Question

The marble statues often slowly get corroded when in open for a long time. Why?

Answer 1

It is due to the inorganic material such as COMPOUNDS of carbon, sulphates , salts acid rains which actually corrodes them

Answer 2

The marble statues often slowly get corroded when in open for a long time bcoz the gaseous oxides CO2, NO2 and SO2 present in the air as chemical pollutants from industries and factories 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.
hope this will help you