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1. Corrosion
Prof. Harish Chopra,
Department of Chemistry,
SLIET, Longowal (Pb) INDIA

2. Introduction
Corrosion, which literally means to "gnaw away", is the
degradation of a metal by its environment.
Corrosion is defined as the deterioration of a material,
usually a metal, because of its reaction with the
environment and which requires the presence of an
anode, a cathode, an electrolyte, and an electrical
circuit.
Most metals, in their natural state, exist in the form of
oxides (or other salts such as carbonates, bicarbonates, sulphides,
chlorides or silicates, etc.) which must be refined to create
the pure metals or alloys, which become useful
structural materials. Corrosion is a natural process and
is a result of the natural tendency of metals to revert to
their more stable compounds, usually oxides. 2

3. In the refining process, energy is added to the ore, to
produce the metal. It is the same energy, which provides
the driving force causing the metal to revert to the more
stable compound. Pure metals and alloys have a much
higher energy state and there is a natural tendency to
return to their lower energy state. Corrosion is the process
nature uses to return metals to their original state. The
rate of corrosion depends upon the environment and the
type of material. It can be very rapid in a highly corrosive
environment or take thousands of years in a slightly
corrosive environment.
.
3

4. Economic Impact
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Secretary, Ministry Chemicals and Fertilizers, Department
of Chemicals and Petrochemicals while releasing 'Impact
India- Cost of Corrosion Study for India' brought out by
NACE International, the CORCON Institute of Corrosion , GAIL
India and Outokumpu India said that “Corrosion and poor
maintenance was one of the major cause of toxic gas
leak in the Union Carbide pesticide plant in Bhopal.
Corrosion has a huge economic and environment impact
on virtually all facets of the worlds infrastructure, from
highways, bridges, and buildings to oil and gas, chemical
processing, water and wastewater systems and
particularly industrial structures. India loses 4.2% of
GDP due to corrosion menace.
Steel News - Published on 21 Feb, 2019

5. Causes of Corrosion
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The following three types of processes
generally cause the corrosion:
❏ Electrochemical corrosion (Wet)
❏ Chemical Corrosion (Dry)
❏ Acidic corrosion
Electrochemical corrosion
The electrochemical corrosion is similar to
the phenomenon which takes place when a
carbon-zinc “dry” cell generates a direct
current. Basically, an anode (negative
electrode), a cathode (positive electrode),
an electrolyte (environment), and a circuit
connecting the anode and the cathode are
required for corrosion to occur.

6. Electrochemical Corrosion
6
A corrosion system can be regarded as a short circuited electrochemical cell in
which dissolution of metal occurs at the anode whereas the corrosion current enters
the electrolyte and flows to the cathode. E.g., if iron (Fe) was exposed to aerated,
corrosive water, the anodic reaction would be:
After dissolution, ferrous ions (Fe2+) generally oxidized to ferric ions (Fe3+) which
combine with hydroxide ions (OH-) formed at the cathode to give a corrosion
product called rust (FeOOH or Fe2O3xH2O). Similarly, zinc gives corrosion
product Zn(OH)2.

7. Electrochemical Corrosion
7
Fundamental electrochemical corrosion of metals in environment
occur at the point where current leaves the anode. Dissimilar
soils may create an environment that enhances corrosion.

8. Chemical Corrosion
[Dry Corrosion]
8
The chemical corrosion is mainly caused due to the direct
reaction of metal surfaces with the atmospheric gases
present in the environment. The most common gases which
cause this type of corrosion are oxygen, carbon dioxide,
hydrogen sulphide, sulphur dioxide, nitrogen, chlorine and
other halogens. The chemical corrosion is of three types;
❏ Oxidative chemical corrosion,
❏ Corrosion by other gases and
❏ Liquid metal corrosion.
.

9. Oxidative Chemical Corrosion
9
Oxidative chemical corrosion is caused by the direct reaction of atmospheric O2
with the metals generally in the absence of moisture (water vapour). Alkali
metals (like Li, Na, K, Rb, etc.) and alkaline earth metals (like Be, Ca, Sr, etc.) are
quickly oxidized even at low temperatures whereas all other metals (except Ag,
Au and Pt) are oxidized at high temperature. At the first instance, oxidation
takes place at the surface of the metal [M] with the formation of metal ion
[Mn+] and electrons (e-). The electrons formed in the oxidation step react with
the atmospheric oxygen to form oxide ions [O2-] which react with the metal
ions to give a layer of metal oxide [M2On] at the surface of the metal.

10. Oxidative Chemical Corrosion
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When the entire surface of the metal is covered with metal oxide,
the further oxidation is restricted. The oxidation of the remaining metal
is possible only if metal ions diffuse out on the surface of the metal or
atmospheric O2 diffuse inside the metal. The diffusion of metal ions towards
the surface of metal is much faster as compared to the diffusion of the O2
inside the metal because of smaller size and higher mobility of metal ions as
compared to O2. So, it is evident the extent of oxidation reaction is
dependent on the type of the metal oxide layer formed on the
surface of the metal.
The 4 (four) types of layers (films) of oxide are generally formed on the
metal.

