                       Arnold, London, 1949
 2 V. Cihal and M. IPrazak             ..   ..     J . Iron & Steel Znst., 1959, 193, 360
   C. Edeleanu . .       ..            ..   ..     J. Iron & Steel Inst., 1958, 188, 122
 3 J. B. Cotton   ..     ..            ..   ..     Chem. and Znd., 1958, p. 68; 1958, p. 492
 4 C. Edeleanu    ..     ..            ..   ..     Corrosion Technology, 1955, 2, 204
 5 C. Edeleanu    ..     ..            ..   ..     J. Iron & Steel Inst., 1957, 185,482
 6 C . Edeleanu   ..     ,.            ..   ..     Metallurgia, 1954, 50, 113
 7 J. D. Sudbury, 0. L. Riggs,and D. A.            Corrosion, 1960, 16, 91
      Shock
  8 D. A. Shock, 0. L. Riggs, and J. D.            Corrosion, 1960, 16,99
     Sudbury
 9 0. L. Riggs, M. 13utchison, and N. L.           Corrosion, 1960, 16,102
      Conger
I0   W. Mueller        ..     ..       ..   ..     Canadian J. of Technology, 1956, 34, 162




Properties of Platinum Metals and Alloys
AN ANNOTATED BIBLIOGRAPHY

   The literature dealing with the properties               The review of this mass of literature ex-
of platinum and the platinum group metals is,            tends to 105 pages and is reasonably compre-
on the whole, sparse and widely scattered. On            hensive. The publication as a whole is likely
this account a recent publication, called a              to prove an invaluable source book to anyone
“technical phasc report”, prepared by R. W.              interested in the literature of the platinum
Douglass, F. C. Holden and R. I. JafTee, of              metals, but it is rather less valuable as a
Battelle Memorial Institute for the U.S.                 critical survey. The brief introductory notes
Office of Naval Research, is particularly                on extraction and benefication are, for
welcome. This was written with the special               instance, misleading as far as modem condi-
intention that it should serve as a guide to             tions are concerned, for today South Africa
planning experimental work on the platinum               is undoubtedly the most significant world
group metals, “revealing”, as the authors put            source of the platinum metals. A few of the
it, “areas where concentrated study is needed            figures quoted for the physical and mechanical
and preventing duplication of previous work”             properties are certainly in error-at least as
and was produced as the first part of a study            far as the pure metals are concerned-and
at Battelle of the metallurgical properties of           need to be treated with much more reserve
the refractory platinum group metals.                    than is accorded them by the authors. How-
   As it is presented, this report provides a            ever, if this is treated as a first-class annotated
very careful survey of the literature of the             bibliography-which         it primarily is-the
past fifty years on the properties of the                report will be found a most useful work of
metals and on the constitution of their                  reference by all interested in the platinum
binary alloys, listing 281 references.                   metals.                                    J. C. C.




Platinum Metals Rev., 1960, 4, ( 3 )                91
                                                                                      BACK        INDEX



           CORROSION PROTECTION OF METALS

   Two methods of combating corrosion which are widely used in New Zealand are cathodic
   protection and chemical inhibitors. Both methods depend on controlling the charge on the
   metal surface, and this can be monitored by measuring the potential of the metal. The
   conditions needed to stop corrosion can then be predicted from an electrochemical phase
   diagram.

   Cathodic protection is effected by forcing the potential to a negative region where the
   metal is completely stable. This can be done by using a sacrificial anode made from a
   more reactive metal, or using an external power supply to change the amount of charge on
   the metal surface. Cathodic protection is well suited to steel structures in marine or
   underground environments.

   There is a class of chemical inhibitors which work by removing electrons from the metal,
   thereby pushing the potential into a positive region where an oxide film spontaneously
   forms. This results in a stable, passive surface with a very low corrosion rate. Industries
   apply this technology in processes where the inhibitor can be conveniently added without
   causing environmental or health problems.


 INTRODUCTION

 When iron or steel is exposed to atmospheric oxygen in the presence of water, the well-
 known rusting process takes place. The metal is degraded to form ferric rust, a red-brown
 compound, which is a sure sign of electrochemical oxidation of the underlying metal.

