An Overview of Corrosion and Protective Coatings
INTRODUCTION and THEORY
Billions of dollars are lost each year due to corrosion. Much of this loss is due to the oxidation of iron and steel, although
many other metals may corrode as well. The problem with iron as well as many other metals is that the oxide formed by
oxidation does not firmly adhere to the surface of the metal and flakes off easily causing "pitting". Extensive pitting of
iron eventually causes structural weakness and disintegration of the metal.
Corrosion which is an electrochemical oxidation reaction occurs in the presence of moisture. For example when iron is
exposed to moist air, it reacts with oxygen to form rust,
Fe2O3 - X H2O
The amount of water complexed with the iron (III) oxide (ferric oxide) varies as indicated by the letter "X". The amount of
water present also determines the color of rust, which may vary from black to yellow to orange brown. The formation of
rust is a very complex process which is thought to begin with the oxidation of iron to ferrous (iron "2+") ions.
Fe Fe2+ + 2e-
Both water and oxygen are required for the next sequence of reactions. The iron (2+) ions are further oxidized to form
ferric ions (iron "3+") ions.
Fe2+ Fe 3+ + 1 e-
The electrons provided from both oxidation steps are used to reduce oxygen as shown.
O2 (g) + 2 H2O 4 OH-
The ferric ions then combine with oxygen to form ferric oxide (iron (III) oxide) which is then hydrated with varying
amounts of water. This overall reaction may be written as:
4Fe2+(aq) + O2(g) + [ 4+ 2XH2O (l)] 2 Fe2O3.XH2O + 8H+(aq)
The formation of rust can occur at some distance away from the actual pitting or erosion of iron as illustrated below.
This is possible because the electrons produced via the initial oxidation of iron can be conducted through the metal and
the iron ions can diffuse through the water layer to another point on the metal surface where oxygen is available. This
process results in an electrochemical cell in which iron serves as the anode, oxygen gas as the cathode, and the
aqueous solution of ions serving as a "salt bridge" as shown below.
The involvement of water accounts for the fact that rusting occurs much more rapidly in moist conditions as compared
to a dry environment. Many other factors affect the rate of corrosion. For example the presence of salt greatly enhances
the rusting of metals. This is due to the fact that the dissolved salt increases the conductivity of the aqueous solution
formed at the surface of the metal and enhances the rate of electrochemical corrosion. This is one reason why iron or
steel tend to corrode much more quickly when exposed to salt or moist salty air near the ocean.
In reality though this is a galvanic process ( a series of spontaneous reactions determined by the potential energies of
the system ) and not a simple reaction as indicated above.
Hence we find that;
1. O2 + 4 e- + 4H+ = 2H2O ( E = + 1.23 V )
2. Fe3+ + e- = Fe2+ ( E = + 0.77 V )
3. Fe3+ + 3e- = Fe ( E = - 0.036 V )
4. Fe2+ + 2e- + Fe ( E = - 0.44 V )
Note : E values are taken from the table of Standard Reduction Potentials.
We find that the greatest potential difference is between equation 1 and equation 4 to give us;
5. 2 Fe + O2 + 4H+ = 2Fe2+ + 2 H2O ( E = E reduction – E oxidation = + 1.67 V )
Therefore we would expect that the most spontaneous will form Fe2+ not Fe3+.
However note equation 2 has an E value below that of equation 1, so any Fe2+ that forms from equation 5 will undergo
further reduction with equation 1 to give
6. 4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O ( E = + 0.36 V )
This reaction is then written to show rust is formed in a hydrated form;
7. 4Fe2+ + O2 + [4+2xH2O] = 4Fe2O3.XH2O + 8H+
In summary when Fe is in the presence of O2, H2O and H+ the favorable galvanic process converts Fe all the way to
Fe2O3, where Fe is in its more stable Fe (III) state.
Hydrated rust is permeable to air and water which allows the base metal to continue to oxidize internally even after a
surface layer of rust has been formed.
The corrosion of aluminium differs from steel or iron in that the aluminium oxide formed on the surface of the aluminium
metal forms a protective corrosion resistant coating, a process known as passivation. Stainless steel similarly resists
oxidation by forming a passivation layer of Chromium (III) Oxide. This is also true of magnesium, copper, titanium and
Galvanisation consists of coating metal with generally a thin layer of zinc by hot-dip or electroplating. Zinc is used
because it is relatively cheap, easy to refine and adheres well to steel. In more corrosive environments cadmium is
Galvanisation often fails at seams, holes and joints where the coating is pierced.