11. Oxidative Chemical Corrosion
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The types of oxide layers (films) formed are:
Stable: A stable layer of metal oxide means formation of an impermeable
coating on the surface of metal which restrain the penetration of atmospheric O2
inside the metal. These types of coatings are formed commonly in Al, Pb, Cu,
Sn, etc. So, with the formation of a stable coating on the surface of the metal
further oxidation is prevented.
Unstable: In this type of layer formation, the oxide layer formed decay to
give metal and oxygen

12. Oxidative Chemical Corrosion
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Volatile: In this type of layer formation, the oxide layer formed volatilizes as
soon as it is formed. This exposes a fresh surface of the metal for further
oxidation by O2. So, formation of volatile layer of metal oxide causes rapid and
continuous corrosion till the whole metal is corroded. E.g., layer formation
(molybdenum oxide, MoO3) is in the case of molybdenum (Mo) metal.
Porous: The oxide layer formed has pores through which atmospheric O2 can
penetrate through the surface of the metal and corrosion continues till the whole
metal is converted into metal oxide

13. Oxidative Chemical Corrosion
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Pilling-Bedworth Rule: The protective or non-protective oxide layer
formation in metals is governed by a rule known as Pilling-Bedworth rule.
According to this rule,
If the volume of the oxide is equal to or greater than the volume
of the metal from which it is formed, the oxide layer is non-
porous and therefore protective in nature
(e.g., Oxides of Cu, In, Al, Ni, Cr)
if the volume of the oxide is less than the volume of the metal
from which it is formed, the oxide layer is porous and therefore
non-protective in nature
(e.g., alkali or alkaline metal oxides).

14. Corrosion by other Metals
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This type of corrosion is caused by gases like CO2, H2S, SO2,
N2, Cl2, and other halogens. The extent of corrosion depends
upon the affinity between the metal and the gas involved and
the type of film formed on the surface of the metal i.e.
protective or non-protective.
I. The extent of corrosion decreases if the layer formed is
protective or non-porous. E.g., attack of Cl2 gas on Ag
metal forms a non-porous layer of AgCl which protects
further corrosion of the Ag metal.
II. The corrosion continues if the layer formed is non-
protective or porous, E.g., attack of Cl2 gas on Sn (tin)
metal forms SnCl4 which is volatile, so, produces a fresh
surface on tin metal for the corrosion to continue.
Similarly, H2S attacks steel at high temperature in
petroleum industry forming FeS scale which is porous
and corrosion continues

15. Liquid Metal Corrosion
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Liquid metal corrosion is caused due to chemical
reaction of solid metal with a flowing liquid metal at
high temperature. Such kind of corrosion occurs in
nuclear power production. Factors affecting liquid metal
corrosion are:

16. Acidic Corrosion
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This theory is applicable to atmospheric rusting of
iron. As per this theory, corrosion (rusting) of iron
takes place due to its reaction with O2, CO2 and
moisture present in the atmosphere.
The following chemical reactions take place during
the process:

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18. Galvanic Corrosion
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This is most common type of corrosion and is also known as bimetallic
corrosion. Galvanic corrosion occurs when two electrochemically dissimilar
metals (say zinc and copper) are metallically connected and exposed to a
corrosive environment. The less noble metal (anode) [the metal higher in
electrochemical series] undergoes corrosion and the more noble metal
(cathode) is cathodically protected by the galvanic current. In case of zinc
and copper, zinc will act as anode (higher in electrochemical series) and
undergoes corrosion whereas copper (lower in electrochemical series) act
as cathode and is not affected.

19. Galvanic Series
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The galvanic series (or
electropotential series) determines
the nobility of metals and semi-
metals. The tendency of a metal to
corrode in a galvanic cell is
determined by its position in the
“galvanic series” of metals and alloys.

20. Concentration Corrosion
20
Concentration-cell corrosion occurs because of differences in the environment
surrounding the metal. It is also called as “crevice corrosion”, “gasket corrosion,”
and “deposit corrosion” because it commonly occurs in localized areas where
small volumes of stagnant solution exist. Normal mechanical construction can
create crevices at sharp corners, spot welds, lap joints, fasteners, flanged
fittings, couplings, threaded joints, and tube-sheet supports.
The most common type of
concentration cell are the
“oxygen” and “metal ion” cells.
Areas on a surface in contact
with an electrolyte having a high
oxygen concentration generally
will be cathodic relative to those
areas where less oxygen is
present (oxygen cell).

21. Pitting Corrosion
21
Pitting corrosion is a randomly occurring, highly localized form of attack on a metal
surface, characterized by the fact that the depth of penetration is much greater than
the diameter of the area affected. Pitting is one of the most destructive forms of
corrosion, which selectively attacks an area of a metal surface where there is a
surface scratch or an emerging dislocation or a compositional heterogeneity which
results in the formation of a hole. Steel and galvanized steel pipes and storage tanks
are susceptible to pitting corrosion by many potable waters.