         4Fe + 3O2 + 2H2O → 4FeO.OH                                                              (1)

 Nearly all metals, with the exception of gold and platinum, will corrode in an oxidising
 environment forming compounds such as oxides, hydroxides and sulphides. The degradation
 of metals by corrosion is a universal reaction, caused by the simple fact that the oxide of a
 metal has a much lower energy than the metal itself. Hence there is a strong driving force
 for the oxidation of metals. For example the familiar metal aluminium, which is used in
 aircraft, window frames and cooking utensils, is attacked by oxygen to form the oxide as
 follows:

         4Al + 3O2 → 2Al2O3                                                                      (2)

 This reaction is strongly exothermic, releasing -1680 kilojoules per mole of oxide. In fact
 the driving force of the reaction is so great that powdered aluminium will burn to produce
 very high temperatures, sufficient to melt steel.

It is important to realise that corrosive attack on a metal can only occur at the surface of the
metal, hence any modification of the surface or its environment can change the rate of
reaction. Thus we have a basis for designing methods to protect metals from corrosion. A




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Table 1 - Corrosion protection techniques
  Concept                                        Industrial Process
  Removal of oxidising agent                     Boiler water treatment
  Prevention of surface reaction                 Cathodic protection - sacrificial anode
                                                                    - impressed current
                                                 Anodic protection
  Inhibition of surface reaction                 Chemical inhibitors
                                                 pH control
  Protective coatings:
   a. Organic                                    Paint
                                                 Claddings

    b. Metallic                                  Electroplating
                                                 Galvanising
                                                 Metal spraying

    c. Non-metallic                              Anodising
                                                 Conversion coatings
  Modification of the metal                      Alloys - stainless steel
                                                        - cupronickel
                                                        - high temperature alloys
  Modification of surface conditions             Maintenance to remove corrosive agents
                                                 Design to avoid crevices
                                                 Design to avoid reactive metal
                                                 combinations

number of such methods have been developed, and they are set out in Table 1. The table
shows a variety of different concepts by which the surface reaction rate can be reduced.
Each of these has given rise to a number of technologies, the majority of which are
represented in New Zealand industry. In some cases these industries are on a very large
scale. For example paint manufacture is a major chemical industry which consumes large
quantities of solvents, resins and pigments. Most paint products in New Zealand are used in
corrosion protection. Other major industries involved in corrosion control include
electroplating, anodising, galvanising and the production of corrosion resistant alloys.

In this article we will concentrate on two important methods of corrosion control used in New
Zealand industry, namely cathodic protection and chemical inhibitors. Other types of corrosion
control technology, such as electroplating and surface coatings, are covered elsewhere.

THE CHEMISTRY OF CORROSION REACTIONS

Corrosion reactions are electrochemical in nature. They involve the transfer of charged ions
across the surface between a metal and the electrolyte solution in which it is immersed.
There are two types of electrode reaction occurring at the metal surface: anodic and


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cathodic. Anodic reactions involve oxidation: electrons appear on the right hand side of the
equation. For example metallic iron can produce ferrous ions by the anodic reaction:

        Fe → Fe2+ + 2e-                                                                                       (3)

In a solution with higher pH, the anodic reaction produces a surface film of ferric oxide
according to reaction (4).

        2Fe + 3H2O → Fe2O3 + 6H+ + 6e-                                                                        (4)

Cathodic reactions involve electrochemical reduction: electrons appear on the left hand side
of the equation. In corrosion processes the most common cathodic reaction is the
electrochemical reduction of dissolved oxygen according to the equation:

        O2 + 2H2O + 4e- → 4OH-                                                                                (5)

Hence the reduction of oxygen at an electrode will cause a rise in pH due to hydroxide ion
production. This can be important in some corrosion processes as will be explained later.

