Which microorganisms cause MIC in steel infrastructure?
MIC in carbon steel and other ferrous metals is primarily associated with three categories of bacteria:
Sulfate-reducing bacteria (SRB)
SRB are anaerobic bacteria — they operate in the absence of oxygen — that use sulfate ions (SO₄²⁻) as an electron acceptor in their metabolism, reducing sulfate to hydrogen sulfide (H₂S). H₂S is directly corrosive to steel, and the biogenic sulfide environment it creates drives aggressive localised pitting. SRB are the most widely documented and economically significant MIC organisms in industrial infrastructure. They are found in:
- Pipeline internals in oil and gas systems where free water is present
- Offshore structure submerged zones and seabed sediments
- Ballast water systems and water injection systems
- Heat exchanger internals and cooling water systems with biofouling
- Buried pipeline externals in anaerobic soils
Acid-producing bacteria (APB)
APB are aerobic or facultatively anaerobic bacteria that produce organic acids (acetic, formic, lactic acid) as metabolic byproducts. These acids locally reduce the pH at the metal surface, accelerating corrosion. APB often work in concert with SRB — APB consume oxygen in the biofilm, creating the anaerobic conditions SRB require, while also producing acids that increase the aggressiveness of the local environment.
Iron-oxidising bacteria (IOB)
IOB are aerobic bacteria that oxidise ferrous iron (Fe²⁺) to ferric iron (Fe³⁺), using the energy released to drive their metabolism. The ferric iron produced accelerates the cathodic reaction in the corrosion cell, increasing the overall corrosion rate. IOB produce voluminous, reddish-brown tubercles (mounds of iron oxides and bacterial biomass) over active corrosion sites — a distinctive visual indicator of MIC.
Where MIC occurs in industrial and marine assets
| Asset / location | Primary MIC organisms | Key conditions driving MIC |
|---|---|---|
| Oil and gas pipeline internals | SRB, APB | Stagnant or slow-flowing water, sulfate in produced water, anaerobic conditions |
| Offshore structure submerged zone | SRB, IOB | Biofilm formation on unprotected steel; sediment accumulation at base of jacket legs |
| Ballast tanks (marine vessels) | SRB, APB | Accumulated sludge, stagnant ballast water, anaerobic conditions at tank bottom |
| Water injection systems (O&G) | SRB, APB | Oxygen ingress, sulfate in seawater, biofouling of injection lines |
| Buried pipelines | SRB | Anaerobic clay soils, high sulfate groundwater, disbonded cathodic protection |
| Cooling water systems | IOB, APB, SRB | Warm water, nutrients, biofouling control failures |
| Potable water infrastructure | IOB, APB | Low flow zones, stagnation, inadequate disinfection |
| Mining process vessels | APB, SRB | Acidic process environments, sulfate-rich slurries |
How MIC damages steel — the mechanisms
MIC produces corrosion through several mechanisms that often operate simultaneously within a biofilm:
- Biogenic H₂S production — SRB-produced H₂S reacts directly with steel, producing iron sulfide (FeS) corrosion products. FeS is cathodic to steel and creates galvanic couples that accelerate further pitting at the surrounding steel surface.
- Local acidification — APB-produced organic acids and the acid environment within active pits reduce pH below 4 in some MIC cases, at which point even passive-state metals begin dissolving rapidly.
- Differential aeration cells — Biofilms restrict oxygen access to the steel beneath them, creating oxygen-concentration cells between the biofilm-covered (anodic) and open (cathodic) surfaces.
- Cathodic depolarisation — Some bacteria directly consume hydrogen that would otherwise accumulate at the cathodic sites of the corrosion cell, removing the polarising effect and accelerating the corrosion reaction rate. This mechanism, while historically debated, is now well-supported by experimental evidence for specific SRB species.
The practical result of these mechanisms is that MIC typically produces aggressive localised pitting — often with a distinctive morphology (irregular, undercut pits with black FeS deposits in SRB MIC; tubercle-covered pits in IOB MIC) — at rates that significantly exceed what the bulk chemical environment would produce.
Detection and monitoring of MIC
MIC detection combines microbiological assessment with corrosion monitoring:
- Bacterial culture testing — Most Probable Number (MPN) or serial dilution culture methods quantify SRB, APB, and IOB populations in produced water, process water, and biofilm samples. Positive cultures confirm bacterial presence but do not directly measure corrosion rate.
