Why do metals change in appearance over time?

The exterior surface of metal can be changed by many chemical reaction takes place the reaction is carried out by presence of oxygen and water in air which Demage shiny surface of metal. The other processes can also be involved as pollution, moisture and environmental change can also effect on the surface of metal.

If metal is keept in open environment for a long time moisture contents in air boats the corrosion and rusting of metal processes.

Hopefully you will understand.