UN1001 REACTOR CHEMISTRY AND CORROSION Section 3 The Eight

Document Sample
UN1001 REACTOR CHEMISTRY AND CORROSION Section 3 The Eight Powered By Docstoc
					            UN1001:
REACTOR CHEMISTRY AND CORROSION
    Section 3: The Eight Forms of Corrosion

                      By

      D.H. Lister & W.G. Cook
   Department of Chemical Engineering
      University of New Brunswick
                                              1
              The Eight Forms of Corrosion

1. Uniform attack (general corrosion);
2. Galvanic corrosion;
3. Crevice corrosion;
4. Pitting;
5. Intergraular attack (“IGA”);
6. Selective leaching;
7. Flow-Accelerated Corrosion;
8. Stress corrosion cracking (“SCC”)


                                             2
      UNIFORM ATTACK or GENERAL CORROSION

• This is the most common form of corrosion.

• A chemical reaction (or electrochemical reaction)
  occurs over entire exposed surface (or large areas)
  more or less uniformly.

• Metal thins … fails.

• Not usually serious and is typically predictable from
  simple tests (e.g., coupon or specimen immersion)

• Can be designed “around” by specifying an adequate
  CORROSION ALLOWANCE for the expected lifetime
  of the component.
                                                          3
• Uniform attack minimized by:

  • specifying proper materials;

  • correctly applying coatings;

  • using corrosion inhibition;

  • protecting cathodically.




                                   4
1800-year-old Roman nail shows
how iron and steel can withstand
burial underground.



Note: Environment is crucial!




                                   5
              ATMOSPHERIC CORROSION

• Usually “uniform”.

• Dry, damp or wet conditions have profound effect on
  corrosion.

•   Dry atmospheres:

    • at ambient temperatures, most metals corrode very slowly;

    • atmospheric oxygen promotes a protective oxide film ... such films
      are defect-free (sort of!), non-porous (more or less!) and self-
      healing;

    • “passivity” of metals like SS, Ti, Cr depends on protective oxide
      films (but such passivity extends to other environments, e.g.,
      aqueous).
                                                                          6
7
• EXAMPLE:

  • Ag & Cu tarnish in dry air with traces of H2S
    (undesirable - aesthetically, technically - affects
    electrical contacts, etc.).

  • The S2- incorporation in the normally-protective oxides
    creates lattice defects which destroy protective nature
    of films … tarnishing.

  • Moisture not required for tarnishing, it can actually
    retard tarnishing of Cu in presence of traces of H2S.


                                                              8
• Damp atmospheres:

  • corrosion increases with moisture content;

  • at critical moisture level (~ 70% RH), an invisible, thin
    film of moisture forms on (metal) surface, provides
    “electrolyte” for current (critical RH depends on
    surface condition: cleanliness, presence of oxide or
    scale, presence of salts or other contaminants that may
    be hygroscopic).



                                                                9
• Wet atmospheres:

   • promote puddles, pockets, visible water layers (from dew, sea
     spray, rain, etc.);

   • crevices, condensation traps, etc., create water pools, and lead to
     “wet atmospheric corrosion” even when rest of surface dry;

   • Corroded weathering steel I-beam. Note how corrosion has thinned
     the bottom of the vertical web where corrosion products have
     fallen and formed a moist corrosive deposit. soluble corrosion
     products increase wet corrosion (dissolved ions increase
     conductivity, sustain higher electrical currents);

   • insoluble corrosion products may retain moisture during alternate
     wet and dry conditions, lead to continuous wet corrosion.

                                                                           10
• Corroded weathering steel
  I-beam.

• Note how corrosion has
  thinned the bottom of the
  vertical web where
  corrosion products have
  fallen and formed a moist
  corrosive deposit.




                        11
 Corroded steel framework on the ceiling of a parking
garage. The seams in this corrugated structure act as
condensation traps and lead to wet atmospheric corrosion.
                                                       12
Corroded weathering steel gutter.

                                    13
Rusting of iron and steel, formation of patina on copper,
  examples of damp  wet corrosion.




                 Corroded regions of a painted highway bridge.
                                                                 14
Corroded weathering steel highway bridge girder.
                                                   15
ATMOSPHERIC CONTAMINANTS

• Wet atmospheric corrosion is often governed by level of
  contaminants.

  e.g., marine salts vary drastically with distance from the sea:

  steel at 25 m from the sea will corrode 12x faster than same
  steel 250 m away.

• Industrial atmospheres are generally more corrosive than rural,
  mainly because of sulfur compounds produced by burning fuels.



                                                                 16
• SO2 selectively adsorbs on metals – under humid conditions
  metal oxide corrosion products catalyze oxidation to SO 3:

               SO2 + 1/2 O2 → SO3 (with a catalyst)

               H2O + SO3      →    H2SO4

• Small additions ( ~ 0.2%) of Cu, Ni or Cr increase resistance of
  steel to sulfur pollution by enhancing the formation of a tighter,
  more protective rust film.



  NOTE: longevity of ancient Fe probably due to SO2 - free
  environments rather than high degree of corrosion resistance.
                                                                17
• Nitrogen compounds promote atmospheric corrosion -
  from fuel burning (NOx as well as SOx), as well as by
  thunderstorms.

