Bacterial anaerobic corrosion

Bacterial anaerobic corrosion is a bacterially-induced oxidation of metals.

Diagram of bacterially-induced corrosion.

In a humid environment and anoxic conditions the corrosion of metals occurs as a result of a redox reaction that generates molecular hydrogen from hydrogen ions, requiring bacteria, unlike anaerobic corrosion that occurs spontaneously.

A base metal, such as iron (Fe) goes into aqueous solution as positively charged cation, Fe²⁺. As the metal is oxidized under anaerobic condition by the protons of water, H⁺ ions are reduced to form molecular H₂. This can be written in the following ways under acidic and neutral conditions respectively:

Fe + 2 H⁺  →  Fe²⁺ + H₂

Fe + 2 H₂O  →  Fe(OH)₂ + H₂

Usually, a thin film of molecular hydrogen forms on the metal. Sulfate-reducing bacteria, oxidize the molecular hydrogen to produce hydrogen sulfide ions (HS⁻) and water:

4 H₂ + SO₄²⁻  →  HS⁻ + 3 H₂O + OH⁻

The iron ions partly precipitate to form iron (II) sulfide. A reaction with water also occurs, producing iron hydroxide.

Fe²⁺ + HS⁻  →  FeS + H⁺

3 Fe²⁺ + 6 H₂O  →  3 Fe(OH)₂ + 6 H⁺

The net equation comes to:

4 Fe + SO₄²⁻ + H⁺ + 3 H₂O  →  FeS + 3 Fe(OH)₂ + OH⁻

This form of corrosion by sulfate-reducing bacteria can, in this way, be far more harmful than anaerobic corrosion.

See also

• Corrosion
• Microbial corrosion

References

• Georg Fuchs (Hrsg.): Allgemeine Mikrobiologie, 8. Auflage. Thieme, Stuttgart 2006. ISBN 9783134446081.
• Holger Brill (Hrsg.): Mikrobielle Materialzerstörung und Materialschutz - Schädigungsmechanismen und Schutzmaßnahmen. Gustav Fischer Verlag Jena, Stuttgart 1995, ISBN 3-334-60940-5.