The impure iron surface behaves like small electrochemical cell in the presence of water containing dissolved oxygen or carbon dioxide. In this cell pure iron acts as an anode and impure iron surface acts as cathode. Moisture having dissolved CO2 or O2 acts as an electrolyte.
The reactions at cathode and anode are as follows:
Anode: 2Fe (s) → 2Fe2+ + 4e-
Cathode: O2(g) + 4H+ (aq) + 4e-→ 2H2O (l)
Overall Reaction: 2Fe (s) + O2(g) + 4H+ (aq) → 2H2O (l)
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