Many other oxidation reactions exist which do not involve iron or produce rust. The terms "rust" and "rusting" only mean oxidation of iron and its resulting products. The best-known of these reactions involve oxygen, hence the name "oxidation". Although rust may generally be termed as "oxidation", that term is much more general and describes a vast number of processes involving the loss of electrons or increased oxidation state, as part of a reaction. Over time, the oxygen combines with the metal forming new compounds collectively called rust. Rust is a general name for a complex of oxides and hydroxides of iron, which occur when iron or some alloys that contain iron are exposed to oxygen and moisture for a long period of time.
Rapid oxidation occurs when heated steel is exposed to air
Rust scale forming and flaking off from a steel bar heated to its forging temperature of 1200☌. Although rusting is generally a negative aspect of iron, a particular form of rusting, known as stable rust, causes the object to have a thin coating of rust over the top, and if kept in low relative humidity, makes the "stable" layer protective to the iron below, but not to the extent of other oxides such as aluminium oxide on aluminium. Rebar used in underwater concrete pillars, which generates green rust, is an example. Other forms of rust include the result of reactions between iron and chloride in an environment deprived of oxygen. Several forms of rust are distinguishable both visually and by spectroscopy, and form under different circumstances. Many other metals undergo similar corrosion, but the resulting oxides are not commonly called "rust". Rusting is the common term for corrosion of elemental iron and its alloys such as steel. Surface rust is commonly flaky and friable, and provides no passivational protection to the underlying iron, unlike the formation of patina on copper surfaces. Given sufficient time, any iron mass, in the presence of water and oxygen, could eventually convert entirely to rust.
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