Question

Briefly explain the electrochemical processes involved in the rusting of iron.

Hint:

Rusting is accelerated by the presence of electrolytes (like salt in seawater) because they increase the conductivity of the aqueous film on the metal surface.

Answer 1

Concept:
Rusting occurs at the surface of iron when it comes into contact with moisture and oxygen, forming an electrochemical cell.
Step 1: At the Anode.
Iron atoms lose electrons and are oxidized to ferrous ions:
Anode: Fe(s) → Fe²⁺(aq) + 2e⁻ (E° = -0.44 V)

Step 2: At the Cathode.
Electrons move through the metal to another spot where they reduce oxygen in the presence of H⁺ ions (from H₂CO₃ formed by dissolved CO₂):
Cathode: O₂(g) + 4H⁺(aq) + 4e⁻ → 2 H₂O(l) (E° = 1.23 V)

Step 3: Formation of Rust.
The ferrous ions are further oxidized by atmospheric oxygen to ferric ions, which deposit as hydrated ferric oxide (rust):
2 Fe²⁺ + 2 H₂O + ½ O₂ → Fe₂O₃ + 4 H⁺
Rust: Fe₂O₃ · x H₂O