a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, carbonate or sulfide. It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment.

1. Corrosion
Pesala Shalal
What is corrosion ?
Corrosion is when a refined metal is naturally converted to a more stable form such as its oxide,
hydroxide or sulphide state this leads to deterioration of the material. Corrosion is usually defined
as the degradation of metals due to an electrochemical process. The formation of rust on iron,
tarnish on silver, and the blue-green patina that develops on copper are all examples of corrosion.
The total cost of corrosion in the United States is significant, with estimates in excess of half a
trillion dollars a year.
Statue of Liberty: Changing Colors
The Statue of Liberty is a landmark every American recognizes. The Statue of Liberty is easily
identified by its height, stance, and unique blue-green color (Figure 1). When this statue was first
delivered from France, its appearance was not green. It was brown, the color of its copper “skin.”
So how did the Statue of Liberty change colors? The change in appearance was a direct result of
corrosion. The copper that is the primary component of the statue slowly underwent oxidation from
the air. The oxidation-reduction reactions of copper metal in the environment occur in several
steps. Copper metal is oxidized to copper(I) oxide (Cu2O), which is red, and then to copper(II)
oxide, which is black
2Cu(s)+12O2(g)Cu2O(s)(red)Cu2O(s)+12O2(g)2CuO(s)(black)2Cu(s)+12O2(g)Cu2O(s)(red)Cu2
O(s)+12O2(g)2CuO(s)(black)
Coal, which was often high in sulfur, was burned extensively in the early part of the last century.
As a result, sulfur trioxide, carbon dioxide, and water all reacted with the CuO
2CuO(s)+CO2(g)+H2O(l)Cu2CO3(OH)2(s)(green)3CuO(s)+2CO2(g)+H2O(l)Cu2(CO3)2(OH)2(s
)(blue)4CuO(s)+SO3(g)+3H2O(l)Cu4SO4(OH)6(s)(green)2CuO(s)+CO2(g)+H2O(l)Cu2CO3(OH)
2(s)(green)3CuO(s)+2CO2(g)+H2O(l)Cu2(CO3)2(OH)2(s)(blue)4CuO(s)+SO3(g)+3H2O(l)Cu4S
O4(OH)6(s)(green)

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These three compounds are responsible for the characteristic blue-green patina seen today.
Fortunately, formation of the patina created a protective layer on the surface, preventing further
corrosion of the copper skin. The formation of the protective layer is a form of passivation, which
is discussed further in a later chapter.
Figure 1. (a) The Statue of Liberty is covered with a copper skin, and was originally brown, as shown in
this painting. (b) Exposure to the elements has resulted in the formation of the blue-green patina seen today.
Perhaps the most familiar example of corrosion is the formation of rust on iron. Iron will rust when
it is exposed to oxygen and water. The main steps in the rusting of iron appear to involve the
following (Figure 2). Once exposed to the atmosphere, iron rapidly oxidizes.
anode:Fe(s)⟶Fe2+(aq)+2e−E∘Fe2+/Fe=−0.44Vanode:Fe(s)⟶Fe2+(aq)+2
e−EFe2+/Fe∘=−0.44V
The electrons reduce oxygen in the air in acidic solutions.
cathode:O2(g)+4H+(aq)+4e−⟶2H2O(l)E∘O2/O2=+1.23Vcathode:O2(g)+4
H+(aq)+4e−⟶2H2O(l)EO2/O2∘=+1.23V
overall:2Fe(s)+O2(g)+4H+(aq)⟶2Fe2+(aq)+2H2O(l)E∘cell=+1.67Voveral
l:2Fe(s)+O2(g)+4H+(aq)⟶2Fe2+(aq)+2H2O(l)Ecell∘=+1.67V

