Very few metals are found naturally in their elemental form. Instead, most metals react with other elements to form compounds. We have to extract and refine these metals to produce the useful elemental forms that we recognise and exhibit the properties we value. Because these metals are so reactive, as soon as they are exposed to the atmosphere they will begin to revert to their natural state as a compound with other elements.
Most of these compounds are oxides. Iron for example exists as sixteen known iron oxides and oxyhydroxides. The most well known of these iron oxides, iron (III) oxide, is more commonly known as rust.
One example for rusting is given by the chemical equation :
4Fe + 3O2 → 2Fe2O3
There are different forms of rust, such as the result of the reaction of iron and chlorine in an environment deprived of oxygen, such as rebar used in underwater concrete pillars, which generates green rust.
Iron corroding in water gives rust. Rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3).
Iron is not the only material subject to attack from the environment. Copper will react with oxygen in the atmosphere to form various oxides such as cupric oxide (Copper(II) oxide) and cuprous oxide (Copper(I) oxide). Cupric oxide is a black substance, whereas cuprous oxide is red in colour. In the presence of water, particularly seawater, copper can combine with chrlorine to produce Copper(II) chloride, commonly known as the green/blue compound verdigris.