Discuss the process of rusting of iron.

Corrosion slowly coats the surfaces of metallic objects with oxides or other salts of the metal. The rusting of iron, tarnishing of silver, development of green coating on copper and bronze are some of the examples of corrosion. It causes enormous damage to buildings, bridges, ships and to all objects made of metals especially that of iron. We lose crores of rupees every year on account of corrosion. In corrosion, a metal is oxidised by loss of electrons to oxygen and formation of oxides. Corrosion of iron (commonly known as rusting) occurs in presence of water and air. The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon. At a particular spot of an object made of iron, oxidation takes place and that spot behaves as anode and we can write the reaction

Anode: 2Fe (s) → 2Fe²⁺ + 4 e^{– ,}  E_{(Fe2+ /Fe)} = – 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⁺ (which 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 with the reaction

 Cathode: O₂(g) + 4H⁺(aq) + 4 e^– → 2H₂O (l ) + 2H₂O, E_H2O = 1.23 V E V

 The overall reaction being: 2Fe(s) + O₂ (g) + 4H⁺ (aq) → 2Fe2 +(aq) + 2H₂O (l) , E(cell) V E =1.67 V

The ferrous ions are further oxidised by atmospheric oxygen to ferric ions which come out as rust in the form of hydrated ferric oxide (Fe₂O₃ .x H₂O) and with further production of hydrogen ions.