The potential difference E across the interface between a metal and a solution is the key
factor controlling both the products of an electrode reaction and rate at which they are
formed. The potential difference itself is caused by layers of charges at the surface:
electrons in the metal and excess anions or cations in the solution, as shown in Figure 1.
This arrangement of charges is known as the double layer or the Helmholtz layer. It is
found not only on metal surfaces but also on other surfaces in contact with solutions such as
colloids and proteins. The state of charging of the Helmholtz layer and hence the magnitude
of the potential E can be changed as a result of using an external electrical current or by
electrode reactions such as those shown in equations (3) to (5). For example, in the
presence of a high concentration of oxygen, the cathodic reaction will remove electrons
from the metal surface hence making the metal more positively charged and increasing the
potential E.


                                                 .       -
                                   Electrons           e        Excess cations
                                                         -      forming surface
                                   forming surface     e
                                   charge               -       charge
                                                       e

                         Metallic iron                                          Water

                                                                        E




                            Figure 1 - Electric double layer at ametal surface
The surface charge on the metal (electrons) is equal and opposite to the excess charge in the solution (cations).
                  The potential difference, E, at the surface is created by the double layer.


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The role of the electrode potential E in defining the products of corrosion reactions can be
readily seen in Figure 2. This figure shows the corrosion products as a function of electrode
potential and pH for iron at room temperature in the presence of water as solvent. At
negative potentials metallic iron itself is the stable form hence in this region no corrosion is
possible, and this is referred to as the immunity condition. At higher potentials and acidic
pH values ferrous ions will form giving rise to active corrosion. Ferric ions are produced
only at high potentials above 0.7 V.



                                        3+                                Passivation
                                   Fe
                        +1
                                                                Fe2O 3

                  E/V          Corrosion
                          0                       2+
                                                 Fe
                                                                         Fe3O4
                         -1
                                     Immunity
                                                           Fe


                                             0         5             10           15
                                                            pH
                                   Figure 2 - Iron equilibrium diagram
     Iron at 25oC in water. The diagram shows the stable forms of the element as a function of E and pH.

If the pH lies on the alkaline side of neutral then insoluble surface oxides will form. The
oxide Fe3O4 , known as magnetite or black iron oxide, is produced at low electrode
potentials. Low potentials are found in relatively stagnant conditions with a low oxygen
partial pressure as in soil or inside boilers which have been treated to remove oxygen. The
characteristic black surface of iron under these conditions is due to magnetite. At more
positive potentials the oxide formed is Fe2O3 and this is usually present as a thin adherent
film. Since this oxide forms at the surface, its presence acts to block the surface reactions
and hence corrosion rates are reduced. This is called passivation and the oxide film on the
surface is known as a passive layer. The corrosion rate is very low in the passivation region
of the diagram. Diagrams of the type shown in Figure 2 are widely used in corrosion
technology to predict the corrosion products which may be formed from a given metal under
conditions specified by the axes of the figure. However the diagram does not tell the rate of
corrosion which may be the most important information required in a practical situation.

In order to understand the rate of the corrosion process we must examine the electrochemical
polarisation curves of the electrode reactions which take place on the metal surface. Figure 3
shows the polarisation curve of iron in an acidic solution at room temperature. The rates of the
electrode processes are controlled by the value of E. Thus, for a cathodic process in acidic
solution producing hydrogen gas by the reduction of hydrogen ions, the more negative the
electrode potential the greater the surface concentration of electrons and the faster the reaction
rate.

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        2H+ + 2e- → H2(g)                                                                                   (6)

Since the reaction rate is proportional to the flow of electrons (measured as a current I) the
diagram shows the magnitude of I as a function of E.


                                                                        O2


                                  Fe2O3

                                 PASSIVE
         Potential


                                                                         ACTIVE
                                                                2+
                                                              Fe

             ECORR


                                                              H2
                                                                   CATHODIC


                                ICORR
                                                        Current
                                     Figure 3- Polarisation of iron
    The diagram shows how the potential, E, of the metal determines the electrochemical reaction rate and
                                            corrosion products.

