Corrosion Science Chemistry Research Article | Page 2

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Corrosion: Understanding the Basics
The Definition of Corrosion
Corrosion can be defined in many ways. Some definitions are very
narrow and deal with a specific form of corrosion, while others are quite
broad and cover many forms of deterioration. The word corrode is de-
rived from the Latin corrodere, which means “to gnaw to pieces.” The
general definition of corrode is to eat into or wear away gradually, as if
by gnawing. For purposes here, corrosion can be defined as a chemical
or electrochemical reaction between a material, usually a metal, and
its environment that produces a deterioration of the material and its proper-
ties.
The environment consists of the entire surrounding in contact with the
material. The primary factors to describe the environment are the follow-
ing: (a) physical state—gas, liquid, or solid; (b) chemical composition—
constituents and concentrations; and (c) temperature. Other factors can
be important in specific cases. Examples of these factors are the relative
velocity of a solution (because of flow or agitation) and mechanical
loads on the material, including residual stress within the material. The
emphasis in this chapter, as well as in other chapters in this book, is on
aqueous corrosion, or corrosion in environments where water is pres-
ent. The deterioration of materials because of a reaction with hot gases,
however, is included in the definition of corrosion given here.
To summarize, corrosion is the deterioration of a metal and is caused
by the reaction of the metal with the environment. Reference to marine
corrosion of a pier piling means that the steel piling corrodes because of
its reaction with the marine environment. The environment is air-
saturated seawater. The environment can be further described by speci-
fying the chemical analysis of the seawater and the temperature and ve-
locity of the seawater at the piling surface.
When corrosion is discussed, it is important to think of a combination
of a material and an environment. The corrosion behavior of a material
cannot be described unless the environment in which the material is to
be exposed is identified. Similarly, the corrosivity or aggressiveness of
an environment cannot be described unless the material that is to be ex-
posed to that environment is identified. In summary, the corrosion be-
havior of the material depends on the environment to which it is sub-
jected, and the corrosivity of an environment depends on the material
exposed to that environment.
It is useful to identify both natural combinations and unnatural combi-
nations in corrosion. Examples of natural or desirable combinations of
material and environment include nickel in caustic environments, lead in
water, and aluminum in atmospheric exposures. In these environ- ments,
the interaction between the metal and the environment does not