Modern coatings add aluminium to the coating as zinc-alume, aluminium will migrate to cover scratches and thus
provide protection for longer. These rely on the aluminium and zinc oxides protecting the once-scratched surface rather
than oxidizing as a sacrificial anode.
There are several other methods available to control corrosion and prevent the formation of rust, colloquially termed
Cathodic protection makes the iron a cathode in a battery formed whenever water contacts the iron and also a sacrificial
anode made from something with a more negative electrode potential, commonly zinc or magnesium. The electrode
itself doesn't react in water, but only provides electrons to prevent the iron rusting.
Bluing is a technique that can provide limited resistance to rusting for small steel items, for it to be successful, water-
displacing oil must be rubbed onto the blued steel.
Corrosion control can be achieved using a coating to isolate the metal from the environment, such as paint. Large
structures with enclosed box sections, such as ships and modern automobiles, often have a wax-based product applied
to these sections. This may contain rust inhibiting chemicals as well as forming a barrier. Covering steel with concrete
provides protection to steel by the high pH environment at the steel-concrete interface. However, if concrete covered
steel does corrode, the rust formed can cause the concrete to spall and fall apart. This creates structural problems and
is also known as concrete cancer.
Rust converters available on the market are usually phosphoric acid based. These work by reacting with the surface
rust to form ferric phosphate;
Fe2O3 + 2 H3PO4 2 FePO4 + 3 H20
Ferric phosphate can act as a passivation layer in that it can inhibit further rusting by preventing oxygen access to the
underlying iron base.
In using these types of products it is necessary that there be extensive rust on the surface for the coating to form. The
resultant phosphate layer is relatively stable but can be quite susceptible to scratching, chemical dissolution and UV
degradation, therefore can become less effective over a short period of time.
Rust Encapsulator is an industrial strength liquid product specially formulated for rust prevention and rust passivation.
When the product is applied to new or non-oxidised metal surface it forms a durable, hard coating that acts to
dramatically reduce or prevent rust from forming. The way this works is that the product coats the metal surface,
polymerises and hardens when the solvent evaporates, forming a plastic type hydrophobic protective film that does not
allow oxygen or water to contact and hence react with the underlying metal.
When applied to a metal surface that is rusty, the product will convert the rust to a stable iron oxide state as well as
encapsulate this and form a protection layer to guard against further corrosion.
This is done in the following sequences.
A proprietary reducing agent ingredient reduces the ‘bad’ ferric rust Fe3+ ( Fe2O3 ) oxide to black passivated ‘harmless’
ferrous rust Fe2+ oxide ( FeO ). By its nature the ferrous rust also binds to the substrate well.
Several proprietary organic oligomer ingredients then react with the rust and polymerisation occurs so in effect the total
process of action is that the Fe2O3 is converted to FeO and then this harmless form of rust is encapsulated tightly within
an 20 micron thick iron-organo-polymer matrix and a hydrophobic protective barrier is formed that will minimise or
prevent further oxidation.
If the coating becomes a little scratched or worn over time, protection is still existent by the bulk coating which will act as
an electron sink, limiting oxidation spread. Also it has been the case that in some instances, in-situ migration ( spread
and repair ) of the polymeric coating to ‘mend’ small scratches and chips have been observed.
This occurs because the intermolecular forces and properties of the coating matrix will inadvertently tend to self level
and cover minor scratches in the coating. This is further accelerated at higher atmospheric temperatures where the
coating displays some physical properties similar to a liquid-transition amorphous solid which allows it to ‘flow’ and
Rust Encapsulator also penetrates most metal surfaces to a depth of up to 10 microns. This then forms a very strong
‘keyed in’ bond with both the metal surface and the 20 micron thick protective coating on the surface. This adds
significantly to the coatings efficacy and endurance.
Due to the polymerised form of the coating as well as the addition of a polyalkylene oxide and a hydroxyl substituted
aromatic carboxylic acid ingredient, the cured coating has excellent resistance to oxidation and UV degradation as well
as high chemical resistance.
The coating is also a stable and solid foundation on which to overcoat with paint if desired, therefore can substitute as a
primer coat in most instances.
