Taj Mahal is an example of corrosion of marble. The discoloration of its marble is due to corrosion.
Air primarily consists of various pollutant gases like carbon dioxide, nitrogen dioxide, Sulphur dioxide etc which mix with moisture in air to form acids such as carbonic acid, nitric acid and sulphuric acid. This acid comes down as acid rain and causes corrosion of marble statues and monuments. Marble which is composed of calcium carbonate reacts with the different acids which is given as follow:-
CO2 + H2O + CaCO3→ Ca (HCO3).
CaCO3 + H2SO4→ CaSO4 + CO + H2O.
The compound formed are water soluble and are easily washed away. Thus blackening of marble surface is the corrosion of marble surface due to the formation of side products of calcium.
