MIC: when bacteria eat your equipment

What is MIC?

Microbiologically influenced corrosion (MIC) is corrosion initiated or accelerated by microorganisms — bacteria, archaea, fungi — living in biofilms on metal surfaces. The microbes do not "eat" metal in any literal sense: they create and sustain local chemical conditions (acids, sulfides, oxygen differentials, depolarized reactions) under which ordinary electrochemical corrosion runs far faster and far more locally than the bulk environment would ever suggest. The result is a familiar and unwelcome signature: deep, localized perforation of equipment whose water chemistry looked harmless.

The mechanism: the biofilm is the weapon

Free-floating bacteria are largely irrelevant to corrosion. The damage begins when they settle and build a biofilm — a slime matrix that glues a community of organisms to the surface. Under that film:

1. The environment separates from the bulk. The biofilm blocks oxygen and traps metabolic products. Underneath it, pH, oxygen and chemistry are set by the colony, not by your water treatment.
2. Differential cells form. Oxygen-rich zones outside versus oxygen-starved zones under the film create concentration cells — the covered spot becomes the anode. This alone is classic under-deposit attack; the biofilm makes it self-sustaining.
3. Specialist organisms accelerate specific reactions. The best-known culprits, sulfate-reducing bacteria (SRB), thrive in the oxygen-free zone under the film and generate hydrogen sulfide, producing corrosive sulfides and characteristic black deposits. Acid-producing bacteria drop the local pH; iron- and manganese-oxidizing bacteria build tubercles that worsen the occlusion.
4. Localization does the rest. As with pitting, a small aggressive anode served by a large surface concentrates the attack — MIC failures are typically deep, cup-shaped or terraced pits under discrete deposits, perforating walls at rates that can reach millimetres per year while average loss stays trivial.

Where MIC strikes

The pattern across industries is consistent — MIC loves water that sits:

Temperature in the mesophilic range (roughly ambient to ~50 °C) suits most culprit organisms, which is why cooling and firewater systems sit in the bullseye.

Diagnosing MIC — and not over-diagnosing it

MIC is both under-recognized and over-blamed: any odd perforation risks being labeled "bacteria" without evidence. A defensible diagnosis triangulates three lines of evidence — morphology (localized, deep pits under deposits or tubercles, sometimes with characteristic sub-surface cavities), microbiology (culture or molecular testing of deposits sampled at the failure, not from bulk water alone), and chemistry/operations (stagnation history, sulfides or black deposits, nutrient sources, absence of a better mechanical or chemical explanation). Any single line alone is suggestive, not conclusive.

What actually controls MIC

• Deny the stagnation. Drain and dry hydrotest and firewater when possible, eliminate dead legs, keep flow above settlement velocities, and clean sludge — a biofilm that cannot establish cannot corrode.
• Mechanical cleaning first. Pigging and cleaning remove the biofilm and deposits; biocides reach what cleaning has exposed. Chemical treatment through an established biofilm mostly kills the top layer of a protected colony.
• Biocide programs with monitoring. Effective when matched to the system and verified (deposit sampling, coupons, bioactivity monitoring) — not as a set-and-forget chemical feed.
• Barrier linings. For chronically vulnerable surfaces — tank bottoms, water boxes, firewater headers — a properly applied internal lining removes the metal-electrolyte-biofilm contact entirely and is often the only durable answer in systems that cannot be kept clean and dry.
• Repair with the mechanism arrested. MIC perforations and deep pits repair well with engineered cold-applied composites and resurfacing — but repairing without cleaning, treating or lining hands the colony a fresh surface. Fix the biology and the geometry together.

FAQ

Can MIC occur in clean, treated water systems? Risk is much lower but not zero — treatment lapses, dead legs and deposits create protected niches. The systems that fail are almost always the ones with a stagnation story.

How fast can MIC perforate a pipe? Localized rates far exceeding general corrosion are well documented — perforation of carbon steel in a few years, sometimes less in ideal (for the bacteria) conditions. The point is not a universal number but the localization: the wall fails at spots while averages look fine.

Does stainless steel resist MIC? Not reliably. Stainless suffers MIC too, particularly under deposits and at welds — organisms that concentrate chlorides or produce oxidants can defeat the passive film, and firewater systems in stainless have famous MIC failure histories.

Is black slime in my system proof of MIC? Black sulfide deposits and rotten-egg odor point to SRB activity — a strong indicator, not yet proof of corrosion causation. Sample the deposit at a damage site and combine with morphology and history before writing the failure report.

Will a biocide alone solve an established MIC problem? Rarely. Established biofilms shield their communities; cleaning first, then treating, then verifying is the sequence that works — with lining or drainage changes where the system invites recolonization.

MIC damage in your tanks or piping?

Induscoat Canada repairs MIC-perforated and pitted equipment with engineered composite systems and applies internal linings that take the metal out of the bacteria's reach — tank bottoms, water systems, exchangers. Send inspection findings and photos through our request a quote page; we respond within 24 business hours.