Rust and stainless steel. Rust being chipped from an iron object (nut and bolt at left) with the tip of a stainless steel screwdriver (right), which is free of rust. Rust is hydrated iron oxide. It forms in a reaction between iron and atmospheric water vapor and oxygen. This corrodes and weakens the original iron structure. Stainless steel is an alloy that contains iron, but also contains over 10 percent chromium. This makes it resistant to ordinary corrosion.