Chemistry – Metals - Corrosion of metals | e-Consult

The rusting of iron is an electrochemical process involving oxidation and reduction. It occurs when iron is exposed to both oxygen and water, creating an electrochemical cell. The iron acts as the anode (where oxidation occurs) and the area of lower oxidation potential acts as the cathode (where reduction occurs). Electrons flow from the anode to the cathode through the iron metal, completing the circuit. The water acts as the electrolyte, allowing the movement of ions.

At the Anode (Oxidation): Iron atoms lose electrons and become iron(II) ions (Fe²⁺). This is oxidation.

Reaction: Fe(s) → Fe²⁺(aq) + 2e^-

At the Cathode (Reduction): Oxygen dissolved in the water gains electrons and is reduced to hydroxide ions (OH^-). This is reduction.

Reaction: O₂(g) + 2H₂O(l) + 4e^- → 4OH^-(aq)

The iron(II) ions (Fe²⁺) then react with hydroxide ions (OH^-) to form iron(II) hydroxide [Fe(OH)₂], which further oxidizes to form hydrated iron(III) oxide (Fe₂O₃·nH₂O), the reddish-brown substance we know as rust.