(i) The slow and steady destruction of a metal by chemical
or electro chemical reaction with the environment.
(ii) When the surface of iron is exposed to moisture and other gases present in the atmosphere, the following chemical reaction takes place.
Fe → Fe2+ + 2e-
O2 + 2H2O + 4e- → 4OH-
O2 + 4H+ + 4e- → 2H2O
The Fe2+ ions are oxidised to Fe3+ ions.
The Fe3+ ions combine OH- ions to form Fe(OH)3 . This becomes rust which is hydrated ferric oxide with the formula Fe2O3.xH2O. It is a reddish brown substance.
