If the penny has a silver color, it is made out of steel with a zinc coating to make it look nicer and protect it from rusting. ... When exposed to moisture the penny would start to rust.

Does a penny rust?

A penny doesn't technically “rust.” The copper plating corrodes, resulting in green surface tarnish. The corrosion is from oxidation -- a chemical reaction between the metal and oxygen, water and carbon dioxide in the air. Rust is the term used to describe this process when it happens to iron instead of other metals.

Why would a penny rust?

The copper in a penny, whether it forms the bulk of the coin or merely a surface layer, turns dull when exposed to the air. The reason is that copper atoms combine with oxygen molecules to form copper oxide, in a chemical process called oxidation. ... When oxidation occurs with iron, the result is called rust.