3. 3
What we call rust is hydrated iron(III) oxide, which forms when iron(II) ions react further with
oxygen.
4Fe2+(aq)+O2(g)+(4+2x)H2O(l)⟶2Fe2O3⋅xH2O(s)+8H+(aq)4Fe2+(aq)
+O2(g)+(4+2x)H2O(l)⟶2Fe2O3⋅xH2O(s)+8H+(aq)
The number of water molecules is variable, so it is represented by x. Unlike the patina on copper,
the formation of rust does not create a protective layer and so corrosion of the iron continues as the
rust flakes off and exposes fresh iron to the atmosphere.
Figure 2. Once the paint is scratched on a painted iron surface, corrosion occurs and rust begins to
form. The speed of the spontaneous reaction is increased in the presence of electrolytes, such as the
sodium chloride used on roads to melt ice and snow or in salt water.
One way to keep iron from corroding is to keep it painted. The layer of paint prevents the water
and oxygen necessary for rust formation from coming into contact with the iron. As long as the
paint remains intact, the iron is protected from corrosion.
Other strategies include alloying the iron with other metals. For example, stainless steel is mostly
iron with a bit of chromium. The chromium tends to collect near the surface, where it forms an
oxide layer that protects the iron.
Zinc-plated or galvanized iron uses a different strategy. Zinc is more easily oxidized than iron
because zinc has a lower reduction potential. Since zinc has a lower reduction potential, it is a
more active metal. Thus, even if the zinc coating is scratched, the zinc will still oxidize before the
iron. This suggests that this approach should work with other active metals.

4. 4
Another important way to protect metal is to make it the cathode in a galvanic cell. This
is cathodic protection and can be used for metals other than just iron. For example, the rusting of
underground iron storage tanks and pipes can be prevented or greatly reduced by connecting them
to a more active metal such as zinc or magnesium (Figure 3). This is also used to protect the metal
parts in water heaters. The more active metals (lower reduction potential) are called sacrificial
anodes because as they get used up as they corrode (oxidize) at the anode. The metal being
protected serves as the cathode, and so does not oxidize (corrode). When the anodes are properly
monitored and periodically replaced, the useful lifetime of the iron storage tank can be greatly
extended.
Figure 3. One way to protect an underground iron storage tank is through cathodic protection.
Using an active metal like zinc or magnesium for the anode effectively makes the storage tank the
cathode, preventing it from corroding (oxidizing)
CORROSION TYPES
UNIFORM CORROSION
Uniform corrosion is considered an even attack across the surface of a material and is the most
common type of corrosion. It is also the most benign as the extent of the attack is relatively easily

5. 5
judged, and the resulting impact on material performance is fairly easily evaluated due to an ability
to consistently reproduce and test the phenomenon. This type of corrosion typically occurs over
relatively large areas of a material’s surface.
PITTING CORROSION
Pitting is one of the most destructive types of corrosion, as it can be hard to predict, detect and
characterize. Pitting is a localized form of corrosion, in which either a local anodic point, or more
commonly a cathodic point, forms a small corrosion cell with the surrounding normal surface. Once
a pit has initiated, it grows into a “hole” or “cavity” that takes on one of a variety of different shapes.
Pits typically penetrate from the surface downward in a vertical direction. Pitting corrosion can be
caused by a local break or damage to the protective oxide film or a protective coating; it can also be
caused by non-uniformities in the metal structure itself. Pitting is dangerous because it can lead to
failure of the structure with a relatively low overall loss of metal.
CREVICE CORROSION
Crevice corrosion is also a localized form of corrosion and usually results from a stagnant
microenvironment in which there is a difference in the concentration of ions between two areas of a
metal. Crevice corrosion occurs in shielded areas such as those under washers, bolt heads, gaskets,
etc. where oxygen is restricted. These smaller areas allow for a corrosive agent to enter but do not
allow enough circulation within, depleting the oxygen content, which prevents re-passivation. As a
stagnant solution builds, pH shifts away from neutral. This growing imbalance between the crevice
(microenvironment) and the external surface (bulk environment) contributes to higher rates of
corrosion. Crevice corrosion can often occur at lower temperatures than pitting. Proper joint design
helps to minimize crevice corrosion.

