# Corrosion

Corrosion is the attack of atmospheric gases on metal surface to change it into salts like oxides, carbonates, sulphides etc. It is a redox reaction and reverse of extraction of metals.

E.g.

Rusting of Iron: It can be explained according to electro-chemical theory as follows:

$\text{At anode:} Fe \to Fe^{+2} + 2e^- \\[3mm] 4Fe^{+2} + O_2 + 4H_2O \to 2Fe_2O_3 + 8H^+$

$\text{At cathode:} H^+ + e^- \to H \\[3mm] 4H + O_2 \to 2H_2O$

Rust is hydrated ferric oxide, i.e., $Fe_2O_3.xH_2O$

Factors increasing corrosion:

(i)     Presence of impurities in the metal

(ii)    Presence of electrolyte e. g. saline Water

(iii)   Presence of moisture or rainy season

Prevention of Corrosion:

It is possible by following methods:

(i)    Barrier protection: Metal surface is coated with oil, paints, grease or by electroplating

(ii)    Sacrificial protection: Here metal is coated by a more electro-positive metal i.e. it is sacrificed, e.g., Zn is coated over iron, it is called Galvanization.

(iii)    Electrical protection: Here metal is connected to a more electro-positive metal by a wire.

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