rust (chemistry)

Reddish-brown oxide of iron formed by the action of moisture and oxygen on the metal. It consists mainly of hydrated iron(III) oxide (Fe₂O₃.H₂O) and iron(III) hydroxide (Fe(OH)₃). Rusting is the commonest form of corrosion.

Rust prevention
There are two main approaches to protect against rusting. Barrier methods introduce a barrier between the metal and the air and moisture to minimize the reaction. This is the commonest method of rust prevention and the barrier may consist of a layer of grease, paint, plastic, or an unreactive metal, such as tin, copper, or chromium.

In sacrificial protection, the iron is actually covered by a more reactive metal, such a zinc (galvanization), or connected to a more reactive metal, such as magnesium, so that as long as the ‘sacrificial’ metal is present it will corrode first.