Microbiologically influenced corrosion, or MIC, is metal loss driven not by chemistry alone but by living microorganisms colonizing pipe and tank surfaces. Bacteria and other microbes settle under deposits, build biofilms, and create local conditions that corrode the steel beneath them far faster than the surrounding metal. This guide explains which organisms cause MIC, the distinctive under-deposit pitting signature it leaves, and why biocide programs and monitoring are needed to keep it in check.
Microbially Influenced Corrosion in one line: Microbiologically influenced corrosion is localized corrosion accelerated by the activity of microorganisms - most notoriously sulfate-reducing bacteria - that colonize metal surfaces, form biofilms and deposits, and create aggressive local chemistry beneath them. It typically shows up as clustered, deep pitting under deposits, tubercles, or biofilm rather than as uniform thinning, and because it is biological it is managed with biocide treatment, cleaning, and microbial monitoring in addition to conventional corrosion control.
MIC is caused by communities of microorganisms rather than a single culprit, but sulfate-reducing bacteria are the classic and most damaging players in oil and gas systems. They thrive in the oxygen-free conditions found under deposits and in stagnant water, where they consume sulfate and produce sulfide as a byproduct. That biological sulfide is corrosive to steel and also feeds a self-sustaining cycle beneath the deposit. Other organisms - iron-oxidizing, acid-producing, and slime-forming types - contribute in different ways, often working together in the same biofilm.
The key to why microbes accelerate corrosion is that they change the local environment at the metal surface. A biofilm and its deposits create a sheltered pocket where oxygen is excluded, acids or sulfides accumulate, and the chemistry right at the steel becomes far more aggressive than the bulk fluid would suggest. The organisms effectively set up a tiny, concentrated corrosion cell and keep it fed, so metal loss proceeds under the deposit while the surrounding surface looks fine.
MIC is most associated with systems that carry or hold water and have low-flow or stagnant zones - produced water lines, water injection systems, storage tanks, and dead legs - because those conditions let biofilms establish and deposits settle. Warm, nutrient-rich, stagnant water is close to ideal for the bacteria that drive it.
The visual signature of MIC is what distinguishes it from other corrosion. It typically produces localized, often clustered pits hidden beneath deposits, tubercles, or slimy biofilm, rather than the broad, even thinning of general corrosion. Lift off a tubercle or deposit and you frequently find a sharp, deep pit in the steel underneath - discrete cavities that can penetrate wall quickly even though the overall metal loss across the surface is modest.
This under-deposit, localized character is exactly what makes MIC dangerous and easy to miss. An average wall-thickness reading can look reassuring while a cluster of hidden pits is driving toward a leak. Because the attack is concentrated in small spots protected by deposits, it can outrun the corrosion allowance locally and cause a pinhole failure long before general thinning would have been a concern. It also tends to appear at low-flow spots, weld heat-affected zones, and other places where deposits and biofilm accumulate.
Confirming that pitting is truly microbial usually takes more than the pit shape alone. Operators look for the association of pitting with biofilm and deposits, sample for the responsible organisms, and consider the operating history - stagnant, water-wet, warm conditions favor MIC. The pitting pattern raises the suspicion; microbial testing supports the diagnosis.
Because MIC is biological, the primary controls target the organisms and the conditions they need. Biocide treatment - dosing chemicals that kill or suppress the microbes - is the front-line tool, and it is used alongside mechanical cleaning such as pigging to remove the deposits and biofilm that shelter the bacteria. Reducing stagnation, controlling water, and eliminating dead legs take away the low-flow, water-wet conditions that let biofilms establish in the first place. Monitoring closes the loop by tracking bacterial populations and corrosion so the biocide program can be tuned rather than run blind.
A cloud SCADA platform such as Merobix supports MIC control on the operating side. Biocide and chemical injection are dosing systems, and continuous monitoring of injection pump status and rates, with alarms when dosing stops or drops, helps ensure the biocide program is actually delivering treatment instead of quietly lapsing - a lapse that lets microbial populations rebound. Trending flow and identifying low-flow or stagnant conditions also helps operators find the dead legs and idle lines where MIC is most likely to take hold.
The microbial sampling and pit inspection that actually diagnose MIC come from field testing and inspection programs, not from SCADA. But keeping the biocide dosing continuous and reliable, and surfacing the stagnant conditions that favor the bacteria, is where remote monitoring reinforces a MIC-management program. Reliable treatment plus visibility into flow conditions is what keeps the biological driver of the corrosion under control between inspections.
Sulfate-reducing bacteria are microbes that thrive without oxygen and consume sulfate, producing corrosive sulfide as a byproduct. They are the classic drivers of microbiologically influenced corrosion in oil and gas water systems because they colonize sheltered spots under deposits and generate an aggressive local chemistry that attacks the steel. Their activity is a major reason biocide programs target anaerobic bacteria.
MIC usually shows up as localized, often clustered deep pits hidden under deposits, tubercles, or biofilm, rather than the broad even thinning of general corrosion. Finding sharp pits beneath deposits, especially in low-flow, water-wet, warm zones, raises suspicion of MIC. Confirmation typically also involves sampling for the responsible microorganisms and reviewing the operating history.
Because it is biological, control combines biocide treatment to kill the organisms, mechanical cleaning such as pigging to remove the deposits and biofilm that shelter them, and reduction of stagnant, water-wet conditions like dead legs. Microbial and corrosion monitoring is used to tune the biocide program. Reliable, continuous chemical dosing is essential, since a lapse lets microbial populations rebound.
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