22. Stress Corrosion
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Stress corrosion cracking (SCC) is an environmentally induced-delayed failure.
It occur when many alloys are subjected to static, surface tensile stresses and
are exposed to certain corrosive environments. Cracks are initiated and
propagated by the combined effect of a surface tensile stress and the
environment. When stress corrosion cracking occurs, the tensile stress
involved is often much less than the yield strength of the material; the
environment is usually one in which the material exhibits good resistance to
general corrosion. Residual stresses are introduced due to cold deformation and
forming, welding, heat treatment, machining and grinding, etc. Such stresses
build-up corrosion products in confined spaces.

23. Stress Corrosion
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SCC is classified as a disastrous form of
corrosion, as the failure may occur unexpectedly,
and the catastrophic nature of this severe form of
corrosion attack has been repeatedly illustrated
in much newsworthy failures, like swimming
pool roof collapse in Switzerland and
Boeing 747 crash in Netherlands.

24. Stress Corrosion
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Chloride SCC: One of the most important forms of stress corrosion that
concerns the nuclear industry is chloride stress corrosion. Chloride
stress corrosion is a type of intergranular corrosion and occurs in
stainless steel under tensile stress in the presence of oxygen, chloride
ions, and high temperature.
Reason: It is thought to start with chromium carbide deposits along the
grain boundaries that leave the metal open to corrosion.
Control: This form of corrosion is controlled by maintaining low chloride ion
and oxygen content in the environment and use of low carbon steels.

25. Stress Corrosion
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Caustic SCC: Caustic embrittlement is a very common form of stress
cracking corrosion occurring in mild steel at high temperatures under alkaline
conditions. It is generally found in steam boilers or heat transfer equipments.
The water present in steam boilers and other heat transfer equipments
becomes alkaline due to the hydrolysis of sodium carbonate which in turn is
formed during the softening of water by lime-soda process.
Due to the presence of sodium hydroxide water becomes caustic and its
concentration increases as the water evaporates. This concentrated soda
attacks the boiler by dissolving its iron as sodium ferroate (Na2FeO2) which
causes the embrittlement of the boiler in the following ways:
This embrittlement can be removed by
adding either sodium sulphate or
tannin or lignin or sodium phosphate.

26. Stress Corrosion
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Brass SCC: Stress corrosion cracking in brass generally takes
place in ammonia or organic amines. The reason for cracking is
due to the formation of complexes by brass components (that is, Cu
and Zn) with ammonia. The Cu and Zn present in brass react with
ammonia to form [Cu(NH3)4]2+ and [Zn(NH3)4]2+ complex ions,
respectively which leads to the dissolution of brass and hence
formation of cracks. This is called as brass stress corrosion cracking.
The most effective means of preventing SCC are:
❏ Proper designing of the equipment with the right materials;
❏ Reduction in stresses;
❏ Removal of critical environmental species such as
hydroxides, chlorides, and oxygen;
❏ Avoiding stagnant areas and crevices in heat exchangers
where chloride and hydroxide might become concentrated.

27. Waterline Corrosion
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When water is stored in a metallic tank, it is
observed that the metal below the waterline
gets corroded. It is because the water below
the waterline is poorly oxygenated and acts
as an anode. The metal above the waterline is
highly oxygenated and acts as a cathode.

28. Passivity
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Passivity or passivation of metals or alloys is defined as, “A relatively
inactive state in which the metal or alloy displays a more noble
behavior than thermodynamic conditions predict or more simply
defined as the reason why the metal does not corrode when it
should or a state in which metal or alloy exhibits much higher
corrosion resistance than expected from its position in the
electrochemical series”. The first "correct" account of passivation of iron
is attributed to Michael Faraday in 1836 when he reported the oxide layer
formation on the surface of iron. Passivity is the result of formation of
a highly protective but (thickness 0.0004 mm) quite invisible,
thin film on the surface of metal or alloy, thus making it nobler,
E.g., The Delhi Iron Pillar (in India, now more than 1600 years of age) is
an historic landmark on which passive surface films are believed to have
played a significant protective role. This remarkable wrought iron structure
has reportedly been subject to formal scientific "passivation" scrutiny and
coupon corrosion monitoring since 1953.

29. Passivity
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The film formed during passivation is insoluble, non-porous and of
such a “self healing nature”, that when broken, it will repair itself on
re-exposure to oxidizing conditions. The common examples of
passive metals and alloys are Ti, Al, Cr and a wide variety of
stainless steel alloys containing Cr.
These exhibit outstanding corrosion resistance in oxidizing
environments, but in reducing environments, they become
chemically active. Based on experiment conducted in aerated 0.5 M
NaCl solution, the passivity of certain metals falls in the following
order:

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References
The some contents are taken from:
Chemistry For Engineers
By
Harish Chopra
Anupama Parmar
[In addition, Internet sources have also been used]

31. Thank You !
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