6. 6
INTERGRANULAR CORROSION
An examination of the microstructure of a metal reveals the grains that form during solidification of
the alloy, as well as the grain boundaries between them. Intergranular corrosion can be caused by
impurities present at these grain boundaries or by the depletion or enrichment of an alloying
element at the grain boundaries. Intergranular corrosion occurs along or adjacent to these grains,
seriously affecting the mechanical properties of the metal while the bulk of the metal remain intact.
An example of intergranular corrosion is carbide precipitation, a chemical reaction that can occur
when a metal is subjected to very high temperatures (e.g., 800°F - 1650°F) and/or localized hot work
such as welding. In stainless steels, during these reactions, carbon “consumes” the chromium,
forming carbides and causing the level of chromium remaining in the alloy to drop below the 11%
needed to sustain the spontaneously-forming passive oxide layer. 304L and 316L are enhanced
chemistries of 304 and 316 stainless that contain lower levels of carbon, and would provide the best
corrosion resistance to carbide precipitation.
STRESS CORROSION CRACKING (SCC)
Stress corrosion cracking (SCC) is a result of the combination of tensile stress and a corrosive
environment, often at elevated temperatures. Stress corrosion may result from external stress such
as actual tensile loads on the metal or expansion/contraction due to rapid temperature changes. It
may also result from residual stress imparted during the manufacturing process such as from cold
forming, welding, machining, grinding, etc. In stress corrosion, the majority of the surface usually
remains intact; however, fine cracks appear in the microstructure, making the corrosion hard to
detect. The cracks typically have a
brittle appearance and form and
spread in a direction
perpendicular to the location of
the stress. Selecting proper
materials for a given environment
(including temperature and
management of external loads)
can mitigate the potential for
catastrophic failure due to SCC.
GALVANIC CORROSION
Galvanic corrosion is the
degradation of one metal near a
joint or juncture that occurs when
two electrochemically dissimilar
metals are in electrical contact in an electrolytic environment; for example, when copper is in
contact with steel in a saltwater environment. However, even when these three conditions are
satisfied, there are many other factors that affect the potential for, and the amount of, corrosion,
such as temperature and surface finish of the metals. Large engineered systems employing many
types of metal in their construction, including various fastener types and materials, are susceptible
to galvanic corrosion if care is not exercised during the design phase. Choosing metals that are as
close together as practicable on the galvanic series helps reduce the risk of galvanic corrosion.

7. 7
How Corrosion Occurs
Corrosion is an electrochemical reaction that appears in several forms, such as chemical corrosion
and atmospheric corrosion, the latter of which is the most common form. When acidic substances
(including water) come in contact with metals, such as iron and/or steel, rust begins to form. Rust
is the result of corroding steel after the iron (Fe) particles have been exposed to oxygen and
moisture (e.g., humidity, vapor, immersion). When steel is exposed to water, the iron particles are
lost to the water’s acidic electrolytes. The iron particles then become oxidized, which results in the
formation of Fe⁺⁺. When Fe⁺⁺ is formed, two electrons are released and flow through the steel to
another area of the steel known as the cathodic area.
Oxygen causes these electrons to rise up and form hydroxyl ions (OH). The hydroxyl ions react
with the FE⁺⁺ to form hydrous iron oxide (FeOH), better known as rust. Where the affected iron
particles were, has now become a corrosion pit, and where they are now, is called the corrosion
product (rust).
Corrosion can happen at any rate, depending on the environment that the metal is in. However,
since atmospheric corrosion is so widespread, it is recommended to take effective precautionary
measures when it comes to corrosion prevention.
This is a corroded tank.

8. 8
Removing and Treating Rust
Depending on the situation and application, you may be able to treat the area that has corroded. If
the affected area is small and treatable, you may require some tools and products to remove it.
Begin by removing the rust from the metal using a tools such as a grinding wheel or needle
gun. Be careful not to cause any additional damage to the metal.
For large corroded areas, you may want to consider a permanent protective coating, such as CSL’s
SI-COAT Anti Corrosion Protective Coating. You will also want to take this time to look at the
application as a whole for other premature signs of corrosion.
How Can I Prevent Corrosion?
One of the best ways to prevent corrosion is to apply an Anti-Corrosion Protective Coating. A
protective coating protects its substrate by preventing contact between the substrate and harsh
environments (atmospheric, chemical, etc.). Here at CSL Silicones Inc, we offer two kinds of anti-
corrosion protective coatings (one is an environmentally responsible Low VOC option!) that are
easily applied using only one coat. The Si-COAT® 579 AC protective coating is cost-effective and
offers long-lasting protection to virtually any substrate.
The coatings are environmentally responsible, have superior temperature resistance (can withstand
temperatures between -76°F and 392°F), will not chalk or fade, have a low film build, require only
a single-coat of application, and have outstanding UV resistance. The 180% elasticity makes the
coating highly flexible, which allows for thermal expansion and contraction of the substrate to
which it is applied.