Anodic reactions are accelerated by increasing potential in the positive sense as shown in
the diagram. Ferrous ions are produced in the active state and this is the region in which
corrosion will take place freely. At higher potentials the reaction passes into the passivation
region (as shown in figure 2) and passivation occurs. This is observed as a very small
current flowing in this region. The metal is protected by the passive film of ferric oxide on
the surface. We see at very positive potentials that the passive electrode surface will act as
an anode to oxidise water to oxygen gas, but this does not occur in normal corroding
systems. To find the corrosion rate under normal conditions we look for the point on the
diagram where the anodic and cathodic reactions intersect. At this point the rates of the
anodic and cathodic reactions are equal and the system is behaving as a closed circuit with
all the electrons produced in the anode reaction being consumed in the cathodic reaction.
This is the situation for an electrically isolated structure made from the metal. The
polarisation diagram can be used to predict changes of corrosion rate as will be discussed in
the next section.

CATHODIC PROTECTION

The principle involved in cathodic protection is to change the electrode potential of the
metallic article or structure so that it lies in the immunity region (shown in Figure 2).


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Within this region the metal is the stable form of the element and corrosion reactions are
therefore impossible. Cathodic protection may be regarded as the most elegant form of
corrosion protection because it renders the metal completely unreactive. It can however be
fairly expensive in the consumption of electric power or the extra metals involved in
controlling the potential within this region. There are two major methods of applying
cathodic protection to a metal structure and these will be discussed below.

In the case of iron or steel immersed in an aqueous solution the electrode potential should be
about -700 mV (standard hydrogen electrode scale) or even more negative than this in order
to ensure the structure remains in the immunity region. The metal surface under cathodic
protection will be completely free from corrosion, but there may be some evolution of
hydrogen gas according to equation (6). In seawater, calcareous deposits may form on the
surface due to the increase in pH which occurs as a result of cathodic reactions. These
deposits are composed of a mixture of calcium and magnesium basic carbonates, produced
by precipitation from the localised zone of alkaline seawater close to the metal surface.
Calcareous deposits of this type are found on the submerged steelwork supporting the Maui
gas platform, which is located 30 km off the coast of Taranaki.

(a) Impressed Current
This technique is widely used for the protection of buried pipelines and the hulls of ships
immersed in seawater. A d.c. electrical circuit is used to apply an electric current to the
metallic structure. The negative terminal of the current source is connected to the metal
requiring protection. The positive terminal is connected to an auxiliary anode immersed in
the same medium to complete the circuit. The electric current charges the structure with
excess electrons and hence changes the electrode potential in the negative direction until the
immunity region is reached. It is important that the anode be completely separated from the
cathode so that a true electric circuit is established with the current flow from the anode to
the cathode taking place through the solution between those electrodes.

Figure 4 shows the layout for a typical impressed current cathodic protection system. The
function of the reference electrode is to monitor the electrode potential of the protected
structure, in this case a buried pipeline, in order to ensure that the immunity region is
reached. The reference electrode is designed to have a constant potential and no current
passes through it. In the case of buried structures the most common reference electrode is
Cu/CuSO4 (saturated), with a potential of +316 mV (standard hydrogen scale). The d.c.
rectifier acts as the power supply and is adjusted so that the potential of the structure is
sufficiently negative to reach the immunity region, as indicated by the reference electrode.
It is usual to apply a surface coating or wrapping to the pipeline before cathodic protection
is used. This will result in a much smaller consumption of electricity since most of the
structure will be effectively protected by the coating. Special anode materials have been
designed to withstand applied currents for very long periods. They normally consist of
platinised titanium or lead alloys connected to an insulated cable positioned some distance
from the structure itself. The buried anodes are distributed at intervals along the pipeline,
normally several kilometres apart and several hundred metres from the nearest point of the
pipeline.




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                                                     DC rectifier
                                E                                                  Soil surface




                                                         Cathode
                                                         (i.e. buried pipe)

               Reference                                                      Anode
               electrode                                   Current




                 Figure 4 - Impressed current cathodic protection of a buried pipeline
   A DC current passes between a buried anode and the pipeline. The pipeline is connected to the negative
            terminal, hence its potential becomes more negativ and it functions as the cathode.

