Microbially Influenced Corrosion
Microbe Information
The mechanisms commonly thought to be involved in MIC include:
- Cathodic depolarization, whereby the cathodic rate limiting step is accelerated
by micro-biological action.
- Formation of occluded surface cells, whereby microorganisms form "patchy" surface
colonies. Sticky polymers attract and aggregate biological and non-biological
species to produce crevices and concentration cells, the basis for accelerated
attack.
- Fixing of anodic reaction sites, whereby microbiological surface colonies
lead to the formation of corrosion pits, driven by microbial activity and
associated with the location of these colonies.
- Underdeposit acid attack, whereby corrosive attack is accelerated by acidic
final products of the MIC "community metabolism", principally short-chain
fatty acids.
Microbes fall into two basic groups, aerobic and anaerobic. These two groups
are based on the kind of environment they prefer, either with or without oxygen.
Slime formers form a diverse group of aerobic bacteria. Common anaerobic bacteria
include Sulfur/sulfate reducing bacteria (SRB's) and organic acid formers.
Microbes tend to form colonies, with different characteristics from the outside
to inside. On the outside, "slimers" may produce polymers (slime) that attract
inorganic material, making the colony look like a pile of mud and debris. These
aerobic organisms can efficiently use up all available oxygen, giving anaerobic
microbes (SRB's) inside the colony a hospitable environment, allowing enhanced
corrosion under the colony.
Microbially influenced corrosion (MIC) is a special danger when steels or
alloys of aluminum and copper are in constant contact with nearly neutral water,
of pH 4 to 9, 50° to 122°F (10° to 50°C), especially when
stagnant. Microbially influenced corrosion mostly takes the form of pitting
corrosion.
Corrosion products and effects include iron sulfates, slime, plugging, and
bacteria growths. Sulfate-reducing bacteria (SRB) are anaerobic bacteria which
metabolize sulfates (SO42-) and produce sulfuric acids
or H2S, thus introducing hydrogen sulfide into the system. SRB colonies
can also form deposits that are conducive to under-deposit corrosion (crevice
corrosion.)
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Where Found
Water storage tanks are a common site where MIC occurs. SRB's can contaminate
tanks, which must then be cleaned and sterilized because it is impossible for
biocides to penetrate the large amounts of sludge and debris in tank bottoms.
Flow lines are another common MIC site, especially at the bottom of the line
where water accumulates. MIC has also been detected at the 3 o'clock and 9
o'clock positions, presumably at the oil and water interface.
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Prevention / Mitigation
To reduce or prevent microbial corrosion:
Drilling - biocides
Production - biocides, chlorine dioxide
Flowlines - biocides, chlorine dioxide
Cost considerations - Continuous vs. batch; EPA; biostat vs. biocide
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Pictures - Click on thumbnail to see larger picture
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Bacterial attack is usually characterized by rounded pits with etched
sides, edges, and bottoms. |
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MIC pits often have a terraced effect. |
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Although MIC normally occurs at the bottom of the line where water accumulates,
it has also been detected at the 3 o'clock and 9 o'clock positions, presumably
at the oil and water interface. |
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One of the quick texts for SRB is the pipe cleaner test. Positive results
are shown in these examples. |
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