Hydrogen ion in water may also
be available due to dissolution of other acidic oxides from the
atmosphere) This spot behaves as cathode with the reaction
Cathode: O2(g) + 4 H+(aq) + 4 e– ¾® 2 H2O (l) 
o
|
|
+
2
2
H
O  
H O
=1 23 V
E
The overall reaction being:
2Fe(s) + O2(g) + 4H+(aq) ¾® 2Fe2 +(aq) + 2 H2O (l)    
o
E(cell)
=1 67 V
The ferrous ions are further oxidised by atmospheric oxygen to
ferric ions which come out as rust in the form of hydrated ferric oxide
(Fe2O3