# Anaerobic corrosion

Hydrogen corrosion is a form of metal corrosion occurring in the presence of anoxic water. Hydrogen corrosion involves a redox reaction that reduces hydrogen ions, forming molecular hydrogen.

Metals enter aqueous solution and are oxidized.

Oxidation reaction (pH independent):

$Fe\;\rightarrow\;Fe^{2+}\;+\;2\,e^-$

Reduction reaction in acid solution:

$2\,H^+\;+\;2\,e^-\rightarrow\;H_2\;$

In an acidic solution, the water molecules are protonated and the hydronium ions (H3O+) are directly reduced into H2.

Reduction reaction in neutral or slightly alkaline solution:

$2\,H_2O\;+\;2\,e^-\;\rightarrow\;H_2\;+\;2\,OH^-$

In a neutral or slightly alkaline solution, the protons of water are reduced into molecular hydrogen giving rise to the production of hydroxide ions responsible of the precipitation of the slightly soluble ferrous hydroxide (Fe(OH)2).

This finally leads to the global reaction of the anaerobic corrosion of iron in water:

$Fe\;+\;2\,H_{2}O\;\rightarrow\;Fe(OH)_2\;+\;H_2\;$

## Transformation of ferrous hydroxide into magnetite

Under anaerobic conditions, the ferrous hydroxide (Fe(OH)2 ) can be oxidized by the protons of water to form magnetite and molecular hydrogen. This process is described by the Schikorr reaction:

3 Fe(OH)2 → Fe3O4 + H2 + 2 H2O
ferrous hydroxide → magnetite + hydrogen + water

The well crystallized magnetite (Fe3O4) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)2 ).

This process also occurs during the anaerobic corrosion of iron and steel in oxygen-free groundwater and in reducing soils below the water table.