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:
On steel food cans and other galvanized objects, the iron is coated with a more reactive metal, normally zinc. Scratches in the zinc expose iron, but because the zinc is acting as the anode the iron is still protected from rusting. This principle is also used for underground steel fuel tanks, which are connected by a ground-wire to a block of more-reactive metal called a "sacrificial anode."