CHEMISTRY OF RUST

Many materials react with oxygen to form a chemical compound that is a combination of that material and oxygen. When iron combines with oxygen, it forms iron oxide, or rust. Iron oxide is a larger molecule than iron, so if iron oxidizes, it often puffs up and may even flake. This is because the rust requires more physical space than the original iron.

Some things cause steel or iron to rust faster than others. Water will cause iron and steel to rust. Dissimilar metals rust faster than single metals because of electrochemical reactions, so steel rusts faster than iron, and joints between dissimilar metals rust very quickly. Salt water will cause rust faster than water because salt water is a better electrical conductor. Like most chemical reactions, heat also speeds rust.