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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 H2 S, thus introducing hydrogen sulfide into the system. SRB colonies can also form deposits that are conducive to under-deposit corrosion (crevice corrosion.)


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.


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


Pictures - Click on thumbnail to see larger picture

Bacterial attack is usually characterized by rounded pits with etched sides, edges, and bottoms.

MIC pits often have a terraced effect.

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.

One of the quick texts for SRB is the pipe cleaner test. Positive results are shown in these examples.

Petroleum Recovery Research Center, Socorro, NM-87801