9. 9
Left: A tank experiencing corrosion.
Right: The same tank following an application of Si-
COAT 579 Anti-Corrosion Protective Coating.
Si-COAT Anti-Corrosion Protective Coatings can be applied to a wide range of applications,
such as structural steel, bridges, machinery and equipment, areas with heavy corrosion, tank
exteriors, metal roofs, cladding, and more.
Si-COAT AC protective coatings are ideally applied to where the necessary coverage is essential
and maximum protection, adhesion, elasticity, and longevity are required.
Difference Between Corrosion and Rusting
Corrosion Rusting
Corrosion is the process of deterioration of metals and
non-metals by oxidation.
Rusting is oxidation of iron (or steel) in
presence of air and moisture.
Corrosion can occur on both metals and non-metals. It
may occur on skin and wood as well.
Rusting occurs on metals only such as iron
and steel.
Corrosion includes rusting. Rusting is a type of corrosion.
It requires exposure of air or chemicals on surface. It requires air and moisture both.
It may require corrosive chemicals such as HCl,
H2SO4 and other strong acids and bases.
It doesn’t require any chemicals.
The compound (or a layer) formed by corrosion can of
different colors such as blue, green etc.
Rusting forms rust which is red orange in
color.

10. 10
What is Rancidity?
Generally, students get confused between corrosion and rancidity. Rancidity is different from
corrosion although it is also an oxidation reaction. Rancidity is oxidation of fats and oils present in
food materials due to which smell and taste of the food material changes.
We can prevent it by adding antioxidants and keeping food in airtight containers.
What Is The Criteria For Selecting Corrosion-Resistant Alloys?
Corrosion resistance is the ability to prevent environmental deterioration by chemical or electro-
chemical reaction. Desirable characteristics of corrosion-resistant alloys, therefore, include high
resistance to overall reactions within the specific environment.
Some of the attractive properties that a metal may feature include:
• Minimized dissolution of the metal in aggressive solutions.
• High resistance to local attack, whether deep penetration in local pitting, networks of local
cracks associated with stress corrosion cracking, or intra-granular corrosion.
• Resistance to enhanced corrosion due to the presence of applied or residual stress or the
application of fluctuating stress.
• Resistance to enhanced corrosion at the interface under load of two contacting and slipping
surfaces.
• Resistance to accelerated local corrosion where mating surfaces of assemblies meet the
corrosive environment.
• Resistance to selective dissolution of a more active constituent of an alloy, leaving behind a
weak deposit of the other material – for example the dezincification of brass.
• Resistance to the combined action of different corrosion sources.
Methods of corrosion prevention
1. BARRIER COATINGS
One of the easiest and cheapest ways to prevent corrosion is to use barrier
coatings like paint, plastic, or powder. Powders, including epoxy, nylon, and
urethane, are heated to the metal surface to create a thin film. Plastic and waxes
are often sprayed onto metal surfaces. Paint acts as a coating to protect the
metal surface from the electrochemical charge that comes from corrosive

11. 11
compounds. Today’s paint systems are actually a combination of different
paint layers that serve different functions. The primer coat acts as an inhibitor,
the intermediate coat adds to the paint’s overall thickness, and the finish coat
provides resistance to environmental factors.
The biggest drawback with coatings is that they often need to be stripped and
reapplied. Coatings that aren’t applied properly can quickly fail and lead to
increased levels of corrosion. Coatings may also contain volatile organic
compounds, which can make them vulnerable to corrosion.
Failing Barrier Coating
2. HOT-DIP GALVANIZATION
This corrosion prevention method involves dipping steel into molten zinc. The
iron in the steel reacts with the zinc to create a tightly bonded alloy coating
which serves as protection. The process has been around for more than 250
years and has been used for corrosion protection of things like artistic
sculptures and playground equipment. Compared to other corrosion prevention

12. 12
methods, galvanization is known for lower initial costs, sustainability, and
versatility.
Unfortunately, galvanization can’t be done on-site, meaning companies have to
pull equipment out of work to be treated. Some equipment may simply be too
large for the process, forcing companies to abandon the idea altogether. In
addition, if the process isn’t done properly, the zinc can chip or peel. And high
exposure to environmental elements can speed up the process of zinc wear,
leading to increased maintenance check-ups. Lastly, the zinc fumes that release
from the galvanizing process are toxic.
3. ALLOYED STEEL (STAINLESS)
Alloyed steel is one of the most effective corrosion prevention methods
around, combining the properties of various metals to provide added strength
and resistance to the resulting product. Corrosion-resistant nickel, for example,
combined with oxidation-resistant chromium results in an alloy that can be
used in oxidized and reduced chemical environments. Different alloys provide
resistance to different conditions, giving companies greater flexibility.
Despite its effectiveness, alloyed steel is very expensive. Companies with
limited financial resources will likely have to turn to other
methods. Monitoring surface conditions are critical, as cracks or scratches can
result in an increase in corrosion. Companies also need to make sure the agents
used in maintenance don’t include corrosion properties.