Impressed current cathodic protection is a specialised technology and can be very effective
if correctly designed and operated. Several warships operated by the Royal New Zealand
Navy have impressed current systems for corrosion control. Other examples are the natural
gas pipelines which distribute methane from the Kapuni and Maui fields. Impressed current
cathodic protection is applied to gas pipelines in Auckland, with deep anode installations at
the Auckland Domain and other points in the region.

(b) Sacrificial Anode
This technique is frequently used for ships in seawater and for offshore oil and gas production
platforms such as the Maui gas platform operated by Shell BP Todd Oil Services Ltd. The
principle here is to use a more reactive metal in contact with the steel structure to drive the
potential in the negative direction until it reaches the immunity region. Figure 5 illustrates the
principle. Zinc is often used as the sacrificial anode. In the absence of zinc the corrosion
potential ECORR is given by the intersection of the anodic and cathodic curves. If a zinc
electrode is now attached, it produces an anodic dissolution current at a more negative potential.
 The intersection with the cathodic curve now occurs at a more negative potential EPROT in the
region in which the steel itself has a negligible corrosion rate. In practice a reference electrode
is used to check that the steel structure has indeed reached the immunity region. A potential of
around -900 mV with respect to the Ag/AgCl reference electrode in seawater is the criterion for
immunity of the steel. In the case of the Maui platform it was not feasible to apply surface
coatings to the steel structure before it was installed, hence the corrosion protection of the 6,000
tonnes of steel forming the tower depends entirely on cathodic protection by sacrificial anodes
made from the aluminium alloy "Alanode". Some 580 tonnes of this alloy has been used to
produce several hundred separate anodes attached to the legs and braces of the tower under the
sea so as to give complete and uniform protection to all parts of the steel structure. Regular
monitoring of the potential of the steel is carried out using submerged reference electrodes of
Ag/AgCl. Aluminium is a sufficiently reactive metal to provide the required corrosion
protection, but a small proportion of indium, about 0.1%, is included in the alloy to provide
efficient anodic action. Pure aluminium alone has such a resistant oxide film that its reactivity


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                                                                     Steel-anodic dissolution

                  Potential

                       ECORR
                                                                      Zinc-anodic dissolution

                       EPROT
                                                                             Oxygen-cathodic
                                                                             reduction

                                                         Current
                             Figure 5 - Cathodic protection by a sacrificial anode
The addition of a sacrificial zinc anode to a steel structure shifts the potential from ECORR to EPROT, where steel
                       is protected from corrosion (anodic current for steel falls to zero).

is insufficient to properly protect the steel structure.

CORROSION INHIBITORS

It is well known in surface chemistry that surface reactions are strongly affected by the
presence of foreign molecules. Corrosion processes, being surface reactions, can be
controlled by compounds known as inhibitors which adsorb on the reacting metal surface.
The term adsorption refers to molecules attached directly to the surface, normally only one
molecular layer thick, and not penetrating into the bulk of the metal itself. The technique of
adding inhibitors to the environment of a metal is a well known method of controlling
corrosion in many branches of technology. A corrosion inhibitor may act in a number of
ways: it may restrict the rate of the anodic process or the cathodic process by simply
blocking active sites on the metal surface. Alternatively it may act by increasing the
potential of the metal surface so that the metal enters the passivation region where a natural
oxide film forms. A further mode of action of some inhibitors is that the inhibiting
compound contributes to the formation of a thin layer on the surface which stifles the
corrosion process.

Table 2 shows some examples of common inhibitor systems classified by their modes of
action. Adsorption inhibitors are used quite widely in many proprietary mixtures which are
marketed to control corrosion. For example, radiator fluids in the cooling circuits of engines
frequently contain amines such as hexylamine C6HI3NH2, or sodium benzoate. These act as
inhibitors of the anodic reaction. Corrosion inhibitors are also used in the metal cleaning
field. For example, it is possible to clean steel articles by immersion in sulfuric acid, H2SO4.
 The acid would normally attack the metal, causing corrosive loss. This can be minimised
by adding antimony trichloride, SbCI3, a specific inhibitor for preventing the corrosion of
steel in acidic media. Oxides and foreign metals such as zinc will readily dissolve in the
presence of SbCl3 , which acts only on the steel itself. Amine inhibitors are sometimes
present in volatile corrosion inhibitors. These are used in packaging materials to prevent
corrosion of steel articles during transport. A good example is the wrapping used on
automobile engines and other machinery during their shipment to New Zealand.

