Corrosion

Corrosion: 

When a piece of iron is left in open for sometime, it acquires a film of brownish substance which is called rust. This process of change of iron to rust is called rusting of iron. Rust is different from iron. This rust is iron oxide. The chemical equation of rusting of iron can be represented as :

Prevention of Rusting:

(i) Galvanisation: It is considered as the best method to prevent rusting. It is the process of deposition of a thin layer of zinc on iron surface. It is done to prevent rusting of iron. Rusting can also be prevented by deposition of thin layer of chromium on iron articles. We use galvanised Iron (G.I) pipes to carry water in taps. It is estimated that about 15% of the total world production of iron is destroyed due to rusting.

(ii) Application of coat of paint or grease on the surface of iron also help in prevention of rusting.

Essential Conditions For Rusting:

(i) Presence of moisture (water vapour)

(ii) Presence of air (or oxygen)

Near the Qutub Minar in Delhi stands a 7 metre high iron pillar braving heat, rain and rough weather for more than 1600 years. It has not rusted even after so many years. Its quality of rust resistant iron is still a mystery for the present day scientists.

Factors that help in speeding up the process of rusting:

(i) Higher moisture content in air increases the rate of rusting.

(ii) Presence of salts (electrolytes) in water also makes the process of rusting faster. Due to presence of many salts in sea water, the ships suffer a lot of damage from rusting inspite of being painted.