Explanation

Rust is a mixture of oxides of iron formed on the surface of iron.

It is a result of a redox reaction between Oxygen and Iron in the presence of water/moisture.

Oxygen being an excellent oxidizing agent first reacts with Iron to oxidize it to a +2 or +3 state

This oxidized iron is highly reactive and thus combines with water/moisture to form hydroxides which on dehydration give oxides, thus forming a reddish-brown deposit of rust.

The formula for rust is Fe₂O₃.nH₂O as exact value of water present in the rust is variable.