Microbially Induced Corrosion

November 15, 2016

Microbially induced corrosion (MIC) is a type of corrosion that is either directly or indirectly caused by microorganisms, usually chemoautotrophs, and there are very few situations in which MIC cannot occur.

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An example of bacterial corrosion Ferrobacillus ferrooxidans oxides iron to both iron oxides and iron hydroxides and is the cause of the iconic “rusticles” that have formed on the wreck of the RMS Titanic over the past century.

Microbially induced corrosion (MIC) is a type of corrosion that is either directly or indirectly caused by microorganisms, usually chemoautotrophs. Chemotrophs are bacteria that obtain their energy by oxidizing electron donors from their environment. These bacteria can either be autotrophic or heterotrophic. Chemoautotrophs create all of their organic compounds from carbon dioxide in addition to harnessing chemical reactions to create energy for themselves. In contrast, chemoheterotrophs cannot fix carbon to form organic compounds like chemoautotrophs.

Chemotrophic Bacteria

Some types of chemotrophic bacteria produce hydrogen sulfide by reducing sulfates. The hydrogen sulfide that is produced from this reaction can then cause sulfide stress cracking, which is a type of hydrogen embrittlement that occurs when both hydrogen sulfide and water are present. High strength steels are particularly vulnerable to sulfide stress cracking because the body centered cubic structure restricts the slip capabilities of the alloy. Similarly, bacteria in the genus Acidithiobacillus produce sulfuric acid and Acidothiobacillus thiooxidans are often found to be the cause of sewer pipe damage for this very reason.

Concentration Cells

Additionally, some bacteria consume the hydrogen formed during galvanic corrosion (which is a form of corrosion caused when two dissimilar metals are placed in contact in the presence of an electrolyte, causing one metal to corrode at the expense of the other metal) to survive. Therefore, bacterial colonies can grow and form concentration cells. These cells can both cause cathodic corrosion and speed up that process in cases where it already occurs.

Bacterial corrosion often appears in the form of pitting corrosion. For example, in cast iron, selective leaching can occur when bacteria consume only the iron and leave behind a matrix of graphite with a much lower mechanical strength. Specifically, Ferrobacillus ferrooxidans oxides iron to both iron oxides and iron hydroxides and this bacterial species is the cause of the iconic “rusticles” that have formed on the wreck of the RMS Titanic over the past century.

Both aerobic and anaerobic bacteria can cause difference types of corrosion. Because of that, there are very few situations in which MIC cannot occur. MIC can be reduced with the use of chemical corrosion inhibitors which alter the environment to the point where corrosive bacteria cannot survive. Therefore, it is important to not only know when MIC might be present, but also to choose the right inhibitor for any situation that may arise.

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