                      N2 + x O2 → 2 NOx;

   nitrogen-based fertilizers (from NH3) increase nitrogen
   pollutants in atmosphere.




                                                             18
• H2S promotes atmospheric corrosion (e.g., Ag, Cu tarnishing)

    • from industry (oil & gas, pulp and paper , etc.);

    • from decomposition of organic S compounds;

    • from sulfate-reducing bacteria (SRB) in polluted rivers etc.


                                 H 2O
                 SRB + SO42-     → H2S




                                                                19
• Dust particles detrimental (stick to metal surfaces, absorb water,
  H2SO4 etc., may contain Cl- … WHICH IS BAD … since it
  breaks down protective oxide films).

• CO2 dissolution in water can give pH ~ 5.6 (in equilibrium with
  normal atmosphere containing CO2) … BUT … CO2 is not
  significant in atmospheric corrosion, in fact sometimes can
  inhibit it (if SO2 is present).




                                                               20
                  ATMOSPHERIC VARIABLES

• Surface temperature very important - as T rises, corrosion
  rate rises - though damp and wet corrosion stop when
  moisture driven off;

• Metal surfaces that retain moisture generally corrode faster
  than rain-washed surfaces; rain flushes impurities off
  surfaces, removes particles, etc. that promote differential
  aeration, etc.;

• Winter exposure generally more severe (more combustion
  products in atmosphere, temperature inversions, etc.),
  though summer gives higher surface temperatures.


                                                             21
                 ATMOSPHERIC VARIABLES

• Relative humidity very important: for clean Fe, critical
  RH  60%

   • above this, rust begins to form slowly from
     deposited water film. At 75 - 80% RH, corrosion
     rate increases rapidly (probably because of capillary
     condensation within the rust layer).

   • if corrosion product rust is microporous, moisture
     will condense at different RHs depending on pore
     size:
       • 1.5 nm - diameter pore (capillary) condenses
         water at 50% RH;
       • 36 nm - diameter pore at 98% RH.
                                                             22
               ATMOSPHERIC VARIABLES


• Note: dust, particles, etc. on surfaces create
  crevices that can condense moisture at various
  RHs.

• Salt or soluble corrosion products will form
  electrolytes in condensed moisture - lower critical
  RH, also increase corrosion.




                                                    23
“ General Corrosion”




                       24
25
26
27
Damp and wet corrosion are described in terms of
  ELECTROCHEMISTRY.

We have seen how a metal dissolution, such as:

       Zn + 2 HCl           ZnCl2 + H2

can be regarded as two reactions:

       Zn  Zn2+ + 2 e-      (oxidation - an ANODIC process)

       2H+ + 2e  H2         (reduction - a CATHODIC process)


                                                           28
BOTH REACTIONS OCCUR SIMULTANEOUSLY
  AND AT THE SAME RATE.

                                  Electrochemical reactions
                                  occurring during corrosion
                                      of zinc in air-free
                                      hydrochloric acid


It follows, that during metallic corrosion …

 THE RATE OF OXIDATION EQUALS THE RATE OF
                REDUCTION.
                                                         29
• Implies that a corroding surface has anodic and cathodic areas for
  UNIFORM CORROSION

• These must be distributed evenly over the surface and in fact must
  move around.

• Some anodic “half reactions” for corrosion:

                      Zn  Zn2+ + 2 e-

                      Na  Na++ e-

                      Fe  Fe2+ + 2 e-

                      Cu  Cu2+ + 2 e-          … etc.

                                                                   30
• Anodic “half” reaction must be balanced by cathodic “half”
  reactions:

• Primary cathodic “half reactions” include:



hydrogen evolution                    2 H + + 2 e-  H 2

oxygen reduction                O 2 + 4 H + + 4 e-  2 H 2 O
(acid solution)

oxygen reduction                O2 + 2 H2O + 4 e-  4 OH-
(neutral or basic solution)

                                                               31
More cathodic half reactions:
metal ion reduction                M3+ + e-  M2+

                          (e.g.   Fe3+ + e-  Fe2+)



metal deposition          (e.g.   Cu+ + e-  Cu)



Note that the flow of charge (i.e., electrons) is a measure of the reaction
  rate (metal dissolution or corrosion rate).

Thus, if the corrosion “current” can be measured, the corrosion rate is
  directly evaluated through Faraday’s Law


                                                                          32
                   Faraday’s Law

                     M wt  I  t
                  m
                      n F
•   m = mass deposited/released (g);
•   Mwt = atomic or molecular weight (g/mol);
•   I = current passed (Amps);
•   t = time current/potential applied (seconds);
•   n = electrons transferred in the half-cell reaction;
•   F = Faraday constant (96485 C/mol).
    • this is the number of charges that must be passed to
      reduce or oxidise one mole of a compound

                                                             33
To illustrate:




                 34
• Say the imeasured = 10-5 amps/cm2

• Using Faraday’s Law the corrosion rate is calculated as:


  m  CR 
           56 g           
                    mol 105 A cm2    
                                      2.9 109 g cm2s
                  2  96485C mol

• Or in a more useable unit … (divide by the density –
  7.86 g/cm3 for iron; convert cm to mm and seconds to
  years)

         CR  116 mm / yr  0.12 mm / yr
                                                             35