Corrosion

Slow formation of undesirable compounds such as oxides, sulphides or carbonates at the surface of metals by reaction with moisture and other atmospheric gases is known as corrosion.

Factors affecting corrosion

i. Reactivity of metals.

ii. Presence of moisture and atmospheric gases like CO₂, SO₂, etc.

iii. Presence of impurities.

iv. Presence of electrolyte.

Electrochemical theory of rusting of iron

An electrochemical cell, also known as corrosion cell, is developed at the surface of iron.

At a particular spot of an object made of iron, oxidation takes place and that spot behaves as anode.

Anode - Pure iron

Anode reaction: 2Fe(s) → 2 Fe²⁺ + 4e^– , = – 0.44 V

Electrons released at anodic spot move through the metal and go to another spot on the metal and reduce oxygen in presence of H⁺.

H⁺ is believed to be available from H₂CO₃ formed due to dissolution of carbon dioxide from air into water. Hydrogen ion in water may also be available due to dissolution of other acidic oxides from the atmosphere.

This spot behaves as cathode.

Cathode - Impure iron surface

Cathode reaction: O₂(g) + 4H⁺(aq) + 4e^– → 2H₂O(l), = 1.23V

The overall reaction:

2Fe(s) + O₂(g) + 4H⁺(aq) → 2Fe²⁺(aq) + 2H₂O(l), E^o_cell =1.67 V

Prevention of rusting of iron:

Rusting of iron can be prevented by the following methods:

1. Barrier protection through coating of paints or electroplating.

2. Through galvanisation or coating of surface with tin metal.

3. By the use of antirust solutions (bis phenol).

4. By cathodic protection in which a metal is protected from corrosion by connecting it to another metal that is more easily oxidised.