What is microbiologically influenced corrosion (MIC)?

When does MIC happen?

Microbes do not add a new kind of chemistry. A 2024 review describes MIC as microorganisms accelerating the two halves of ordinary corrosion: metal dissolving at the anode and the reaction at the cathode ⁵. API RP 571, third edition (2020), gives it its own section, 3.45, among the damage mechanisms of fixed equipment ¹. It takes hold where four conditions meet:

• Water that stands or moves slowly. MIC typically forms pits under biofilms and deposits, especially in stagnant water with no oxygen ⁷; inside pipelines it is more common in liquid lines than in gas lines ³.
• Food and something to breathe. Produced oil and water supply carbon, while the steel and the seawater injected for secondary recovery supply iron and sulfate; temperatures of 15 to 35 °C in oil-handling facilities suit microbial growth ⁶.
• Low oxygen near the metal. Sulfate-reducing bacteria (SRB) work without oxygen, and bacteria that use oxygen higher up in a line can consume it and so favour SRB below them ⁵.
• Wet soil and failed coating, on the outside. External MIC needs nutrients, which bogs, landfills and refuse sites provide ³; in incident data it is most strongly linked to clay soils and tape coatings, though it has occurred with many soils and coatings ⁴.

Which microbes cause MIC?

No single species explains MIC. Corrosive biofilms are communities whose members work together, and one group can switch between several reactions ⁶. The groups below are the ones the sources name most often:

Group	Oxygen	What it does to steel	Clue in the field or laboratory
Sulfate-reducing bacteria (SRB)	Works without it	Turns sulfate into sulfide and can draw electrons straight from the metal, which favours pits	Sulfur or iron sulfide in the deposits; a rotten-egg smell in an excavation
Acid-producing bacteria (APB)	Not specified; many are fermenters	Produce acids under the biofilm	Acid in the biofilm, low pH in the fluid
Sulfur-oxidizing and iron-oxidizing bacteria	Need it	Consume oxygen and make acids that speed metal oxidation	Found where oxygen reaches the surface; can let SRB thrive beneath
Methanogens and other archaea	Work without it	Found with sulfate reducers in corrosive offshore biofilms	Seen only with molecular methods such as quantitative PCR or gene sequencing

The table draws on the 2024 review, the PHMSA report of 2008 and the 2016 offshore study ^{5 3 6}.

Field rates can far exceed laboratory ones. In 2014, a 10.9 mm steel pipe carrying oily seawater at 25 °C on an offshore facility in the Gulf of Mexico developed a pinhole leak after two years, about 5.45 mm/y; the highest rate the authors found reported for a single bacterial strain in the laboratory was 0.88 mm/y ⁶.

How common is MIC in pipeline incidents?

US incident data give a sense of scale. A 2019 presentation by a specialist at Canada's National Energy Board, now the Canada Energy Regulator, broke down incidents on US hazardous liquid pipelines reported to the Pipeline and Hazardous Materials Safety Administration (PHMSA) from 2010 to early 2019 ⁴:

Incident cause	Incidents	With MIC	Share with MIC
Internal corrosion	439	192	44%
External corrosion	278	38	14%
All causes	3,549	230	6.5%

Counts are from the presentation; the shares for internal and external corrosion are calculated from them ⁴. For 2002 to 2009, the same source puts MIC at 4.8 percent of all hazardous liquid incidents, and it cautions that MIC can go unrecognized or unreported ⁴. The presenter's views were stated not to be the board's ⁴.

How is MIC found?

For the inside of pipelines, AMPP keeps a dedicated test method, NACE TM0212-2018, Detection, Testing, and Evaluation of Microbiologically Influenced Corrosion on Internal Surfaces of Pipelines ². The guiding rule, quoted from NACE TM0106-2016 for buried pipe, is that MIC is concluded from the weight of circumstantial evidence rather than from one test ⁴.

Inside a line, the biofilm itself is the best sample. Its decay products point to the bacteria at work: sulfur suggests SRB, and acid or low pH suggests APB ³. Research laboratories add scanning electron microscopy, energy-dispersive spectroscopy and microsensors that read pH, hydrogen sulfide and oxygen at different depths of a biofilm ⁵. DNA methods, such as quantitative PCR and high-throughput sequencing, show which microbes are present in a biofilm ⁶.

Outside a buried line, quick kits and laboratory tests check soil samples for bacterial activity ³. Investigators also gather coating type and condition, soil and water table data, oxygen conditions at pipe depth, above-ground coating surveys and in-line inspection results ⁴.

How is MIC controlled?

Inside pipelines, the PHMSA report lists biocides injected into the flow, cleaning pigs that remove deposits, and, for gas lines, drying the gas and removing places where liquid can collect ³. The 2024 review adds protective and antibacterial coatings, and notes that bacteria can become resistant after long use of the same chemical treatment ⁵.

On the outside, cathodic protection needs a stricter target where MIC is involved: about -950 mV against a copper/copper sulfate electrode, or a 300 mV or greater shift, in place of the usual -850 mV or 100 mV shift ³.

Is MIC the same as pitting corrosion?

No. Pitting is a form, what the damage looks like; MIC is a cause, and it usually leaves pits under biofilms and deposits ⁷. Many pits have nothing to do with microbes: stainless steel pits in chloride water with an oxidizer present, no biofilm needed ⁷. Telling the two apart is the point of the evidence-based diagnosis above. See pitting corrosion and what is corrosion?.