Microbiologically Induced Corrosion (MIC) 2026 — Detection + Mitigation Decoded

MIC in tanks and piping: how to confirm it and how to find it

Short answer: microbiologically influenced corrosion (MIC) is corrosion that is started or accelerated by microorganisms living in a biofilm on the metal surface, most often sulfate-reducing bacteria (SRB) under stagnant water or deposits. In storage tanks it concentrates in the water and sludge layer on the floor and in sumps; in piping it concentrates in dead legs, low points, fire water and lines left full of hydrotest water. Confirming MIC needs microbiological sampling taken before cleaning destroys the evidence, combined with NDE that measures the pitting: floor MFL scanning with UT prove-up in tanks, and UT scanning or radiography in piping.

The sections below add the field detail that a damage-mechanism summary leaves out: the organisms involved, where MIC actually appears, why "bacteria present" does not prove MIC, the sampling sequence, which NDE methods measure MIC pitting and where each struggles, and what the owner's API 653 or API 570 inspector needs from the inspection. It is written for tank and terminal operators, refinery and chemical plant integrity teams, and fire protection system owners in the USA and Canada.

What MIC is, in practical terms

Microorganisms do not eat steel. They change the chemistry at the metal surface. A biofilm, a slimy layer of microbes and the material they secrete, forms on surfaces exposed to water with little or no flow. Under the biofilm, conditions can be very different from the bulk fluid: oxygen can be absent even when the fluid above contains oxygen, acids and sulfides can accumulate, and differences between covered and uncovered areas create local corrosion cells.

Several groups of organisms are associated with MIC. Sulfate-reducing bacteria are anaerobic, convert sulfate to sulfide and are linked to the majority of reported MIC failures in oil and gas systems. Acid-producing bacteria generate organic acids under the biofilm. Iron- and manganese-oxidising bacteria create deposits (tubercles) that shield the surface and set up concentration cells. Methanogens and other archaea are also implicated in some systems. Real systems usually contain a consortium of these rather than a single species, which is part of why diagnosis is not simple.

The damage is localized: deep pits, often cup-shaped or with undercut, sometimes clustered under nodules or deposits, with a black, sulfide-rich corrosion product when SRB are involved. On austenitic stainless steel welds, MIC from untreated hydrotest or cooling water has produced pits with very small surface openings over much larger subsurface cavities, a pattern documented in Nickel Institute publications on MIC of stainless steels. That morphology matters for inspection, because a tiny surface opening is easy to miss visually.

Where MIC shows up in storage tanks

• The floor under the water bottom. Crude oil, refined products and biofuel blends carry water that settles on the tank floor. The water and sludge layer, with nutrients from the hydrocarbon and sulfate from the water, is the classic MIC habitat. Pitting on the product side of the floor plates is often clustered in low areas and along the lap welds where water pools.
• Sumps and the area around water draws. Water collects there by design, and draining is often incomplete.
• Under sludge and deposits. Thick sludge shields the floor from inhibitors and biocides and creates the oxygen-free conditions SRB need.
• Critical zone and annular plates. MIC at the shell-to-bottom junction is particularly serious because of the stresses there.
• Bottom-side (soil-side) of the floor. Soil-side corrosion has many causes, and MIC is one of them where the foundation stays wet and cathodic protection is inadequate. The NDE approach is the same, but confirmation sampling is much harder.
• Water tanks and fire water tanks. Untreated or poorly treated water storage can host MIC on walls and floors.

For tanks, the integrity question is not just "is it MIC?" but "how deep are the pits, and what is the remaining floor thickness?" That answer comes from NDE, and it feeds the owner's API 653 assessment of the floor and the next internal inspection date.

Where MIC shows up in piping

• Dead legs and low-flow branches, including bypasses, drains and standby lines where water settles and flow rarely disturbs a biofilm.
• Fire water systems, both underground mains and above-ground sprinkler and deluge piping, which sit full of stagnant water for long periods. MIC in fire sprinkler systems is well documented in fire protection literature.
• Equipment left wet after hydrotest. Hydrotest water left standing in piping, tanks or exchangers, especially untreated or poorly treated water, is a recognised cause of MIC, including on 300-series stainless steel welds.
• Cooling water systems, including exchanger shells and tubes, where biocide treatment lapses.
• Produced water, water injection and gathering lines in upstream operations, where the pipeline-focused AMPP standard TM0212 is the reference for detection, testing and evaluation of MIC on internal surfaces.
• Buried piping on the soil side where coatings have failed and cathodic protection is insufficient.

