Rusting
The corrosion of iron in the presence of both oxygen and water to form hydrated iron(III) oxide.
Formula / equation
4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O
Unit
Dimensionless
In depth
Iron is oxidised at anodic regions while oxygen is reduced at cathodic regions, with water completing the circuit. Because rust is flaky and porous rather than adherent, it offers no protection and corrosion continues into the metal until the object is destroyed.
Examples in the real world
A car body corroding fastest where road salt collects; iron in dry air or in boiled deoxygenated water does not rust.