The second class of inhibitors are those which cause the potential of the metals to rise into
Table 2 - Corrosion inhibitors

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         Mode of action                                       Examples
           Adsorption                        amines                            RNH2
                                            thiourea                         NH2CSNH2
                                       antimony trichloride                    SbCl3
                                            benzoate                         C6H5COO-
           Passivating                        nitrite                           NO2-
                                            chromate                            CrO42-
                                             red lead                           Pb3O4
                                        calcium plumbate                       Ca2PbO4
          Surface layer                    phosphate                           H2PO4-
                                            silicate                          H2SiO42-
                                           hydroxide                            OH-
                                          bicarbonate                          HCO3-
                                       hexametaphosphate                      Na6(PO3)6

the passivation region. They are all oxidising agents, containing elements in their higher
oxidation states. For example nitrite, which is used as an additive in cooling fluid circuits
for the control of corrosion of steel, is a mild oxidising agent which can raise the potential of
steel into the passivation region. A traditional pigment used in paints is red lead, Pb3O4,
containing lead in the tetravalent stale, and the formula can be written as plumbous
plumbate Pb(II)2Pb(IV)O4. The plumbate ion is an active oxidising agent and serves to
promote passivation of the underlying metal. The modern pigment calcium plumbate, often
used in paint formulations, contains the same plumbate ion PbO44- in a different compound.
Likewise zinc chromate ZnCrO4 is also widely used in corrosion control as a passivating
inhibitor. The passivating inhibitors all share the common property of conferring protection
on a metal by using its own natural oxide film.

The last category of corrosion inhibitors are those which form a surface layer of a foreign
chemical compound provided by the inhibitor itself. For example phosphate is widely used
as an additive in boiler water or cooling circuits and in pickling baths for metals. Phosphate
produces a surface layer of ferric phosphate FePO4 on steel which provides a measure of
corrosion protection and is an excellent base for paints. Chromate is an extremely important
industrial inhibitor in spite of its toxicity and unfavourable environmental problems.
Chromate works in two ways, the high oxidation state Cr(VI) causes the metal to pass into
the passivation region (see Figure 2) and the product of oxidation by chromate is chromic
oxide Cr2O3 which itself forms an inert, relatively insoluble surface film. In practice
chromate treatment of steels produces a mixed film of ferric and chromic oxides which is
highly resistant to corrosion. An example of the use of chromate was the Marsden B
thermal power station, now retired. Large quantities of cooling water are circulated in the
plant and sodium chromate, added at a level of about 400 mg/L, was formerly used as a
corrosion inhibitor. It proved to be very effective in protecting the steel; but changes in
environmental regulations meant that it was no longer possible to permit discharge of
chromium at a level above 5 µg/L. This ruled out the use of sodium chromate as an inhibitor
at Marsden B and it was replaced by a new inhibitor system involving the use of an organic
zinc phosphate mixture. Some of the other inhibitors listed in this category of surface film
builders are very important industrially. The commercial inhibitor Calgon is a solution of
sodium hexametaphosphate, a condensed phosphate polymer based on the unit (-PO3-)n.

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Hexametaphosphate functions as a corrosion inhibitor because it has a high affinity for
metal cations such as calcium, zinc, copper and ferrous ions. Under some conditions it acts
to dissolve substances containing these cations and hence has a cleaning effect, assisting the
removal of scale deposits. But at the surface itself an insoluble layer of a ferrous
hexametaphosphate is deposited and will act as a corrosion inhibitor. Calgon therefore is
used as an inhibitor in potable water systems (drinking water) because it is non-toxic and is
widely used in large institutions such as hotels and hospitals. We must not neglect to
mention the simple hydroxide ion as a corrosion inhibitor. In the presence of hydroxide,
and hence high pH, metal oxides and hydroxides are insoluble, and these are effective in
controlling corrosion. For example, the common building material ferroconcrete involves
placing highly alkaline fresh concrete (pH above 12) in contact with steel reinforcing. The
high hydroxide concentration ensures effective corrosion inhibition by passivation of the
steel surface, and a strong bond is formed between the concrete and the steel.

