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'''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):''
:<math>Fe\;\rightarrow\;Fe^{2+}\;+\;2\,e^-</math>

''Reduction reaction in acid solution:''
:<math>2\,H^+\;+\;2\,e^-\rightarrow\;H_2\;</math>

In an acidic solution, the water molecules are [[protonated]] and the [[hydronium]] ions (H<sub>3</sub>O<sup>+</sup>) are directly reduced into H<sub>2</sub>.

''Reduction reaction in neutral or slightly alkaline solution:''
:<math>2\,H_2O\;+\;2\,e^-\;\rightarrow\;H_2\;+\;2\,OH^-</math>

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)<sub>2</sub>).

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

:<math>Fe\;+\;2\,H_{2}O\;\rightarrow\;Fe(OH)_2\;+\;H_2\;</math>

==Transformation of ferrous hydroxide into magnetite==
Under anaerobic conditions, the [[ferrous hydroxide]] (Fe(OH)<sub>2</sub> ) can be oxidized by the [[proton]]s of water to form [[magnetite]] and molecular [[hydrogen]].
This process is described by the [[Schikorr reaction]]:

:''3 Fe(OH)<sub>2</sub> → Fe<sub>3</sub>O<sub>4</sub> + H<sub>2</sub> + 2 H<sub>2</sub>O''
:''ferrous hydroxide → magnetite + hydrogen + water''

The well crystallized magnetite (Fe<sub>3</sub>O<sub>4</sub>) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)<sub>2</sub> ).

This process also occurs during the [[wikt:anaerobic|anaerobic]] corrosion of [[iron]] and [[steel]] in [[Anoxic waters|oxygen-free]] [[groundwater]] and in reducing [[soil]]s below the [[water table]].

==See also==
*[[Hydrogen]]
*[[Redox reaction]]
*[[Electrochemistry]]

{{DEFAULTSORT:Anaerobic Corrosion}}
[[Category:Corrosion]]
[[Category:Hydrogen production]]