A brief introduction to corrosion and types of corrosion, such as pitting corrosion.
Intergranular and transgranular corrosion,
1. N. Sankar, M.E
Shree Sathyam Engineering and Technology
Department of Mechanical Engineering,
Corrosion is the deterioration or destruction of
metals and alloys in the presence of an
environment by chemical or electrochemical means
In the broad sense, corrosion may be defined as
“The destruction of a material by chemical,
electrochemical, or metallurgical interaction
between the environment and the material”
The basic cause of corrosion is the instability of
metals in their refined forms.
The metals tend to revert to their natural states
through the process of corrosion.
Corrosion can be classified in different ways, such as
Chemical and electrochemical
High temperature and low temperature
Wet corrosion and dry corrosion
Dry corrosion occurs in the absence of aqueous
environment, usually in the presence of gases and
vapors, mainly at high temperatures.
Electrochemical nature of corrosion can be understood
by examining zinc dissolution in dilute hydrochloric
Zn + 2HCl = ZnCl2 + H2
Anodic reaction is Zn = Zn++ + 2e with the reduction
of 2H+ + 2e = H2 at cathodic areas on the surface of
There are two half reactions constituting the net cell
5. Electrochemical principles
corrosion is essentially an electrochemical process
resulting in part or all of the metal being transformed
from the metallic to the ionic state
•If a piece of ordinary iron is placed in a
solution of hydrochloric acid vigorous
of hydrogen gas is observed
•On the surface of the metal there
numerous tiny anode and cathode
areas caused by inclusions in the
metal, surface imperfections,
•This condition is shown schematically in Fig. at the
anode, positive-charged iron atoms detach themselves
from the solid surface and enter the solution as positive
ions, while the negative charges, in the form of electrons,
6. Factors Influencing Corrosion
One of the most important factor in influencing
corrosion is the difference in electrical potential of
dissimilar metals when coupled together and
immersed in an electrolyte
This potential is due to the chemical natures of the
anodic and cathodic regions.
Some indication of which metals may be anodic as
compared with hydrogen is given by the standard
The most important types are
Intergranular and transgranular corrosion,
8. Pitting corrosion
Pitting is one of the most destructive types of
corrosion, as it can be hard to predict, detect and
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
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
10. Cavitations corrosion
Cavitation corrosion, illustrated schematically in Fig.
Is caused by the collapse of bubbles and cavities
within a liquid.
Vibrating motion between a surface and a liquid is
such that repeated loads are applied to the surface,
causing very high stresses when these bubbles form
and collapse regularly.
11. •Figure shows numerous small pits formed by
corrosion on the surface of a cast-iron sleeve.
•This type of corrosion may be minimized or
eliminated by switching to a more resistant material or
by using a protective coating.
•These collapses produce
high stress impacts which
remove particles of the
forming deep pits,
12. 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
13. 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.
14. Fretting corrosion.
Fretting corrosion is a common type of surface
damage produced by vibration which results in
striking or rubbing at the interface of closefitting,
highly loaded surfaces.
Such corrosion is common at surfaces of clamped
or press fits, splines, keyways, and other close-
fitting parts to minute relative movement.
Fretting corrosion ruins bearings, destroys
dimensions, and reduces fatigue strength
This type of corrosion is a mechanical-chemical
When two components rub to gather, adhesive
forces cause small particles of the surface to
15. With continued slight motion, the welded particles tear
away from the opposing surfaces and react
chemically with the atmosphere, forming debris or
powder in the joint.
Figure shows fretting corrosion on the shaft of an oil
pump gear during fatigue testing.
There are several ways in which fretting corrosion
may be overcome. The most obvious way is to
The source of vibration by tighter clamping or more
16. 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-
17. As a stagnant solution builds, pH shifts away from
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
18. Intergranular and transgranular
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
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
Intergranular corrosion occurs along or adjacent to
these grains, seriously affecting the mechanical
properties of the metal while the bulk of the metal
19. 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
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
20. Stress corrosion
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
It may also result from residual stress imparted during
the manufacturing process such as from cold forming,
welding, machining, grinding, etc
21. In stress corrosion, the majority of the surface
usually remains intact; however, fine cracks appear
in the microstructure, making the corrosion hard to
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.