What is corrosion? Explain the electro- chemical theory of rusting of iron and write the reactions involved in the rusting of iron?

Explanation

The process of slowly eating away of the metal due to attack of the moisture and atmospheric gases on the surface of the metal resulting into the formation of compound such as oxides, sulphides, carbonates, sulphates etc. is called corrosion. The electrochemical phenomenon of rusting of iron can be described as:
At anode: `Fe(s)` undergoes oxidation to releases electrons.
`Fe(s) toFe^(2+) (aq)+2e^(-)`
At cathode: The electrons which are released participate in the reduction reaction and combine with `H^(+)` ions released from carbonic acid `(H_2CO_3)` formed by the combination of `CO_2 and H_2O` present.
`H_2O+CO_2 leftrightarrow H_2CO_3`
`H_2CO_3 leftrightarrow 2H^(+)+ CO_3^(2-)`
`H_2O leftrightarrow H^(+)+ OH^(-)` (reduction)
`H^(+)+ e^(-) to H`
THe hydrogen atoms combine with oxygen either dissolved in water or present in air to form `H_2O` molecule.
`4H+O_2 to 2H_2O`
Thus net result of the redox reaction is to oxidise iron to `Fe^(2+)`.
These further reaction with oxygen and water to form ferric oxide as follows:
`4Fe^(2+) (aq)+O_2+4H_2O to 2Fe_2O_3+8H^(+) (aq)`
Ferric oxide undergoes hydration to form the hydrate oxide also called rust. Protons that are released further take part in th redox reaction.
`Fe_2O_3+xH_2O to Fe_2O_3.xH_2O_(hydrate ferric o x ide)`