Many metals are chemically active elements and easily react with substances like water , air ( oxygen ), acids, etc. Rusting is slow chemical reaction between iron and oxygen . We have observed iron nail that are shiny when it was new, but get coated with a reddish brown powder when left for some time ( in presence of moisture it is faster) . This process is commonly known as rusting of iron. This reaction is called oxidation reaction. That is to say Iron reacts with air (oxygen ) in presence of moisture ,it forms chemical compound ferrous oxide. In absence of moisture ,reaction is slow but rusting will take place.