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Mechanisms of
Microbiologically Induced Corrosion (MIC)
MIC is a common problem in
industrial processes due to the presence of microbes, adequate
nutrients and corrosive byproducts. The following table indicates
typical mechanisms and manifestations of MIC in industrial
environment.
Table from: D. Pope and E. Morris, materials
Performance, Vol. 34, No. 5, NACE International, May (1995) p24.
TABLE 1 - Mechanisms
Potentially Involved in Cases of MIC
Cathodic depolarization
- The classic mechanism for MIC of steel and
iron proposed by von Wolzgen Kuhr in 1934
- This mechanism is based on the idea that the
rate-limiting step in corrosion is the dissociation of hydrogen from
the cathodic site.
- It is thought that sulfate-reducing bacteria (SRB)
consume hydrogen through the action of their hydrogenase enzymes,
and thus “depolarize” the cathode, accelerating corrosion.
- Some investigators still believe that this
mechanism is the important one for MIC of iron and steels, despite
the fact that numerous experiments using SRB in pure culture gave
corrosion rates far less than those seen at field sites and less
than those measured in experiments using MIC communities.
Formation of occluded area on metal
surface
- This mechanism is based on the observation
that when microorganisms form colonies on the surface of a metal,
they do not form uniform layers, but rather, local “community
centers.”
- The sites chosen for initial colonization may
be related to such metallurgical features as roughness, preexisting
corrosion sites, inclusions, or surface charge.
- Once the colony has formed, it produces sticky
polymers which tend to attract and aggregate other biological and
nonbiological (metals and chloride, for example) species to the
colonization sites.
- This, in addition to the metabolism of
available oxygen, iron, manganese, etc., results in conditions
within and under the colonies very different from those on the
surrounding metal.
- This leads to the formation of crevices and
oxygen and ion concentration cells, allowing corrosion to proceed.
Fixing the anodic sites
- This parallels the development of the occluded
cell. The presence and activities of the microbes creates a
condition under t he colony in which incipient pitting leads to
pitting driven principally by microbiological activities.
- This is made possible by the fact that most of
the microbiological community usually remains fixed to the
colonization site (although progeny may find other colonization
sites).
- This causes the anodic site to become “fixed.”
This is a principal reason for the fact that more than 90% of MIC is
seen as pitting-type corrosion
Underdeposit acid attack
- Most of the final products of MIC community
metabolism are short-chain fatty acids (acetic acid is the most
common).
- Acetic acid is very aggressive to carbon steel
when concentrated under a colony or other deposit.
- This is the case both at field sites and in
the laboratory.
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