| Thermodynamic Corrosion Cycle: A further corrosion cycle, often used as introduction to corrosion theory, is a
thermodynamic one. Almost all metals and alloys used in service are actually in an
unstable thermodynamic state. There is thus a fundamental thermodynamic tendency for them
to return to a stable state through corrosion processes. In essence, corrosion reverts the
metals/alloys back to the stable state of the ores from which they were derived. Corrosion
products tend to be thermodynamically stable species, similar to the original ores.
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| In view of this cycle, corrosion has been described by
Fontana as "extractive metallurgy in reverse". While corrosion scientists and
engineers cannot rewrite the fundamental laws of thermodynamics and ultimately cannot
avoid this fundamental cycle, much can be done to reduce rate of degradation to acceptable
levels. Corrosion monitoring is important for measuring the rate of degradation and
ensuring that the rate of corrosion remains within acceptable limits (through suitable
corrosion control methods). |
Thermodynamics is concerned with energy states. The original metallic ores are said to be
in a state of low energy. External energy is applied in the conversion of the ores to
usable metals and alloys, transforming them to a higher energy state. They tend to revert
to a lower (more stable) energy state by reacting with a corrosive environment. While
thermodynamics can predict whether a corrosion reaction will take place, it does not
provide an indication of the rate of corrosion reactions. The rate of reactions is
described by kinetic theory.
References/Literature:
M.G. Fontana: "Corrosion Engineering", McGraw-Hill, New York,
1986.
K.R. Trethewey and J. Chamberlain: "Corrosion for Science and
Engineering 2nd Edn.", Longman (England), 1995.
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