Corrosion of iron

Aug 26, 2022, 16:45 IST

About Corrosion of iron

Corrosion Is defined as the gradual transformation of a metal Into its combined state because of the reaction with the environment. Metals are usually extracted from their ores. Nature tries to convert them again into the ore form. The process by which the metals have the tendency to go back to their combined state, is termed corrosion.

Reaction of Corrosion of iron

When iron is exposed to moist air, it is found covered with a reddish-brown coating which can easily be detached. The reddish-brown coating is called ‘rust’. Thus, the corrosion of iron or formation of the rust is called rusting. The composition of the rust is not certain but it mainly contains hydrated ferric oxide, 2Fe2O3.3H2O together with a small quantity of ferrous carbonate. The rust is formed by the action of water on iron in presence of dissolved oxygen and carbon dioxide. It has been observed that impure iron is more prone to rusting.

The following are the favourable conditions for the rusting of iron:

(i) Presence of moisture

(ii) Presence of a weakly acidic atmosphere

(iii) Presence of impurity in the iron.

Mechanism of Corrosion of iron

Various theories have been proposed to explain the phenomenon of rusting of iron but the accepted theory is the modern electrochemical theory. When impure iron comes in contact with water containing dissolved carbon dioxide, a voltaic cell is set up. The iron and other impurities act as electrodes while water having dissolved oxygen and carbon dioxide acts as an electrolyte. Iron atoms pass into solution as ferrous ions.

Fe  Fe2+ + 2e

Iron, thus, acts as anode.

The impurities act as cathode. At the cathode, the electrons are used in forming hydroxyl ions.

H2O + O + 2e  2OH

In presence of dissolved oxygen, ferrous ions are oxidised to ferric ions which combine with hydroxyl ions to form ferric hydroxide.

Fe3+ + 3OH  Fe(OH)3

[2Fe2+ + H2O + O  2Fe3+ + 2OH]

Corrosion or rusting is a surface phenomenon and thus, the protection of the surface prevents the corrosion. Iron can be protected from the rusting by use of following methods

(i) Applying paints, lacquers and enamels on the surface of iron.

(ii) By forming a firm and coherent protective coating of ferrosoferric oxide. This is done by passing steam over hot iron.

(iii) By coating a thin film of zinc, tin, nickel, chromium, aluminium, etc.