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.

A rusty bolt on a bridge over a small stream

Rust is the substance formed when iron compounds corrode in the presence of water and oxygen. 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 hematite 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 summarised 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 nearly never used without alloying.

When steel contacts water, an electrochemical process starts. On the surface of the metal, iron is oxidised 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 -> 4OH^-

The hydroxide ions react with the iron(II) ions and more dissolved oxygen to form iron oxide. The hydration is variable, however in its most general form:

Fe²⁺ + 2OH^- -> Fe(OH)₂

4Fe(OH)₂ + O₂ -> 2(Fe₂O₃.xH₂O) + 2H₂O

Hence, rust is hydrated iron(III) oxide. Corrosion tends to progress faster in seawater than fresh water due to higher concentration of sodium chloride ions, making the solution more conductive. Rusting is also accelerated in the presence of acids, but inhibited by alkalis. Rust can often be removed through electrolysis, however the base metal object can not be restored through this method.

Corrosion of aluminum is different from steel or iron, in that aluminum oxide formed on the surface of aluminum metal forms a protective, corrosion resistant, coating.

Hydrated iron oxide is permeable to air and water, meaning that the metal continues to corrode after rust has formed. The iron mass eventually converts entirely to rust, and disintegrates. There are several methods available to control corrosion and prevent the formation of rust. Galvanising consists of coating metal with a thin layer of another metal, such as zinc. The electrochemical potential of zinc is more negative than steel (or iron) and will provide cathodic protection to the underlying steel. Typically, zinc is applied by either hot-dip galvanizing or electrogalvanizing. A nice thing about galvanising is that a scratch on a galvanised piece of iron will not lead to rust at the scratch. The zinc layer acts as a galvanic anode.

Cathodic protection is a method to control corrosion and the formation of rust using electrochemical techniques.

Corrosion control can be done using a coating to isolate the metal from the environment.

Covering steel with concrete provides protection to steel by the high pH environment at the steel-concrete interface. However, if concrete covered steel does corrode, the rust formed can cause the concrete to spall and fall apart, this will create structural problems.