Corrosion is degradation of materials’ properties due to interactions
with their environments, and corrosion of most metals (and many
materials for that matter) is inevitable. While primarily associated
with metallic materials, all material types are susceptible to
degradation. Degradation of polymeric insulating coatings on
wiring has been a concern in aging aircraft. Even ceramics can
undergo degradation by selective dissolution. Like death and
taxes, corrosion is something we hope to avoid; but ultimately it is
something we must learn to deal with. The fundamental cause or
driving force for all corrosion is the lowering of a system’s Gibbs
energy. As Fig. 1 illustrates, the production of almost all metals
(and engineering components made of metals) involves adding
energy to the system. As a result of this uphill thermodynamic
struggle, the metal has a strong driving force to return to its
native, low energy oxide state. This return to the native oxide
state is what we call corrosion and even though it is inevitable,
substantial barriers (corrosion control methods) can be used to
slow its progress toward the equilibrium state. Thus it is the rate of
the approach to equilibrium that is often of interest. This rate is
controlled not only by the nature of the metal surface, but also by
the nature of the environment as well as the evolution of both.
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