The electrochemistry of rust formation
Iron is one of the most important raw materials for industry. When this metal corrodes, it is colloquially referred to as ‘rusting’. The process creates a mix of multiple compounds, including iron oxides, iron hydroxides and hydrated iron oxides.
This transformation takes place as a multi-stage electrochemical process in the presence of water and oxygen in a neutral environment. First iron gets oxidised and oxygen gets reduced in the water. As a short-lived intermediate product iron(III) hydroxide is created.
2 Fe + O2 + 2 H2O → 2 Fe(OH)2
Iron hydroxide contains bivalent iron (Fe2+) and is not yet ‘rust’ in the conventional sense. The actual rust is created as a result of further oxidation and dehydration reactions.
4 Fe(OH)2 + O2 + 2 H2O → 4 Fe(OH)3
Fe(OH)3 → FeO(OH) + H2O
2 FeO(OH) + O2 → Fe2O3 + H2O
The corrosion products formed are insoluble in water. They are precipitated and form the characteristic porous layer of rust on the metal.