Question

the marble statues often slowly get corroded when kept in open for long time. Why?

Answer 1

Marbel get corrode due to reacting with chemicals in air and also due to acid rains caused due to micing of acidic gas in cloud..

Answer 2

Answer: Corrosion is the process of slow eating away of metal due to attack of atmospheric gases on the surface of metal resulting into formation of compounds such as oxides, sulfides , carbonates etc is called as corrosion.

Marble statues are made up of calcium carbonate $CaCO_3$. When these marble statues are kept in open for long, calcium carbonate start reacting with the acids present in the rain or the reactive gases and start converting to other compounds and thus get corroded.

For example:  

$CaCO_3+2HNO_3\rightarrow Ca(NO_3)_2+H_2O+CO_2$