Define corrosion. What is the chemical formula of rust?

Explanation

Corrosion. It is a process of cating away of metals when exposed to the atmosphere surrounding it. Many metals when exposed to the atmosphere, react with air or water in the environment to form undesirable compound on their surfaces. In case of iron, the corrosion is called rusting. The red or orange coating that forms on the surface of iron when exposed to air and moisture is called rust. Chemically rust is a hydrated form of ferric oxide, `Fe_(2)O_(3).xH_(2)O`.
Factors which affect corrosion. The main factors which affect corrosion are :
(i) Position of metals in e.m.f. series. The reactivity of a metal depends upon its position in the electrochemical series. More the reactivity of the metal, more will be the possibility of the metal getting corroded.
(ii) Presence of impurities in metals. The impurities help in setting up a sort of voltaic cells, which increase the speed of corrosion.
(iii) Presence of electrolytes. Presence of electrolytes in water also increases the rate of corrosion. For example, corrosion of iron in sea water takes place to larger extent than in distilled water.
(iv) Presence of `CO_(2)` in water. Presence of `CO_(2)` in natural water increases rusting of iron. Water containing `CO_(2)`, acts as an electrolyte and increases the flow of electrons from one place to another.
(v) Presence of protective coatings. When the iron surface is coated with layers of metals more active than iron, then the rate of corrosion is retarded. For example, coating of zinc on iron prevents rusting.