CONCLUSION

Corrosion can be controlled effectively by cathodic protection or inhibitors, provided the
chemical and electrical conditions are monitored in a scientific manner. The same can be
said for all of the anti-corrosion technologies listed in Table 1. The costs of stopping
corrosion can be quite high, but these costs must be faced by many industries if they wish to
achieve a high level of performance. The key factor is the scientific knowledge on which
the technologies are based.




Article written by Graeme Wright (Chemistry Department, University of Auckland)




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                  WELDING RESEARCH




                                                                                                                                             RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT
                  SUPPLEMENT TO THE WELDING JOURNAL, FEBRUARY 1999
                  Sponsored by the American Welding Society and the Welding Research Council




    Hydrogen-Induced Cracking along the Fusion
        Boundary of Dissimilar Metal Welds

The susceptibility of dissimilar austenitic/ferritic combinations to hydrogen-induced
             cracking near the fusion boundary has been investigated


                                      BY M. D. ROWE, T. W. NELSON AND J. C. LIPPOLD

ABSTRACT. Presented here are the re-           Introduction                                    cracking has occurred during fabrica-
sults from a series of experiments in                                                          tion, prior to exposure to a hydrogen en-
which dissimilar metal welds were made             Dissimilar metal welds are used ex-         vironment. The fact that disbonding can
using the gas tungsten arc welding             tensively in the power generation, petro-       occur without prolonged exposure to hy-
process with pure argon or argon-6% hy-        chemical and heavy fabrication indus-           drogen in service suggests that either hy-
drogen shielding gas. The objective was        tries. Numerous instances of cracking           drogen is not necessary for disbonding to
to determine if cracking near the fusion       along the dissimilar metal fusion bound-        occur, or hydrogen absorbed during
boundary of dissimilar metal welds could       ary have been reported, particularly in         welding can cause cracking near the dis-
be caused by hydrogen absorbed during          cladding applications where a corrosion-        similar metal fusion boundary.
welding and to characterize the mi-            resistant austenitic alloy is applied to a          The fusion boundary microstructure
crostructures in which cracking oc-            ferritic structural steel. Often this crack-    in dissimilar welds often possesses some
curred. Welds consisted of ER308 and           ing, or disbonding, has been associated         unique features. Normal epitaxial nucle-
ER309LSi austenitic stainless steel and        with exposure to hydrogen in service            ation during solidification along the fu-
ERNiCr-3 nickel-based filler metals de-        and, as a result, the mechanism has been        sion boundary gives rise to grain bound-
posited on A36 steel base metal. Crack-        described by various authors as a form of       aries that are continuous from the base
ing was observed in welds made with all        hydrogen-induced cracking (Refs. 1–13).         metal into weld metal across the fusion
three filler metals. A ferrofluid color met-   This type of cracking has been repro-           boundary. These boundaries are roughly
allography technique revealed that             duced in the laboratory by exposing             perpendicular to the fusion boundary
cracking was confined to regions in the        austenitic cladding to hydrogen, either in      and have been referred to as “Type I”
weld metal containing martensite. Mi-          an autoclave or by cathodic charging            boundaries. In dissimilar welds, where
crohardness indentations indicated that        (Refs. 1–3, 7, 8, 11–13).                       an austenitic weld metal and ferritic base
martensitic regions in which cracking oc-          In practice, however, this form of          metal exist, a second type of boundary
curred had hardness values from 400 to                                                         that runs roughly parallel to the fusion
550 HV. Cracks did not extend into bulk                                                        boundary is often observed. This has
weld metal with hardness less than 350                                                         been referred to as a “Type II” boundary
HV. Martensite formed near the fusion                                                          (Ref. 6). These boundaries typically have
boundary in all three filler metals due to           KEY WORDS                                 no continuity across the fusion boundary
regions of locally increased base metal                                                        to grain boundaries in the base metal.
dilution.                                               Hydrogen                               Several investigators have reported that
                                                        Weld Cracking                          hydrogen-induced disbonding typically
M. D. ROWE, T. W. NELSON and J. C. LIP-
                                                        Dissimilar Metal                       follows Type II grain boundaries (Refs.
POLD were all with the Welding and Joining
Metallurgy Group, The Ohio State University,            Austenitic Stainless                   1–4, 7, 8, 12,13). The disbonding phe-
Columbus, Ohio, at the time this paper was              Filler Metals                          nomenon that occurs following fabrica-
written. Currently, M. D. ROWE is a graduate            Nickel-Based Filler                    tion and prior to service has also been as-
student at the Colorado School of Mines,                GTAW                                   sociated with these Type II boundaries.
Golden, Col., and T. W. NELSON is an Assis-             Martensite                                 An additional complication in
tant Professor at Brigham Young University,                                                    austenitic/ferritic dissimilar welds is the
Provo, Utah.                                                                                   dramatic transition in composition and