CHEMICAL and PHYSICAL PROPERTIES OF RUST ENCAPSULATOR
Pored concrete has a pH of around 11 but becomes more acidic after aging due to the development of carbonation or
intrusion of chloride or other negatively charged ions where the pH can drop to 8 after 12 months.
The Rust Encapsulator coating on metal is chemically resistant within a pH range of 3-14. Beyond this pH range the
coating can become microscopically etched for dwell times over 14 days. This will tend to reduce the efficacy of the
coating. In the natural world exposure of the coating to pH values beyond those limits are extremely unlikely for the
majority of its applications.
Rust Encapsulator is essentially an organo-polymer displaying similar chemical and physical characteristics to inert
plastics. This includes flexibility, good UV and heat resistance, acid, alkaline and petrochemical resistance as well as its
thermal, electrical and acoustic properties.
The adhesion of pored concrete to metal that has been coated with Rust Encapsulator has been investigated. It was
observed that there was no measurable difference in bond strength between concrete poured to treated (fully cured)
and untreated metal. Also the coatings presence onto the metal within the concrete does not affect any of the normal
properties of the concrete such as dry times, compressive and tensile strengths. These test results were significant
because if the coating lowered the adhesion of concrete / grout to the treated substrate, it could not be used in the
remediation work to repair spalling or concrete cancer. Since the day of the report (19 97) several hundred of these
types of remediation projects have been successfully completed.
In the event that the coating is applied to metal surfaces that have any oils or silicone contamination, the coating will not
adhere to the metal adequately and protection may be lost in those areas. Also the coating has to be totally cured
(minimum 48 hours at 25 OC ) before concrete or paint is allowed to be in contact with it.
If welding is necessary after application of the coating, the coating would have to be removed by angle grinder where
the weld is going to be made for adequate weld strength, and then the welded area can be re-coated.
If overcoating is preferred after application of Rust Encapsulator, there is no need for a primer coat. Firstly the product is
self priming to apply. Then once applied and fully cured, you can paint over preferably using oil based paint. It is
recommended to add in 10 % Rust Encapsulator as an admixture to oil based paints if increased adhesion and hence
paint longevity is required.
Application to powder coated or painted metal surfaces that have undergone UV degradation (whitening, fading or
efflorescence) will rejuvenate to near original appearance and sheen. If using the product as a rejuvenator in this way, it
must not be applied to a smooth surface in direct sunlight; otherwise the coating may become sticky and unworkable
because the solvent will evaporate too quickly to allow the applicator to spread evenly over the surface. If it is necessary
to apply in an ambient temperature over 25 OC, it is recommended to dilute the product 1:1 with white spirits and to do
multiple coats (2 hours apart) until the desired level of gloss and film thickness is achieved.
HMAS ‘Cape Don’ vessel: Restoration of left side deck. Excess rust was removed and then the area treated with Rust
Encapsulator. You can see where the product has been applied; it has neutralised and encapsulated the rust forming
the gloss protective coating. Later on the floor was overcoated with an epoxy anti slip paint.
Opposite view of the treated and non treated area
These metal surfaces were extremely rusty. After treatment they were overcoated with black oil based paint. This is 6
years after treatment and the rust hasn’t reappeared even in this marine environment.
HMAS ‘Cape Don’ vessel: A Section of steel support structure. The entire external steel areas including this one,
needed to be painted every year.
Right hand section was treated with Rust Encapsulator and over-coated with oil based paint. Left hand side was left
untreated but overcoated yearly using the same paint. This is 6 years after treatment with Rust Encapsulator.
The left segment and right segment of this piece of cast iron is treated with Rust Encapsulator whilst the middle section
remained untreated. This was then left outdoors for 24 months to determine the rust inhibiting properties of the coating.
The whole section of this iron sample was treated with Rust Encapsulator and over-coated with oil based paint. It was
observed that the rust was neutralised and encapsulated and no rust reappeared through the paint. This was left
outdoors and the picture is taken 24 months after treatment.
Existing metal water tower needed to be painted every year to temporarily conceal the rust marks. The structure also
started to show signs of significant pitting and detrimental structural modifications.
The water tower was treated with Rust Encapsulator and over-coated with oil based paint. This is 3 years after
treatment. There has been no need to re-paint since treatment even in this coastal environment. Any further detrimental
structural changes relating to its corrosion has been stopped.
News article, In the local newspaper highlighting the success of the project after treatment with Rust Encapsulator