If your piping programme already treats dead legs as a separate circuit, MIC is one more reason to keep it that way; see the guide on API 570 dead leg inspection for how those circuits are defined and inspected.

Bacteria present is not MIC confirmed

The most common diagnostic error is to find SRB in a water sample and declare MIC, or to find a deep pit and assume MIC because it "looks like bacteria". Neither is enough. Microorganisms are present in almost every untreated water system, and deep pitting has many other causes: under-deposit corrosion, oxygen pitting, chloride pitting, acid attack and galvanic corrosion can all produce similar pits.

A published UK Health and Safety Executive investigation of an onshore pipeline MIC failure set out a lines-of-evidence approach that integrity teams find useful. In short, MIC is confirmed when several independent lines agree:

• Operating conditions that favour microbes: water present, stagnant, low or intermittent flow, temperature in the range where the organisms thrive, nutrients available.
• Microbiological evidence from samples of the deposit or biofilm at the corroded surface, not just the bulk water, using culture methods (serial dilution) and molecular methods such as quantitative PCR, with microscopy techniques such as DAPI staining or FISH where needed.
• Chemical evidence in the corrosion product, for example high sulfide content for SRB-related attack.
• Morphology of the damage: pit shape, clustering, undercutting, association with deposits and with the water-wetted zone.
• Wall-loss data from UT, radiography or MFL that shows the extent and rate of damage over time.

No single line is decisive. Molecular methods tell you what organisms are present and roughly how many, not whether they are corroding the steel at a given rate. Morphology alone can mislead. The value is in the combination.

Sampling sequence: why timing decides the diagnosis

Biofilm evidence is fragile. Draining, water washing, steam cleaning, chemical cleaning, gas-freeing and abrasive blasting for inspection all remove or kill the biofilm and alter the corrosion product. Once a tank floor has been blasted for MFL scanning, there is nothing left to sample. That is why the order of work matters:

1. Before opening: take water-bottom and sludge samples through sample points or water draws, and record water chemistry (pH, sulfate, sulfide, chloride) and temperature.
2. At first entry, before cleaning the floor: once the tank is safe to enter under the site's confined space and gas testing rules, sample deposits and biofilm directly at representative pits, photograph the undisturbed condition and map sludge depth.
3. Preserve samples correctly: follow the laboratory's instructions for containers, temperature and time to analysis. Field testing kits limit degradation; molecular samples may need preservation.
4. Then clean and prepare for NDE: the floor can be cleaned and blasted for MFL scanning and UT prove-up.
5. Correlate: overlay the microbiology results on the NDE map. Clusters of deep pits in the water-wetted zone with positive microbiology build a strong case; isolated pits elsewhere may have another cause.

Microbiological sampling and analysis are a laboratory and corrosion-specialist scope. The NDE contractor's role is to coordinate access, avoid destroying the evidence before it is collected, and supply the wall-loss data that completes the picture.

NDE methods that measure MIC damage

Because MIC is localized pitting, methods that cover area are essential and spot thickness readings are of limited value. The table compares the common choices.

For tank floors, the standard approach is MFL scanning of the accessible floor followed by UT prove-up of every indication above the reporting threshold set by the owner's inspector. The companion articles on tank floor scanning, MFL versus UT and on API 653 floor pitting and remaining thickness explain how the data feeds the floor assessment. Note that API 653 Addendum 4 (July 2025) revised text on tank-bottom thickness measurement, so check the current edition and addendum before writing the scope.

A worked example, described qualitatively

Consider a crude oil tank in a Gulf Coast terminal opened for its scheduled API 653 internal inspection. Water draws had been irregular, and the operator had seen black sludge on previous cleanings. The owner wanted to know whether floor pitting was MIC and how much floor life remained.

The plan sampled the water bottom through the water draw before the tank was taken out of service, then, at first safe entry, sampled sludge and deposits at three locations near the sump and along a low area of the floor, photographing pits before any cleaning. After cleaning and blasting, the floor was scanned with MFL and every indication above the agreed threshold was proved up with UT. The results showed clusters of deep product-side pits in the low area and around the sump, matching the sludge map. The laboratory reported high SRB counts by qPCR in the deposit samples and sulfide-rich corrosion product.

With these lines of evidence aligned, the owner's API 653 inspector assessed the remaining floor thickness against the code and decided on repairs: patch plates in the worst area, an internal lining, and a requirement for regular water draining and biocide treatment as part of the tank's operating procedures. The inspection interval was then set by the inspector under API 653 based on the measured data and the mitigation in place. The NDE contractor supplied the floor map, UT prove-up data and photographs; the laboratory supplied the microbiology; the decisions stayed with the owner.

