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=Bacterial Anaerobic Corrosion=

Bacterial Anaerobic Corrosion is the bacterially-induced oxidation of metals. Corrosion of metals typically alters the metal to a form that is more stable. Thus, Bacterial Anaerobic Corrosion typically occurs in conditions favorable to the corrosion of the underlying substrate. In humid, anoxic conditions the corrosion of metals occurs as a result of a redox reaction. This redox reaction generates molecular hydrogen from local hydrogen ions. Conversely, anaerobic corrosion occurs spontaneously. Anaerobic corrosion primarily occurs on metallic substrates but may also occur on concrete.



Bacterial Anaerobic Corrosion Explained
Bacterial Anaerobic Corrosion typically impacts metallic substrates but may also occur in concrete. Corrosion of concrete mediums leads to considerable losses in industrial settings. When considering the corrosion of concrete there is significant documentation of structural degradation in concrete wastewater infrastructure where wastewater is collected or treated. Similarly, there is a focus towards biofilms in piping exposed to wastewater when considering bacterial anaerobic corrosion of metals.

For bacterial anaerobic corrosion there is general corrosion of substrates as well as another form of corrosion known as pitting. An underlying commonality of either general or pitting corrosion is that the process of breakdown occurs within aqueous conditions. Bacteria tend to form biofilms as their primary means of corroding metals and these bacteria vary drastically across different settings. In municipal wastewater we find that Desulfovibrio desulfuricans is the main contributor to corrosion.

Chemistry of Bacterial Anaerobic Corrosion
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:


 * Biofilm components in streams.jpg + 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 (SRB), 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. Another reaction occurs between iron and water 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 SRB can, in this way, be far more harmful than anaerobic corrosion.

Biofilms and Bacterial Anaerobic Corrosion
There is varying impact on local corrosion noted from biofilms formed of diverse microbial communities. For instance, when isolating a sample of biofilm from a pipe within the first week of growth the corrosion of said pipe accelerated yet by the end of a month the same biofilm began to act as a protective layer for the pipe. Thus, we anticipate variation between corrosion in similar environments and that variation might be attributed to the local bacterial communities. Even with this variation of local biota, the significance of biofilms in mediating corrosion is in how said biofilms alter the electrochemical processes at the interface of the underlying substrate.