Rust is a general term for a series of iron oxides, usually red oxides, formed by the reaction of iron with oxygen in the presence of water or air moisture. Several forms of rust are distinguishable visually and by spectroscopy, and form under different circumstances. Rust consists of hydrated iron(III) oxides Fe2O3·nH2O, iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3. Rusting is the common term for corrosion of iron and its alloys, such as steel. Other metals undergo equivalent corrosion, but the resulting oxides are not commonly called rust. Given sufficient time, oxygen, and water, any iron mass eventually converts entirely to rust and disintegrates. The corrosion of aluminium is extremely slow because the resulting aluminium oxide forms a conformal coating, which protects the remaining aluminium. This process is known as passivation.

When in contact with water and oxygen, or other strong oxidant or acids, iron will rust. If salt is present, for example, in salt water, the metal rusts more quickly. This chemical reaction is used in the production of handwarmers Iron metal is relatively unaffected by pure water or by dry oxygen. As with other metals, a tightly adhering oxide coating, a passivation layer, protects the bulk iron from further oxidation. Thus, the conversion of the passivating iron oxide layer to rust results from the combined action of two agents, usually oxygen and water. Other degrading solutions are sulfur dioxide in water and carbon dioxide in water. Under these corrosive conditions, iron(III) species are formed. Unlike iron(II) oxides, iron(III) oxides are not passivating because these materials do not adhere to the bulk metal. As these iron(III) compounds form and flake off from the surface, fresh iron is exposed, and the corrosion process continues until all of the iron(0) is either consumed or all of the oxygen, water, carbon dioxide, or sulfur dioxide in the system are removed or consumed.

The rusting of iron is an electrochemical process that begins with the transfer of electrons from iron to oxygen. The rate of corrosion is affected by water and accelerated by electrolytes, as illustrated by the effects of road salt (calcium chloride) on the corrosion of automobiles. The key reaction is the reduction of oxygen.

Rust is permeable to air and water, therefore the interior iron continues to corrode. Rust prevention thus requires coatings that preclude rust formation. Stainless steel forms a passivation layer of chromium(III) oxide. Similar passivation behavior occurs with magnesium, copper, titanium, zinc, aluminium.

Rust is associated with degradation of iron-based tools and structures. As rust has a much higher volume than the originating mass of iron, its build-up can also cause failure by forcing apart adjacent parts — a phenomenon sometimes known as "rust smacking". It was the cause of the collapse of the Mianus river bridge in 1983, when the bearings rusted internally and pushed one corner of the road slab off its support. Three drivers on the roadway at the time died as the slab fell into the river below. The following National Transportation Safety Board (NTSB) investigation showed that a drain in the road had been blocked for road re-surfacing, and had not been unblocked so that runoff water penetrated the support hangers. It was also difficult for maintenance engineers to see the bearings from the inspection walkway. Rust was also an important factor in the Silver Bridge disaster of 1967 in West Virginia, when a steel suspension bridge collapsed in less than a minute, killing 46 drivers and passengers on the bridge at the time.

Kinzua Bridge in Pennsylvania was blown down by a tornado in 2003 largely because the central base bolts holding the structure to the ground had rusted away, leaving the bridge resting by gravity alone.

Similarly corrosion of concrete-covered steel and iron can cause the concrete to spall, creating severe structural problems. It is one of the most common failure modes of reinforced concrete bridges.

For More Information: Corrosion & Rust K-12 Experiments & Background Information

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