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Rust formation on iron

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The Corrosion College

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.