- Molecular methods — qPCR (quantitative polymerase chain reaction) provides rapid, sensitive identification and quantification of specific MIC-relevant organisms from water samples. Increasingly used for continuous monitoring in oil and gas production.
- Corrosion monitoring — Coupon exposure, electrical resistance probes, and linear polarisation resistance (LPR) instruments provide corrosion rate data; elevated rates in known-MIC environments corroborate microbiological data.
- Ultrasonic inspection — External UT scanning of pipeline sections and vessel walls detects localised wall loss attributable to MIC pitting.
How protective coatings address MIC
Protective coatings address MIC through barrier protection — preventing the microorganisms and their metabolic products from reaching the steel surface. A well-applied, intact coating with adequate adhesion to properly prepared steel provides the primary defence against both abiotic and biotic corrosion.
Key coating system considerations for MIC environments:
- Coating continuity is critical — Any holiday, mechanical damage, or adhesion failure creates an entry point for bacteria and their corrosive metabolites. MIC is particularly aggressive at coating defect sites because disbonded coating creates the anaerobic, nutrient-accumulating crevice environment that SRB prefer. Holiday testing to NACE SP0188 / ISO 29601 standards is mandatory for immersion-service MIC coatings.
- Coating resistance to H₂S and organic acids — Some standard epoxy formulations are susceptible to softening and degradation in high-H₂S environments. Specify coating products tested and approved for sour service exposure.
- Biocide treatment of the substrate before coating — In systems with established biofilm and MIC, biocide treatment (pigging with biocide slug, or manual cleaning with biocide solution) before surface preparation and coating application is standard practice to eliminate the existing biological community before sealing the steel surface.
- Surface preparation quality — Biofilm residues and MIC corrosion products must be physically removed before coating application. The minimum surface preparation is SSPC-SP10 / Sa 2½. Soluble salt testing after preparation is essential — MIC corrosion products are rich in sulfides and chlorides that must be confirmed below specification limits before coating.
Surface preparation for MIC-affected steel
Steel with established MIC damage presents similar surface preparation challenges to steel with aggressive pitting corrosion from abiotic chloride attack — because MIC produces pitting as its primary damage morphology. The approach:
- Biocide treatment — Apply biocide solution to kill active bacteria before mechanical preparation. This prevents redistribution of viable bacteria during preparation.
- Bulk removal of MIC deposits — Iron sulfide deposits, tubercles, and corrosion crust must be removed before blast or mechanical preparation. The Tercoo® is effective for mechanical removal of MIC-related corrosion deposits from steel surfaces in maintenance applications.
- Blast or mechanical preparation to target grade — SP-10 / Sa 2½ for most protective coating systems; SP-5 / Sa 3 for thermal spray coatings.
- Soluble salt testing — Iron sulfide and associated chlorides and sulfates in MIC-affected steel must be below specification limits before coating. Treat with water wash if required.
- Apply coating within minimum time — Rapid recolonisation of bare steel surfaces by bacteria can occur in MIC-active systems. Apply primer as soon as possible after surface preparation is complete and verified.
Key takeaways
- MIC is corrosion initiated or accelerated by the metabolic activity of bacteria — primarily sulfate-reducing bacteria (SRB), acid-producing bacteria (APB), and iron-oxidising bacteria (IOB). It is estimated to account for approximately 20% of all corrosion costs globally, with estimates reaching up to 40% in the oil and gas industry.
- MIC produces aggressive localised pitting with characteristic morphology — black FeS deposits (SRB), tubercles (IOB) — at rates significantly higher than the bulk abiotic corrosion rate.
- Protective coatings address MIC through barrier protection. Coating continuity is critical — any disbondment or holiday creates the conditions (anaerobic, nutrient-rich crevice) that SRB prefer.
- Surface preparation of MIC-affected steel requires biocide treatment before mechanical preparation, thorough removal of MIC deposits (Tercoo® is appropriate for maintenance scenarios), SP-10 preparation, soluble salt testing, and rapid primer application.
- Coating products specified for MIC environments should be tested and approved for resistance to H₂S and organic acids, and holiday-tested to immersion-service standards before putting the asset back in service.