Microbiologically Influenced Corrosion (MIC): Detection and Control

Microbiologically Influenced Corrosion (MIC): Detection and Control is localised metal loss driven or accelerated by the metabolic activity of micro-organisms living in biofilms and deposits on the metal surface. The bacteria do not attack metal directly. They alter the local chemistry beneath a slime layer or tubercle, creating conditions of low pH, differential aeration and reactive sulphur species that drive rapid, deep pitting well out of proportion to the bulk fluid corrosivity.

Why microbes accelerate corrosion

A biofilm is a structured community of cells held in a matrix of extracellular polymeric substance. Once established, it isolates the metal from the bulk water, sets up oxygen concentration cells and concentrates aggressive ions. Sulphate-reducing bacteria (SRB) thrive in the anaerobic zone beneath the film, reducing sulphate to sulphide and producing corrosive iron sulphide films. Aerobic iron and manganese oxidisers build porous tubercles that shield anaerobic pockets underneath. The result is deep under-deposit pitting, the same geometry seen in classical pitting corrosion, but sustained biologically.

Where MIC thrives

MIC needs water, nutrients and stagnant or low-flow conditions. High-risk assets include:

• Cooling water systems, especially dead legs and low-velocity headers.
• Firewater and deluge lines that sit full and stagnant for long periods.
• Storage tank bottoms with settled water, sludge and sediment.
• Buried and submerged pipelines, and the underside of tank floors.
• Hydrotest water left in a system after commissioning.
• Occluded geometries where deposits collect, overlapping with crevice corrosion under gaskets, deposits and weld backing rings.

Weld seams and heat-affected zones are common initiation sites because their microstructure and surface roughness favour biofilm attachment.

Organisms, mechanisms and control

Different microbial groups drive MIC by distinct routes, and control follows the mechanism. The table below summarises the main actors.

Recognising MIC in the field

MIC has a characteristic signature rather than a single unambiguous test. Look for a combination of features:

• Discrete pits and cavities under tubercles, slime or sludge, often with clean metal between them.
• On carbon steel, black iron-sulphide corrosion product that smells of hydrogen sulphide and effervesces, giving off that gas, on contact with dilute acid.
• Reddish-brown tubercles with a hard oxidised crust and a soft, acidic, dark core.
• On stainless steel, pitting and undercut cavities concentrated at or beside weld seams.
• Cup-shaped or striated pit walls revealed after cleaning back the deposits.

Confirmation combines deposit and pit-morphology analysis, culture or ATP testing for viable organisms, and molecular methods such as qPCR to quantify SRB and other groups. No single indicator is proof; the case is built from water chemistry, deposit analysis, biology and metallurgy together.

Monitoring programmes

Effective MIC management is a monitoring discipline, not a one-off inspection. Useful tools include biofilm coupons and side-stream corrosion coupons, ATP and sessile-cell counts taken from surfaces rather than bulk water, qPCR panels, and periodic borescope or ultrasonic thickness surveys at dead legs and low points. Trending these results turns scattered readings into an early-warning signal before a leak occurs.

Control and prevention

Control attacks the conditions the biofilm needs:

• Keep it moving. Maintain adequate velocity, typically at or above roughly 1 m/s in cooling water, and design out dead legs where deposits settle.
• Keep it clean. Mechanical cleaning, pigging and regular desludging remove the deposits that shelter anaerobes.
• Dose biocides. Alternate oxidising biocides such as chlorine or bromine with non-oxidising biocides, and add dispersants so biocide reaches cells inside the matrix.
• Manage layup. Drain and dry after hydrotesting, or treat and preserve stagnant water; never leave untreated water in a system.
• Protect surfaces. Coatings and, for buried or submerged assets, cathodic protection reduce the exposed steel available for colonisation.

Because MIC is driven by conditions, prevention is far cheaper than repair. A CMMS such as Fabrico lets teams schedule coupon pulls, biocide dosing checks and thickness surveys as recurring work orders, and trend the readings against asset history so a rising SRB count triggers action, not a failure. Book a Fabrico demo to see how corrosion monitoring fits a maintenance plan.

Frequently Asked Questions

Is MIC a distinct corrosion mechanism or just accelerated pitting?

MIC is not a separate electrochemical mechanism. Micro-organisms create and sustain the local conditions, low pH, sulphide and oxygen cells, that drive pitting and crevice attack. The corrosion itself is conventional; the biology keeps it going and localises it.

Which metals are affected?

Carbon and low-alloy steels are most commonly affected, but austenitic stainless steels suffer weld-seam pitting under manganese and iron oxidisers, and copper alloys can also be attacked. Almost any structural alloy in stagnant, deposit-laden water is at some risk.

Can biocides alone stop MIC?

Not reliably. Biocide cannot easily penetrate an established biofilm or reach cells sheltered under tubercles. Effective control pairs biocide dosing with mechanical cleaning, dispersants and adequate flow so the film is removed and cannot re-establish.

How is MIC confirmed rather than assumed?

Confirmation needs multiple lines of evidence: pit morphology, deposit and corrosion-product analysis, viable-organism testing by culture, ATP or qPCR, and water chemistry. A diagnosis rests on the whole picture, since no single test proves MIC on its own.