Taj mahal is an example of corrosion of marble.The discoloration of its marble is due to corrosion. The gaseous oxides (CO2, NO2 and SO2 ) present in the air as chemical pollutants mix with water in air to form acids such as carbonic acid, nitric acid and sulphuric acid.This acid comes down as acid rain and cause corrosion of marble statues. Calcium carbonate present in marble is transformed into other compounds by reaction with acids as follows:
The compounds formed are water soluble and are easily washed away. Blackening of marble surfaces is due to the presence of carbonate, sulphate and nitrate salts.
