is defined as the damage or deterioration of a material (usually a
metal) due to a reaction or interaction with the environment.
cause of corrosion is the instability of metal in its refined form. The
process of corrosion is the tendency of a metal to revert to its natural
state. What dictates the level of corrosion is the combination of the
material type and the environment it is exposed to.
are corrosive in some manner. Understanding the environment helps to determine
what factors contribute to corrosion activity and what the appropriate
control methods could be.
environments can be placed into four major categories: liquid, underground,
atmospheric and high temperature. In most industrial applications, the
process system is exposed to many, if not all of these environments.
material that is inert in one environment may not be in another. It is
for this reason that material selection is important to ensure that adequate
performance characteristics, especially life span, are obtained. Cost
and availability dictate the materials that are used in industrial processes.
This trade-off is what causes most corrosion problems.
exception of some forms of high-temperature corrosion, all forms of corrosion
occur through the action of the electrochemical cell. This cell contains
what is known as an oxidation/reduction reaction. In this reaction, an
exchange of electrons (due to a difference in potential) occurs, where
an anode is the site of oxidation and a cathode is the site of reduction.
The electrons given off at the anode travel through the metal to the cathode,
where they are consumed in a reduction reaction.
is often classified as wet or dry. Wet
corrosion occurs when a liquid phase is present and dry corrosion occurs
in the absence of a liquid phase or above the dew point of the environment.
In most cases,
the combination of the metals found in equipment and structures, combined
with the wide range of possible environments, will result in more than
one form of corrosion within a system.
most common industry classifications of corrosion are as follows:
General Attack (Uniform) Corrosion
This is the most common form of corrosion. It occurs when
a chemical or electrochemical attack occurs over a large area
in a uniform manner. This is often referred to as a general
wall loss or thinning.
This occurs through the electrochemical cell. It requires electron
flow, and is characterized by the presence of an anode (negative),
cathode (positive) and an electrolyte. Most corrosion occurs
at the anode, although some corrosion will occur at the cathode.
Depending on the cell configuration, the corrosion may be localized
Crevice corrosion is a highly localized attack occurring
in a crevice or an otherwise shielded area when a material
is exposed to a stagnant corrosive media. Common locations
for crevice corrosion are:
Crevices(such as under bolt or rivet heads)
A highly localized corrosion attack that results in holes
is referred to as pitting. Pits may be isolated or localized
and of virtually any configuration. They occur at defects
or imperfections in a protective or passive film.
Filiform corrosion is a special form of oxygen cell corrosion
occurring beneath organic or metallic coatings on materials.
The attack results in a fine network of random “threads”
of corrosion product developed beneath the coating material.
Its cause is associated with mild surface contamination
of solid particles or residue deposited on the metal surface
All materials, with the exception of amorphous materials (such
as plastic), are composed of grains and grain boundaries. Intergranular
corrosion occurs when the grain boundaries are attacked in preference
to the material matrix. The only difference between this and
uniform corrosion is that the grains remain undamaged. Metallographic
examination is usually the only way to identify this corrosion
Dealloying (Selective Leaching)
Most materials are made up of a combination of several elements.
Dealloying occurs when one of the elements is removed from the
metal matrix, leaving an altered residual structure. It is commonly
identifiable by a color change or a drastic change in mechanical
Velocity Effects Corrosion
Erosion occurs when the velocity of the fluid is sufficient
to remove protective films from the metal surface. It is
often a combination of erosion (mechanical damage) with
corrosion (electrochemical damage). It is characterized
by patterns that are shaped by the flow of a fluid past
Impingement is localized erosion/corrosion caused by turbulence
or impinging flow. Entrained air bubbles and suspended solids
tend to accelerate this action. It is commonly found in
pumps, valves and at elbows and tees in pipelines.
This is a mechanical damage process
caused by collapsing bubbles in a flowing liquid. Cavitation
usually results in the formation of deep aligned pits in
areas of turbulent flow
Environmental Corrosion Cracking
Stress Corrosion Cracking
Stress corrosion occurs when a material is exposed to a
corrosive media while a force (stress) or pressure is applied.
The material usually remains undamaged with the exception
of cracks that grow through the material matrix. These cracks
are usually very fine, visible only under microscopic conditions,
but often network through the material, ultimately causing
Hydrogen-Induced Cracking (HIC) and Sulfide Stress
Hydrogen-induced cracking results from the combined action
of a tensile stress and hydrogen in the metal. It results
in the brittle failure of otherwise ductile materials when
exposed to an environment where hydrogen can enter the metal.
Sulfide stress cracking is a type of HIC in which sulfide
is the primary poison for the hydrogen evolution. SSC of
medium-strength steels has been a continuing source of trouble
in oil fields.
Liquid Metal Embrittlement (LME)
This is defined as the decrease in strength or ductility
of a metal or alloy as a result of contact with a liquid
metal. Unlike fracture by SSC, cracking begins immediately
upon the application of stress if the liquid metal has wetted
the solid material.
Fretting occurs when motion between surfaces either removes
protective films or mechanically removes material from surfaces
in relative motion. It usually involves the motion of two surfaces
that were not intended to move in that fashion.
High Temperature Corrosion
High-temperature corrosion is a form of material degradation
that occurs at elevated temperatures, often beginning with
temperatures in the range of 30-40% of the melting point of
the alloy. Direct chemical reactions, not the electrochemical
cell reaction, are responsible for the corrosion.
In itself, biological corrosion is not a mechanism but a cause.
The presence of microorganisms can lead to any of the above
mechanisms occurring. Corrosion caused by microorganisms is
usually indistinguishable from other sources; it is often
determined by sampling the process condition for evidence
of microbiological activity.
Corrosion Engineering. Mars G. Fontana, McGraw-Hill Book Co.,
Basic Corrosion Course. NACE ETC-10 Committee, 1996 Rev.