Why Metals Corrode
 Metals
corrode because we use them in environments where they are chemically
unstable. Only copper and the precious metals (gold, silver, platinum,
etc.) are found in nature in their metallic state. All other metals, to
include iron-the metal most commonly used-are processed from
minerals or ores into metals which are inherently unstable in their
environments.
This golden
statue in Bangkok, Thailand, is made of the only metal which is
thermodynamically stable in room temperature air. All other metals are
unstable and have a tendency to revert to their more stable mineral
forms. Some metals form protective ceramic films (passive films) on
their surfaces and these prevent, or slow down, their corrosion process.
The woman in the picture below is wearing anodized titanium earrings.
The thickness of the titanium oxide on the metal surface refracts
the light and causes the rainbow colors on her earrings. Her husband
is wearing stainless steel eyeglasses. The passive film that formed
on his eyeglasses is only about a dozen atoms thick, but this passive
film is so protective that his eyeglasses are protected from corrosion.
We can prevent corrosion by using metals that form naturally protective
passive films, but these alloys are usually expensive, so we have
developed other means of corrosion control.
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