The cost of corrosion to US industries and the American public is
currently estimated at $170 billion per year. Although corrosion is
only nature`s method of recycling, or of returning a metal to its
lowest energy form, it is an insidious enemy that destroys our cars,
our plumbing, our buildings, our bridges, our engines, and our
Corrosion can often be predictable or it can be totally unpredictable
and catastrophic, such as the hydrogen embrittlement or stress
corrosion of critical structural members and pressure vessels in the
aerospace and chemical processing industries.
Corrosion involves the interaction (reaction) between a metal or alloy
and its environment. Corrosion is affected by the properties of both
the metal or alloy and the environment. In this discussion, only the
environmental variables will be addressed, the more important of which
- pH (acidity)
- Oxidizing power (potential)
- Temperature (heat transfer)
- Velocity (fluid flow)
- Concentration (solution constituents)
The concept of pH is complex. It is related to, but not synonymous
with, hydrogen concentration or amount of acid.
While corrosion obeys well-known laws of electrochemistry and
thermodynamics, many variables that influence the behavior of a metal
in its environment can result in accelerated corrosion or failure in
one case and complete protection in another, similar case.
Avoiding detrimental corrosion requires the interdisciplinary approach
of the designer, the metallurgist, and the chemist. Sooner or later,
nearly everyone in these fields will be faced with major corrosion
issues. It is necessary to learn to recognize the forms of corrosion
and the parameters that must be controlled to avoid or mitigate
The theory of corrosion from the thermodynamic and kinetic points of
view covers the principles of electrochemistry, diffusion, and
dissolution as they apply to aqueous corrosion and high-temperature
corrosion in salts, liquid metals, and gases.
We can face the various forms of corrosion, and we must know how to
recognize them, as well as the driving conditions or parameters that
influence each form of the corrosion, for it is the control of these
parameters which can minimize or eliminate corrosion.
All corrosion processes show some common features. Thermodynamic
principles can be applied to determine which processes can occur and
how strong the tendency is for the changes to take place. Kinetic laws
then describe the rates of the reactions. There are, however,
substantial differences in the fundamentals of corrosion in such
environments as aqueous solutions, non-aqueous liquids, and gases.
Corrosion and Environment
Corrosion in aqueous solutions
. Although atmospheric air is the
most common environment, aqueous solutions, including natural waters,
atmospheric moisture, and rain, as well as man-made solutions, are the
environments most frequently associated with corrosion problems.
Because of the ionic conductivity of the environment, corrosion is due
to electrochemical reactions and is strongly effected by such factors
as the electrode potential and acidity of the solution.
Corrosion of metals in aqueous environments. This type of
corrosion is almost always electrochemical in nature. It occurs when two or more
electrochemical reactions take place on a metal surface. As a result,
some of the elements of the metal or alloy change from a metallic
state into a non-metallic state.
The products of corrosion may be dissolved species or solid corrosion
products. In either case, the energy of the system is lowered as the
metal converts to a lower-energy form. Rusting of steel is the
best-known example of a conversion of the metal (iron) into a
nonmetallic corrosion product (rust). The change in the energy of the
system is the driving force for the corrosion process and is a subject
Thermodynamics examines and quantifies the tendency for corrosion and
its partial processes to occur. It does not predict if the changes
actually will occur and at what rate. Thermodynamics can predict,
however, under what conditions the metal is stable and corrosion
Corrosion in Molten Salts and Liquid Metals. These are more
specific but important areas of corrosion in liquid environments.
Both have been strongly associated with the nuclear industry, for
which much of the research has been performed, but there are numerous
non-nuclear applications as well.
Corrosion in Gases. In gaseous corrosion, the environment is
nonconductive, and the ionic processes are restricted to the surface
of the metal and the corrosion product layers.
Because the reaction rates of industrial metals with common gases are
low at room temperature, gaseous corrosion, generically called
oxidation, is usually an industrial problem only at high temperatures
when diffusion processes are dominant.
Forms of Corrosion
Over the years, corrosion scientists and engineers have recognized
that corrosion manifests itself in forms that have certain
similarities and therefore can be categorized into specific groups.
However, many of these forms are not unique but involve mechanisms
that have over lapping characteristics that may influence or control
initiation or propagation of a specific type of corrosion.
The most familiar and often used categorization of corrosion is:
uniform attack, crevice corrosion, pitting, intergranular corrosion,
selective leaching, erosion corrosion, stress corrosion, and hydrogen
damage. This classification of corrosion was based on visual
characteristics of the morphology of attack.
Other prominent corrosion authors have avoided a classification format
and have simply discussed the classical types of corrosion (for
example, pitting and crevice corrosion) as they relate to specific
metals and alloys.
Forms of corrosion are:
- Atmospheric corrosion
- Galvanic corrosion
- Stray-current corrosion
- General biological corrosion
- Molten salt corrosion
- Corrosion in liquid metals
- Other forms(a)
- Filiform corrosion
- Crevice corrosion
- Pitting corrosion
- Localized biological corrosion
Metallurgically influenced corrosion
- Intergranular corrosion
- Dealloying corrosion
Mechanically assisted degradation
- Erosion corrosion
- Fretting corrosion
- Cavitation and water drop impingement
- Corrosion fatigue
Environmentally induced cracking
- Stress-corrosion cracking
- Hydrogen damage
- Liquid metal embrittlement
- Solid metal induced embrittlement
General Corrosion. General corrosion is defined as corrosive
attack dominated by uniform thinning. Although high-temperature attack
in gaseous environments, liquid metals, and molten salts may manifest
itself as various forms of corrosion, such as stress-corrosion
cracking and de-alloying, high-temperature attack has been
incorporated under the term "General Corrosion" because it is often
dominated by uniform thinning.
Localized Corrosion. The forms of corrosion under this category
need no explanation, even though other forms could be placed in this
category. It should be noted, however, that localized biological
corrosion often causes or accelerates pitting or crevice corrosion.
Metallurgically influenced corrosion was so classified as a
result of the significant role that metallurgy plays in these forms of
attack. It is well understood that metallurgy is important in all
forms of corrosion, but this classification is meant to emphasize its
role in these specific forms of attack.
Mechanically assisted degradation groups those forms of
corrosion that contain a mechanical component, such as velocity,
abrasion, and hydrodynamics, that has a significant effect on the
corrosion behavior. Corrosion fatigue was included in this category
because of the dynamic stress state; however, it could easily be
categorized as a form of environmentally induced cracking.
The environmentally induced cracking follows the current trend in the
literature of combining forms of cracking that are produced by
corrosion in the presence of stress.