Corrosion and different types of corrosion.

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FORMS OF CORROSION
Introduction
Corrosion is the gradual disintegration, decaying or deterioration of a
metal by chemical or electrochemical reaction with its environment.
It is also called ‘Weeping of Metals’. The metals like Iron which have
to be extracted from their compounds (ores) tend to go back to their
original forms by the process of atmospheric oxidation (corrosion).
The attack produces oxide, hydrated oxide, hydroxide, carbonate,
basic carbonates, basic sulphate or sulphide of metal and this depends
on the nature of metal, environment and conditions and duration of
exposure.
Types of Corrosion
The various types or forms of corrosion are as follows:-
1. Uniform and Galvanic Corrosion
It is also called general corrosion and the most common type of
corrosion. The surface effect produced by most direct chemical
attacks (like by an acid) is a uniform etching of the metal. It can
be dry or wet, chemical or electrochemical. When two different
metals (like Zn and Cu) are electrically connected and exposed to
an electrolyte, the metal higher in electrochemical series
undergoes corrosion and this is called Galvanic Corrosion.
For example, when Aluminium or Magnesium alloys are in
contact with steel, galvanic corrosion can occur and accelerate the
corrosion of the Aluminium or Magnesium.
In Galvanic Corrosion, a uniform decrease in the volume of a
metal takes place as a result of chemical action and soluble
corrosion products are formed.

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Uniform Corrosion can be controlled by using chemical resistant
coatings. While Galvanic Corrosion can be controlled 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.
Figure 1 Galvanic Corrosion
2. Erosion Corrosion
It is caused by the combined effect of the abrading action of
turbulent flow of gases, vapour and liquids and the mechanical
rubbing action of solids over a metal surface. In other words, it is
the result of a combination of an aggressive chemical
environment and high fluid-surface velocities. Thus it can be due
to fast fluid flow past a stationary object or quick motion of an
object in a stationary fluid.
The major cause of this corrosion is the removal of protective
surface film.
It can be minimized by using harder metals and design changes to
avoid excess of friction and using proper lubrication.
3. Crevice Corrosion
Also called Contact Corrosion, it is a local corrosion and is
usually created by dirt deposits, corrosion products, crack in paint
coatings, etc. It is usually attributed to changes in acidity of
crevice, lack of O2 in the species in the crevice and concentration
of detrimental ionic species in the crevice.

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It can be controlled by selection of resistant materials (like
Molybdenum-containing grades of stainless steel have increased
crevice corrosion resistance), proper design to minimize crevice
and maintaining clean surfaces.
4. Pitting Corrosion
Pitting corrosion is a localized corrosion that occurs at
microscopic defects on a metal surface. The pits are often found
underneath surface deposits caused by corrosion product
accumulation. Owing to the differential amount of O2 in contact
with the metal surface, the small part, beneath impurity becomes
anodic, while the surrounding larger part becomes the cathodic.
This corrosion results in the formation of pinholes, pits and
cavities in the metal.
Pitting corrosion is due to breakdown or cracking of protective
film on a metal at specific point. This breakdown may be caused
by surface roughness, non-uniform finish, alternating stresses,
chemical attack, etc.
Figure 2 Pitting Corrosion
Methods that can be used to control pitting include maintaining
clean surfaces, application of a protective coating (a pure and
homogeneous metal with highly polished surface are resistant)
and use of cathodic protection.

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5. Intergranular Corrosion
Intergranular corrosion is an attack on or adjacent to the grain
boundaries of a metal or alloy; usually leaving the grain interiors
untouched or only slightly attacked. This causes brittleness or
weakness of underlying metal.
Intergranular corrosion is due to the fact that the grain boundaries
contain material which shows electrode potential more anodic
than that at the grain centre in the particular corroding medium.
This may occur due to the precipitation of certain compounds at
the grain boundaries.
Figure 3 Intergranular Corrosion
The remedy for this type of corrosion in alloys like Al-Cu alloy,
stainless steel, etc. is proper heat treatment and rapid quenching
to prevent heterogeneous precipitation that takes place due to
slow cooling.
6. Exfoliating and Selective Leaching
Exfoliating and selective leaching is a sub-surface corrosion
which starts on clean surface and spreads below it, i.e., whole
layers of material are corroded. This attack is usually recognized
from a flaky or blistering surface.
This form of corrosion is seen in Aluminium alloys and can be
avoided by heat treatment and proper alloying.
Selective Leaching or parting is the removal of one of the
elements in an alloy. It leads to a highly porous metal with poor

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mechanical properties. The most common example of this is de-
zincification from Cu-Zn alloys.
The prevention method is use of non-susceptible alloys.
7. 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. It involves a
localized electrochemical corrosion occurring along narrow paths
forming anodic areas with respect to the more cathodic areas at
metal surface. Presence of stress produces strains, which result in
localized zones of higher electrode potential.
Figure 4 Stress Corrosion Cracking
8. Waterline Corrosion
Waterline corrosion results from differential aeration leading to
the formation of oxygen concentration cells. Maximum corrosion
takes place in a steel tank containing water along a line just
beneath the water level, because the access of O2 is much less
there. The area above the waterline is highly oxygenated and
hence acts as cathode, and thus it is not corroded.
Figure 5 Water Corrosion

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This form of corrosion is also seen in marine ships and can be
prevented to a large extent by painting the sides of ships.
9. Soil Corrosion
Soil corrosion is purely electrochemical in nature and the type of
soil determines the corrosive conditions and the extent of
corrosion:-
a) Gravelly or sandy soils are very porous and strongly aerated,
i.e., the conditions are similar to those under wet condition.
Thus the rate of corrosion will be governed by the amount of
moisture content in the soil.
b) Water logged soils have low amount of free oxygen but
provide conditions for growth of various bacteria and micro-
organisms, thus leading to microbiological corrosion.
c) In acidic soils, the corrosion is hydrogen evolution type and
depends on the pH of soil, presence of salt and O2.
d) Intermediate character soil may cause localized and intense
corrosion.
This form of corrosion is reported in water mains, electric cables
and other underground structures embedded in the soil.
10. Microbiological Corrosion
Microbiological Corrosion or Microbial Corrosion or
Microbiologically-Influenced Corrosion (MIC) is caused by the
presence and metabolic activities of micro-organisms.
Anaerobic bacteria like Microspora or Vibrio desulfuricans
reduce sulphates to sulphur and convert oxygen to such forms
which brings about localized corrosion. Most MIC takes the form
of pits that form underneath colonies of living organic matter and
mineral and biodeposits. This biofilm creates an environment (by
maintaining concentration gradients of dissolved salts, gases,
acids, etc.) where conditions become quite corrosive and
corrosion is accelerated.

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The control of MIC involves the use of corrosion resistant alloys,
use of biocides and cleaning methods that remove deposits from
metal surfaces.
11. Stray Current Corrosion
Metal structures like water pipes, gas pipes and cable sheaths
adjacent to D.C. circuits may get corroded due to leakages from
main circuit.
References
1. Advanced Physical Chemistry; Gurtu, J.N. and Gurtu, A.
2. Applied Chemistry for Home Science and Allied Sciences; Jacob,
Thankamma.