Corrosion. View of a corroding steel ring. The exterior of the ring has red patches of rust forming. Steel is an alloy of iron with a small percentage of carbon and other elements. The iron in the steel reacts with oxygen when water is present to form rust (hydrated iron oxide). Rust is bulkier, weaker and more brittle than the steel. It is also porous, allowing more oxygen and water to attack the underlying metal. Steel becomes resistant to rust if chromium, nickel and other elements are included in the alloy.