This article is about a type of chemical corrosion. For the fungus, see rust (fungus). For the person, see Mathias Rust. For the town in Austria, see Rust, Austria.
Rust is the substance formed when iron compounds corrode in the presence of oxygen and water. It is a mixture of iron oxides and hydroxides. Rusting is a common term for corrosion, and usually corrosion of steel.
Iron is found naturally in the ore haematite as iron oxide, and metallic iron tends to return to a similar state when exposed to air, (hydrogen, oxygen, nitrogen, etc.) and water. This corrosion is due to the oxidation reaction when iron metal returns to an energetically favourable state. Energy is given off when rust forms. The process of rusting can be summarized as three basic stages: The formation of iron(II) ions from the metal; the formation of hydroxide ions; and their reaction together, with the addition of oxygen, to create rust.
Iron is the main component of steel and the corrosion of steel is observed more frequently, since iron is rarely used without alloying in the present day.
When steel contacts water, an electrochemical process starts. On the surface of the metal, iron is oxidized to iron(II): Fe → Fe2+ + 2e−
The electrons released travel to the edges of the water droplet, where there is plenty of dissolved oxygen. They reduce the oxygen and water to hydroxide ions: 4e− + O2 + 2H2O → 4−OH