MIC is the mechanism that turns up a year after commissioning, in a system nobody thought was at risk, and the cause is usually water that was left in a line after a test and never drained.
Bacteria do not eat steel. What they do is create local chemistry at the metal surface that corrodes it — sulphate-reducing bacteria producing hydrogen sulphide under a biofilm, acid-producing bacteria dropping the local pH, iron-oxidising bacteria building tubercles that then sustain an oxygen concentration cell underneath.
The result is aggressive, extremely localised attack. MIC pits are characteristically deep relative to their width, often with overhanging edges, and frequently clustered.
MIC needs water, nutrients, and time without disturbance. That combination gives a short list.
Untreated water, at ambient temperature, in the dark, with no flow, left in a new system for months while construction finishes. It is close to an ideal culture medium. MIC found in a system less than two years old is very often traceable to a hydrotest that was never properly drained, dried and preserved — and by then the damage is real, in new steel, and nobody expects it.
Nothing about MIC is uniquely diagnostic on its own, but the combination is recognisable:
| Sign | What you see |
|---|---|
| Pit shape | Deep relative to width; often undercut or with overhanging lips; sometimes hemispherical cups within a larger pit |
| Distribution | Clustered in low-flow zones, not spread evenly |
| Tubercles | Hard reddish-brown mounds; break one open and there is a pit underneath |
| Deposits | Black, sometimes smelling of sulphide, over the pit |
| Location | Six o'clock in horizontal lines; dead legs; behind baffles; at weld roots where the profile traps water |
Weld roots deserve a note of their own. The slight geometry change at the root, plus the different microstructure of weld metal and heat-affected zone, makes the root a preferred site. MIC concentrated along a weld seam in a water line is a common pattern.
Start with what does not. MIC produces pits, and every method that averages or screens by cross-section is blind to them — guided wave, pulsed eddy current, and a spot thickness grid all report a heavily pitted line as sound. That is covered in more detail in how to detect pitting.
If the equipment is open and clean, this is unbeatable and cheap. Tubercles and pits are visible, and a pit gauge measures depth directly. The limitation is that it needs entry and cleaning — and cleaning too aggressively destroys the evidence before anybody has looked at it.
From outside, with a fine enough scan pitch to catch pits that may be only a few millimetres across. Coarse scanning steps over them. Discuss the pitch against the expected pit size rather than accepting a default.
Exchanger and cooler bundles in cooling water service are a classic MIC location. Eddy current or RFT to screen the bundle, IRIS on the tubes that screen badly, to get a real wall profile.
Soil-side MIC on a tank floor is found by the standard floor scan, with UT prove-up at the indications. The parts the scanner covers badly — the critical zone at the shell, lap welds, under patch plates — are also places water collects.
Pit morphology is suggestive, not conclusive. If the answer changes what you do — and it does, because the treatment for MIC is biocide and flow, not a coating — then confirm it:
Inspection finds MIC after it has happened. What prevents it is entirely operational:
Where pits are found, they go to API 579 Part 6 if they are still discrete, and to Part 5 if they have coalesced into a continuous thin area. And because MIC is usually still active when it is found, the remaining life question matters more than the pass or fail today.
MIC, under-deposit corrosion, chloride pitting and ammonium chloride side by side — what distinguishes each one and which method finds it.
Open the mechanism libraryWho writes this. A mechanical engineer with twelve years in oil and gas — in-line inspection, fired heater and furnace inspection, and pipeline integrity. What is here comes from the published standards and from what those years in the field actually looked like. It is not written by an API-certified inspector.
This is not an assessment. Nothing on this site may be used to justify a decision about real equipment. Assessing plant requires the current editions of the applicable codes, data from a licensed source, and a competent engineer who signs for the answer. · Integrity Field Guide