The process of slowly eating away of the metal due to the attack of atmosphere gases on the surface of the metal resulting into the formation of oxides, sulphides, etc. is called corrosion.

 Factors affecting corrosion :

• Reactivity of the metal
• Presence of impurities
• Presence of air and moisture
• Strains in metals
• Presence of electrolytes.

Mechanism of corrosion :

At anode : Fe(s) → Fe²⁺(aq) + 2e^-

At cathode : H₂O + CO₂  → H₂CO₃

H₂CO₃ ⇌ 2H⁺ + CO₃^2-

2H⁺(aq) + 1/2O₂(g) + 2e^- → H₂O(l)

2Fe²⁺(aq) + 1/2O₂(g) + 2H₂O(l) → Fe₂O₃(s) + 4H⁺(aq)

Fe₂O₃(s) + xH₂O(l) → Fe₂O₃.xH₂O(aq)