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Microbiologically influenced corrosion: what it means for the inspector

Long Seam editorial teamSources checked 3 min read

Microbiologically influenced corrosion (MIC) is corrosion that starts or speeds up because of microorganisms living on a metal surface. Colonies and the deposits they build create small local environments that corrode the metal beneath them. The usual result is pitting under mounds of deposit, in water that is stagnant or slow-moving.

Cross-section of a steel surface under stagnant water, with deposit mounds containing microbes sitting on the surface and a small deep pit in the steel directly beneath each mound, clean steel between the mounds
Pits form directly under the deposit mounds. The steel between them can look clean.

How it works

Microorganisms do not eat steel. What they do is change the conditions at the surface. A colony settles, builds a film or mound of deposit, and the chemistry underneath becomes different from the bulk water. Oxygen is used up, acids or other corrosive byproducts form, and a small separate corrosion cell develops. The result is a local attack under each colony, often deep and narrow.

Several conditions favour it. Water that is stagnant or moving slowly gives microbes time to settle. Deposits, sludge and tubercles give them shelter. Equipment that is hydrotested and left wet, or idle lines with trapped water, often sets up exactly these conditions. Water temperature and nutrient content also matter, because the microbes need conditions they can live in.

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Where it shows and how it is found

MIC is associated with water-handling systems and with places where water collects.

  • Cooling water systems, fire water lines and tanks.
  • Low points, dead legs and idle equipment where water sits.
  • Equipment that was hydrotested and not drained and dried promptly.

Typical signs are mounds or tubercles on the surface, slimy or crusty deposit, and pits with a distinctive shape: often a wide mouth or cup-shaped pit under the deposit, in otherwise sound metal. Inspectors clean the surface and look at the metal under the deposit, then measure pit depth with a gauge. UT readings at random spots can miss it, because the attack is so local. Chemical and biological testing of the water or the deposit can support the diagnosis, but a visual look under the deposit is often the first clue.

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Mix-ups to avoid

  • MIC versus general corrosion. MIC is local and tied to colonies. General corrosion is broad.
  • MIC versus condensate corrosion. Condensate corrosion is driven by dissolved gases in steam condensate. MIC is driven by deposits and organisms in water that is not necessarily acidic.
  • MIC versus under-deposit corrosion in general. Not every pit under a deposit is microbial. Confirming needs deposit and water analysis, not just a look.

On the API exam

Microbiologically influenced corrosion is not among the 15 mechanism paragraphs on API's effectivity sheet for September 2026 to May 2027. API 510 questions on RP 571 come only from the listed portions, so use this page as background and for field practice. It is still worth knowing the pattern, because stagnant water, deposits and pitting under deposit come up in inspection work well beyond the exam. Check the sheet for your own window.

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Practise turning thickness readings into a rate with the thickness survey analyzer. Compare it with General vs localized corrosion and Boiler water and condensate corrosion, or browse every term in the damage mechanism glossary.

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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