13. 13
Pipeline using Cathodic Protection
4. CATHODIC PROTECTION
Cathodic protection protects against galvanic corrosion, which occurs
when two different metals are put together and exposed to a corrosive
electrolyte. To prevent this, the active sites on the metal surface need
to be converted to passive sites by providing electrons from another
source, typically with galvanic anodes attached on or near the surface.
Metals used for anodes include aluminum, magnesium, or zinc.
While cathodic protection is highly effective, anodes need to be
checked often which can drive up costs of maintenance. They also
increase the weight on the attached structure and aren’t always
effective in high-resistivity environments. Finally, anodes lead to
increased water flow on ships and other underwater equipment.

14. 14
What Are the Dangers of Corrosion?
Things that are corrosive can destroy (or at least damage) metal. But that’s not all. Eventually,
corrosives can cause damage to not only metal but the human digestive tract, respiratory tract,
eyes, and skin. The effects of corrosion can threaten our very lives.
The Dangers of Corrosion
When materials interact with the environment around them, deterioration caused by corrosion can
occur. In commercial applications, this usually pertains to damage to steel, iron, or metal from
road, rail, and bridge maintenance, warehouse cylinders and tanks, utilities, and pipework. Once
corrosion starts, structural failure is almost certainly inevitable. The main dangers and effects of
corrosion are as follows:
• High risk of employee injury or injury to the general public
• Loss of industrial equipment’s time availability
• When appearance is affected, corrosion can reduce the overall value of a building or
location
• Pipe blockages or mechanical damage to pumps, valves, etc.
• Damage to surroundings or danger to individuals because of leaking gases or liquids
• Contamination of fluids in pipes or vessels
Aside from industrial dangers, corrosion affects our lives as we travel to school, work, and for
leisure. Endangering public safety and resulting in significant repair costs are the effects of
corrosion on bridges, parking structures, buildings, electrical towers, highways, etc. Should these
collapse, because of a weak, corroded section, disaster could result.

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The Economic Impact and Effects of Corrosion
Annually, the United States economy is impacted by corrosion to the tune of literally hundreds of
billions of dollars. Practically speaking, however, if corrosion-resistant materials and coatings were
used more frequently in technical practices, a good one-third of those costs could be reduced.
In our daily lives, the effects of corrosion are both indirect and direct. Indirectly, corrosion is
passed on to customers through services, goods, suppliers, and producers. Directly, our very
possessions can suffer the effects of corrosion. At home, metal tools, outdoor furniture, charcoal
grills, and body panels on our automobiles corrode on a regular basis. There are precautions and
preventative methods for nearly all of these inconveniences. Certain corrosion deterrents are
already built into many of our household products such as dryers, washers, ranges, furnaces, water
heaters, and more.
CONCLUSION
In aqueous environments, metals may be exposed to not only uniform corrosion, but also to various
types of local corrosion including pitting, crevice, intergranular, stress, and galvanic corrosion. In
areas where corrosion is a concern, stainless steel products offer value and protection against these
threats. Stainless’ favorable chemical composition makes it resistant to many common corrosives
while remaining significantly more affordable than specialty alloys such as titanium and Inconel®
alloys.
Stainless steel is a highly alloyed, low-carbon steel with a high (at least 11%) chromium content.
When exposed to an oxygenated environment, the chromium reacts to form a passive oxide layer
on the metal’s surface, slowing further oxidation and providing a self-healing quality, which helps
resist uniform and local corrosion. Nickel helps to stabilize the microstructure, increasing SCC
resistance. Manganese, in moderate quantities and in association with nickel, will perform many
functions attributable to nickel and helps prevent pitting. The addition of molybdenum (the
additional element in Type 316 SS that increases its performance with respect to Type 304 SS),
helps increase resistance to pitting and crevice corrosion. Reduced levels of carbon, such as those
found in 304L and 316L will help prevent intergranular corrosion. Lastly, nitrogen, although not a
major element of stainless steel’s composition, increases pitting resistance. Choosing stainless steel
can help greatly reduce the risk of corrosion and yield long-term savings by avoiding the costs
associated with reinstallation of inferior products.