Mitigation the inspection programme should verify

• Water management. Regular water draws on tanks, drainage and drying after hydrotest, and flushing of dead legs and fire water lines. API 653 internal inspections are an opportunity to check that water draws and sumps actually function.
• Hydrotest water quality and lay-up. Clean, treated test water, prompt draining and drying, particularly for stainless steel systems.
• Biocide treatment in cooling water, fire water and some product and produced-water systems, with monitoring to show it is effective.
• Internal linings on tank floors and lower shell courses, inspected under the owner's lining programme.
• Cathodic protection for soil-side floor and buried piping, with survey records reviewed at each inspection.
• Design changes that remove dead legs or low points where water collects.

Mitigation is a process and operations responsibility. The inspection record should note whether the measures were found in place, because the inspector's interval decision depends on them.

Common mistakes on MIC investigations

• Cleaning before sampling. The single most common reason a MIC investigation ends inconclusively.
• Sampling only bulk water. Planktonic (free-floating) counts in water say little about the sessile population in the biofilm at the pit.
• Calling every deep pit MIC. Other mechanisms create similar pits; the mitigation for each is different.
• Using spot UT on a pitted floor. Random point readings can miss pits entirely; scanning is needed.
• Ignoring the soil side. Product-side MIC is visible; soil-side damage only appears in MFL and UT data.
• Assuming stainless steel is immune. Hydrotest-water MIC of 300-series stainless steel welds is well documented.
• No repeat sampling. One sample is a snapshot; trends over time are more reliable for judging whether mitigation works.

Regulatory overlay in the USA and Canada

Field-erected aboveground storage tanks in petroleum service are commonly inspected under API 653, and many owners also need the tank inspection programme to satisfy the US EPA Spill Prevention, Control and Countermeasure rule (40 CFR 112), which requires integrity testing of bulk storage containers according to industry standards; shop-built tanks are often inspected under STI SP001. Tanks and piping inside OSHA PSM-covered processes fall under the mechanical integrity element of 29 CFR 1910.119. Buried fire water systems are usually governed by fire codes and the authority having jurisdiction rather than API codes, although the same NDE methods apply. Pipelines regulated by PHMSA follow 49 CFR 192 or 195 integrity management rules. In Canada, provincial regulators and fire codes apply, and tank requirements can differ by province. Confirm the rules for your site with your jurisdiction.

How Atlantis supports MIC inspection

Atlantis NDT performs the NDE side of a MIC investigation within the owner's inspection plan: tank floor MFL scanning with UT prove-up, corrosion mapping of annular plates, sumps and shell courses, UT scanning and radiography of piping, dead legs and fire water lines, and guided wave screening of long runs. Work is carried out by ASNT-certified technicians under ASNT Level III oversight, and we schedule the NDE so that microbiological sampling by your laboratory can be done before cleaning. Results go to the owner's API-certified inspector, who remains inspector of record. Microbiological testing is a laboratory scope, and Atlantis does not perform risk-based inspection or fitness-for-service assessments. See aboveground storage tank inspection and magnetic flux leakage testing, or request a quote for MIC tank floor inspection. Quotes on request. For piping, ask us to scope dead leg and fire water NDE.

More questions about MIC inspection

How do you confirm microbiologically influenced corrosion?

By combining several lines of evidence: conditions that favour microbes, microbiological testing of deposits at the corroded surface (culture and molecular methods such as qPCR), corrosion product chemistry, pit morphology and wall-loss data from NDE. Bacteria alone, or pit shape alone, is not proof.

What does MIC look like on a tank floor?

Typically clusters of deep, often cup-shaped pits in the water-wetted low areas and around sumps, frequently under black sludge with a sulfide smell. Similar pits can come from other mechanisms, so sampling is needed to confirm.

Can MFL detect MIC pitting on a tank bottom?

MFL floor scanning detects metal loss from MIC pitting on both the product side and the soil side and flags areas for follow-up. UT prove-up then measures remaining thickness at each indication.

Why must MIC samples be taken before cleaning?

Cleaning, steaming and blasting remove or kill the biofilm and change the corrosion product. Once the floor is blasted, the microbiological evidence is gone.

What is AMPP TM0212?

It is the AMPP (formerly NACE) test method for detection, testing and evaluation of MIC on internal surfaces of pipelines. It is widely used as a reference for sampling and analysis.

Who decides repairs after MIC is found in a tank?

The owner and its API-certified tank inspector, using the API 653 floor assessment and the owner's procedures. The NDE contractor and laboratory provide the data.

Opening a tank soon? Plan the floor scan around your MIC sampling with us.