For iron rust to occur the metal has to be in contact with oxygen and water. The chemical reactions for this process are relatively complex and not all of them are completely understood. It is believed the causes are the following:
Electron transfer (reduction-oxidation)
:One area on the surface of the metal acts as the anode, which is where the oxidation (corrosion) occurs. At the anode, the metal gives up electrons.
::Fe → Fe + 2 e
:Electrons are transferred from iron, reducing oxygen in the atmosphere into water on the cathode, which is placed in another region of the metal.
::O + 4 H + 4 e → 2 HO
:Global reaction for the process:
::2 Fe + O + 4 H → 2 Fe + 2 HO
:Standard emf for iron rusting:
::E° = E° (cathode) − E° (anode)
::E° = 1.23V − (−0.44 V) = 1.67 V
Iron corrosion takes place in an acid medium; H ions come from reaction between carbon dioxide in the atmosphere and water, forming carbonic acid. Fe ions oxidize further, following this equation:
: 4 Fe + O + (4+2) HO → 2 FeO·HO + 8 H
Iron(III) oxide hydrate is known as rust. The concentration of water associated with iron oxide varies, thus the chemical formula is represented by FeO·HO.
An electric circuit is formed as passage of electrons and ions occurs; thus if an electrolyte is present it will facilitate oxidation, explaining why rusting is quicker in salt water.