ABSTRACT
The corrosion of a material may be defined as the irreversible reaction of the material with its
environment, usually resulting in degradation of the material and its properties. In this chapter, we
will be concerned only with the corrosion of metals in aqueous environments under near-ambient
conditions, and we will ignore topics such as high-temperature corrosion, erosion corrosion, environ-
mentally induced cracking, and nonaqueous corrosion. The corrosion of a metal may be viewed as
extractive metallurgy in reverse. Considerable energy must be expended to extract and purify a metal
such as Fe or Al from its ore (i.e., its oxide forms) and, thus, corrosion is the thermodynamically driven
process by which the metals revert to their oxide forms. For example, the energy obtainable from the
oxidation of 1 mole (55.85 g) of pure Fe to g-FeOOH is 81 kcal, sufficient to power a 100 W light bulb
for nearly 1 h. Thus, it is virtually impossible to completely stop such a thermodynamically favorable
process. As long as we continue to use active (i.e., corrodible) metals for the construction of bridges,
buildings, automobiles, airplanes, ships, industrial reactors, and other objects, there will be corrosion
problems and associated costs. It is estimated that the annual cost of corrosion and its control for a
developed country is approximately 3%–4% of the country’s gross domestic product; for the United
States alone, that amounts to $300 billion per year [1]. As there is not much that can be done to alter the thermodynamics of corrosion of structural (active)
metals in natural environments, corrosion control strategies typically focus on slowing the rate of the
corrosion process. A very common corrosion control strategy is to apply one or more layers of a coating
to the metal. Such a coating may simply serve as a barrier between the metal and its environment,
retarding the rate at which water, oxygen, or ions from the environment reach the metal surface. On the
other hand, a coating may function as more than just a barrier. The coating may be an active coating in
the sense that it contains or consists of a material than can interact chemically or electrochemically with
the metal, altering its corrosion behavior. For example, coatings containing hexavalent chromium are
often used to control the corrosion of aluminum alloys [2], whereas coatings containing Zn particles are
commonly used on steel [3]. Such coatings function through (often complex) electrochemical or
chemical interactions of the active species (Cr(VI) or Zn) and their reaction products with the metal
surface. By this definition, conducting polymer coatings are active coatings.
