<P> Given sufficient time, oxygen, and water, any iron mass will eventually convert entirely to rust and disintegrate . Surface rust is flaky and friable, and it provides no protection to the underlying iron, unlike the formation of patina on copper surfaces . Rusting is the common term for corrosion of iron and its alloys, such as steel . Many other metals undergo similar corrosion, but the resulting oxides are not commonly called rust . </P> <P> Other forms of rust exist, like the result of reactions between iron and chloride in an environment deprived of oxygen . Rebar used in underwater concrete pillars, which generates green rust, is an example . 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 aluminum . </P> <P> Rust is another name for iron oxide, which occurs when iron or an alloy that contains iron, like steel, is exposed to oxygen and moisture for a long period of time . Over time, the oxygen combines with the metal at an atomic level, forming a new compound called an oxide and weakening the bonds of the metal itself . Although some people refer to rust generally as "oxidation", that term is much more general; although rust forms when iron undergoes oxidation, not all oxidation forms rust . Only iron or alloys that contain iron can rust, but other metals can corrode in similar ways . </P> <P> The main catalyst for the rusting process is water . Iron or steel structures might appear to be solid, but water molecules can penetrate the microscopic pits and cracks in any exposed metal . The hydrogen atoms present in water molecules can combine with other elements to form acids, which will eventually cause more metal to be exposed . If chloride ions are present, as is the case with saltwater, the corrosion is likely to occur more quickly . Meanwhile, the oxygen atoms combine with metallic atoms to form the destructive oxide compound . As the atoms combine, they weaken the metal, making the structure brittle and crumbly . </P>

Rust on an iron nail element or compound