Rust

Rust is a reddish metal decay that occurs in iron.

Rust is a corrosion, or metal decay, in iron or iron alloys when they are exposed to oxygen such as in water and in the air. The exposure to oxygen oxidizes the iron to create iron oxide which turns the iron reddish or brownish in color. As the iron oxidizes, it becomes weaker.

Some materials are added to iron, such as cobalt to make stainless steel, to slow down the oxidation, but it will not prevent it. Other approaches are to cover the iron with zinc to make galvanized steel, or with paint, plastic, oil, grease, or other material, to block the exposure.