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RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT




                                                                                      Fig. 1 — Schaeffler constitution diagram (Ref. 14) showing predicted mi-    Fig. 2 — Plan view, ER308 filler metal, longitudinal strain, 30% dilu-
                                                                                      crostructures and minimum dilutions necessary to form martensite for the    tion, Ar-6%H2 shielding gas, chromic acid/nital etch, showing crack-
                                                                                      filler metal/base metal combinations used in this investigation.            ing near the fusion boundary and associated with light-etching bands
                                                                                                                                                                  in the weld metal.


                                                                                      microstructure that occurs adjacent to the       similar metal welds and aid in develop-
                                                                                      fusion boundary. This transition can be il-      ment of sound welding procedures.
                                                                                      lustrated using the Schaeffler Constitution
                                                                                      Diagram (Ref. 14). If a tie line is drawn on     Experimental Procedure
                                                                                      this diagram (Fig. 1) from a ferritic steel
                                                                                      base metal to an austenitic stainless steel      Materials
                                                                                      filler metal (such as Type 308 or 309LSi)
                                                                                      or a nickel-based filler metal (such as ER-         The filler materials selected for this in-
                                                                                      NiCr-3), it can be seen that intermediate        vestigation are commonly used in indus-
                                                                                      compositions along the tie line between          try for dissimilar metal welding. Type
                                                                                      the end points will promote martensitic          308, 309LSi and ERNiCr-3 filler metals
                                                                                      and austenitic plus martensitic mi-              were selected to cover a range of com-
                                                                                      crostructures. In practice this transition       positions and microstructures. A36 steel
                                                                                      occurs over a very short distance (less          was selected as the base metal. The
                                                                                      than 1 mm) from the fusion boundary into         chemical compositions of the materials
                                                                                                                                       are listed in Table 1.
                                                                                      the weld metal, and results in a localized
                                                                                      martensitic band along the fusion bound-
                                                                                                                                       Welding Procedures
                                                                                      ary. Cracking has been reported in the
                                                                                      martensitic transition zone near the fu-             The gas tungsten arc welding (GTAW)
                                                                                      sion boundary (Refs. 3, 12). Often, the          process was selected because it allows
                                                                                      Type II boundaries described previously          for close control of dilution and the ad-
                                                                                      reside in this martensitic region.               dition of hydrogen through the shielding          Table 2. Following welding, the weld-
                                                                                          In order to more carefully study the ef-     gas. Shielding gases consisting of pure           ment was left rigidly restrained for up to
                                                                                                                                       argon and Ar-6%H2 were used. Both a               four days, then inspected for cracking
                                                                                      fect of fusion boundary microstructure
                                                                                                                                       multipass and single-pass welding pro-            using side-bend tests, and by sectioning
                                                                                      on hydrogen-induced cracking in dis-
                                                                                                                                       cedure were developed to assess the ef-           and metallography.
                                                                                      similar welds, a number of dissimilar
                                                                                                                                       fect of hydrogen introduction through the             A single-pass procedure followed by