|MadSci Network: Chemistry|
Corrosion of iron can be considered as an electrochemical process. The so-called rusting takes place in the presence of water and oxygen. There are two reactions taking place.
Anode reaction: Fe (s) -> Fe2+(aq) + 2 e-
Cathode reaction: O2(g) + 2 H2O + 4 e- -> 4 OH-(aq)
Humid Fe(OH)2 is not stable in air, and rust (Fe2O3 · x H2O) is formed.
Iron sometimes is coated with a less reactive material, as is the case with chromium, for instance on automobile trim. In this case, scratches in the chromium coating will cause the iron to act as the anode and rust faster. Generally speaking, corrosion is enhanced when iron is in electrically conducting contact with the more noble metal.
Dan Berger adds:
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