Corrosion is a process of formation of
the compound of pure metal by the
chemical reaction between metallic
surface and its environment.
It is an oxidation process. It causes
loss of metal.
Hence, disintegration of a metal by its
surrounding chemicals through a
chemical reaction on the surface of
the metal is called corrosion.
Corrosion comes in many different forms
and can be classified by the cause of the
chemical deterioration of a metal.
Example: Formation of rust on the
surface of iron, formation of green film on
the surface of copper.
The responsible factors for the corrosion
of a metal are the metal itself, the
environmental chemicals, temperature
and the design.
TYEPS OF CORROSION
Listed below are 10 common types of
I. General Attack Corrosion
II. Localized Corrosion
III. Galvanic corrosion
IV. Environmental Craking
V. Flow Assisted Corrosion
VI. Intergranular (Intercrystalline )
IX. High Temperature Corrosion
1. General Attack Corrosion
Also known as
attack corrosion is the
most common type of
corrosion and is
caused by a chemical
reaction that results in
the deterioration of
the entire exposed
surface of a metal.
• Ultimately, the metal deteriorates to the
point of failure.
• General attack corrosion accounts for
the greatest amount of metal
destruction by corrosion, but is
considered as a safe form of corrosion,
due to the fact that it is predictable,
manageable and often preventable.
2. Localized Corrosion
Unlike general attack corrosion,
localized corrosion specifically targets
one area of the metal structure.
Localized corrosion is classified as
one of three types:
a) Pitting Corrosion
b) Crevice Corrosion
c) Filiform Corrosion
a) Pitting corrosion
Pitting results when a
small hole, or cavity,
forms in the metal,
usually as a result of
de-passivation of a
small area. This area
becomes anodic, while
part of the remaining
cathodic, producing a
The deterioration of this small area
penetrates the metal and can lead to
This form of corrosion is often difficult
to detect due to the fact that it is
usually relatively small and may be
covered and hidden by corrosion-
b) Crevice Corrosion
Similar to pitting, crevice corrosion occurs
at a specific location. This type of corrosion
is often associated with a stagnant micro-
environment, like those found under
gaskets and washers and clamps. Acidic
conditions, or a depletion of oxygen in a
crevice can lead to crevice corrosion.
c) Filiform Corrosion
Occurring under painted
or plated surfaces when
water breaches the
coating, filiform corrosion
begins at small defects
in the coating and
spreads to cause
3. Galvanic Corrosion
Galvanic corrosion, or dissimiliar metal
corrosion, occurs when two different
metals are located together in a
corrosive electrolyte. A galvanic couple
forms between the two metals, where one
metal becomes the anode and the other
the cathode. The anode, or sacrificial
metal, corrodes and deteriorates faster
than it would alone, while the cathode
deteriorates more slowly than it would
Three conditions must exist for galvanic
corrosion to occur:
Electrochemically dissimilar metals must be
The metals must be in electrical contact
The metals must be exposed to an
4. Environmental Cracking
cracking is a
that can result from a
combination of environmental
conditions affecting the metal.
Chemical, temperature and stress-
related conditions can result in the
following types of environmental
a) Stress Corrosion Cracking (SCC)
b) Corrosion fatigue
c) Hydrogen-induced cracking
d) Liquid metal embrittlement
5. Flow-Assisted Corrosion (FAC)
Flow-assisted corrosion, or flow-
accelerated corrosion, results when a
protective layer of oxide on a metal
surface is dissolved or removed by wind
or water, exposing the underlying metal
to further corrode and deteriorate.
6. Transgranular and Intergranular
Intergranular corrosion is a
chemical or electrochemical
attack on the grain boundaries
of a metal. This often occurs
due to impurities in the metal,
which tend to be present in
higher contents near grain
boundaries. These boundaries
can be more vulnerable to
corrosion than the bulk of the
De-alloying, or selective leaching, is
the selective corrosion of a specific
element in an alloy. The most common
type of de-alloying is de-zincification of
unstabilized brass. The result of
corrosion in such cases is a deteriorated
and porous copper.
9. Fretting Corrosion:
Fretting corrosion occurs as a result of
repeated wearing, weight and/or vibration on
an uneven, rough surface. Corrosion,
resulting in pits and grooves, occurs on the
surface. Fretting corrosion is often found in
rotation and impact machinery, bolted
assemblies and bearings, as well as to
surfaces exposed to vibration during
10. High-Temperature Corrosion
Fuels used in gas turbines, diesel engines and
other machinery, which contain vanadium or
sulfates can, during combustion, form compounds
with a low melting point. These compounds are
very corrosive towards metal alloys normally
resistant to high temperatures and corrosion,
including stainless steel.
High temperature corrosion can also be caused by
high temperature oxidization, sulfidation and
The rate of corrosion is the speed at
which a metal deteriorates in a specific
environment. The rate, or speed, is
dependent upon environmental
conditions as well as the type, and
condition, of the metal.
In order to calculate the rate of
corrosion, the following information must
a) Weight loss (the decrease in metal
weight during the reference time
b) Density (density of the metal)
c) Area (total initial surface area of the
d) Time (the length of the reference time
Electrochemical methods provide an
alternative to traditional
methods used to determine the rate of
corrosion. Direct and
quantitative determination of
corrosion rates can be
determined from simple
electrochemical measurement like a
linear sweep voltammetry (LSV).
The corrosion rate depends on the
kinetics of both anodic (oxidation)
and cathodic (reduction) reactions.
According to Faraday's law, there is
a linear relationship between the
metal dissolution rate or corrosion
rate, RM, and the
corrosion current icorr
where M is the atomic weight of the
metal, ρ is the density, n is the
charge number which indicates the
number of electrons exchanged in the
dissolution reaction and F is the
Faraday constant, (96.485 C/mol).
The ratio M/n is also sometime
referred to as equivalent weight.
Calculation of corrosion rates requires
the determination of corrosion
currents. When reaction mechanisms
for the corrosion reaction are known,
the corrosion currents can be calculated
using Tafel Slope Analysis.
The NOVA software provides a
convenient interface for making Tafel
plots, calculating Tafel slopes and
Selecting the Corrosion Rate, Tafel Slope
analysis tool from the Analysis menu, a Tafel
plot is added as shown in Figure.
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