Why does iron rust?

Rust is the reddish-brown iron oxide that forms when iron or steel undergoes oxidation in the presence of oxygen and water. The reaction is an electrochemical process, so tiny surface differences can create microscopic corrosion cells. Salt water speeds rusting because dissolved ions make it easier for electrons to move, which is why ships and coastal structures corrode faster. Rusting has been a major engineering problem since iron became widely used, and protective coatings, galvanizing and stainless steel were developed to reduce it.