The surface of iron with lower concentration of oxygen becomes an anode (negative terminal) where oxidation occurs. Iron atom loses electrons and oxidised to form iron(II) ion, Fe 2+, Electrons flow through iron to the edge of the water droplets where the concentration of oxygen is higher. Iron surface becames cathode (positive terminal) where reduction occurs. Oxygen molecules, O2 gains electrons and reduced to form hydroxide ions, OH -, Iron (II) ion, Fe 2+ produced combines with hydroxide ions, OH - to from iron(II) hydroxide, Iron(II)ion, Fe 2+ is green but rust is brown because iron(II) hydroxide, Fe(OH) 2 undergoes further oxidation by oxygen to form hydrated iron(III) oxide, Fe 2O3. xH2O (rust).

Mechanism Rusting of Iron

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