In brief

• MIC is not a single mechanism but a set of overlapping pathways coupling microbiology and electrochemistry.
• Diagnosis demands Multiple Lines of Evidence (MLOE): combining microbiological, chemical, metallurgical, and operational data.
• Material composition, system design, and flow conditions can amplify or mitigate microbial corrosion effects.
• Effective control requires a continuous cycle: threat assessment → mitigation → monitoring.

What MIC is — and what it is not

MIC refers to corrosion influenced by the presence or activity of microorganisms. It is not a unique corrosion pattern or a singular cause. Rather, microorganisms alter anodic and cathodic processes at metal surfaces by changing local chemistry, electron transfer, or film formation. Importantly, microbial activity can sometimes reduce corrosion rates; therefore, “microbiologically induced” is not always an accurate term.

Primary MIC mechanisms

1. Differential aeration and deposit-driven MIC

Biofilms and (bio)mineral deposits create oxygen gradients that establish localized electrochemical cells. These differential aeration effects often lead to pitting or crevice corrosion beneath deposits or patchy biofilms.

2. Electrical MIC (EMIC)

In electrical MIC, microorganisms exchange electrons directly or indirectly with the metal surface. This extracellular electron transfer (EET) can occur via contact structures (e.g., nanowires, cytochromes) or through soluble redox mediators. The result is altered cathodic kinetics, sometimes enabling corrosion even in the absence of conventional oxidants.

3. Metabolite-driven MIC (MMIC)

Microbial metabolites such as sulfide, organic acids, or ammonia modify the local chemical environment, shifting pH and altering complexation equilibria. These processes can destabilize passive films or accelerate metal dissolution even without direct microbial contact.

The once-popular “cathodic depolarization” theory is now recognized as overly simplistic and mainly of historical interest; modern understanding emphasizes coupled electrochemical–biological interactions.

Why fragmented disciplines limit progress

MIC research and field management are often split between microbiology, corrosion science, materials engineering, and operations. Without shared terminology and integrated data, diagnoses remain uncertain and mitigation measures are inconsistent. The review stresses that cross-disciplinary interpretation — supported by standardized testing — is essential for credible threat assessment.

Diagnosis through Multiple Lines of Evidence (MLOE)

No single test can confirm or exclude MIC. A robust assessment integrates four complementary evidence streams:

• Bulk medium: chemical composition (ions, gases), pH, redox potential, temperature, and flow regime.
• Interface: biofilm and deposit composition directly at the metal surface, where local chemistry diverges from the bulk.
• Metal: morphology and metallurgy of corrosion features, including inclusions and passive film integrity.
• Operations & design: flow patterns, dead legs, cleaning routines, temperature cycles, and system history.

Materials, design, and environment

MIC is predominantly localized: pits and crevices act as focal points for biofilm attachment and chemical gradients. Even stainless steels are susceptible when biofilms disrupt passivity, particularly under chloride-rich conditions. Corrosion-resistant alloys with higher PREN values show improved performance, but alloy chemistry alone is no guarantee. Design factors such as weld geometry, heat-affected zones, and stagnant regions often control where MIC initiates.

From threat to control

1. Threat assessment

Combine operational, microbiological, and materials data to identify plausible MIC mechanisms. Traditional CO₂ and H₂S corrosion models help contextualize risk, but quantitative MIC rate models are still under development. Thus, mechanistic reasoning supported by MLOE remains the preferred approach.

2. Mitigation and prevention

Mitigation strategies integrate multiple barriers: design modification, cleaning and pigging, biocide and inhibitor programs, coatings or liners, and cathodic protection. Compatibility between chemical treatments and microbial ecology is critical to avoid resistance or unintended side effects.

3. Monitoring

Monitoring combines electrochemical or weight-loss measurements with microbiological and chemical indicators. Short-term (hours to days) monitoring targets biofilm growth and activity; long-term (months to years) monitoring assesses damage accumulation. Emerging sensors and molecular tools now enable more direct insight into microbial contributions.

What this means for asset integrity

• Measure where corrosion actually occurs — directly at metal interfaces, not just in bulk fluids.
• Design for accessibility and cleaning to minimize stagnant zones and allow representative sampling.
• Integrate microbial, chemical, and metallurgical data into one diagnostic framework.
• Consider transient operational states (aerobic/anaerobic shifts, temperature cycling) as key MIC drivers.

How MICBUSTERS supports quantitative field diagnosis

MICBUSTERS brings microbiological quantification directly to the field. Using on-site quantitative Polymerase Chain Reaction (qPCR), we detect and measure microbial DNA associated with MIC mechanisms — such as sulfate-reducing, nitrate-reducing, and acid-producing bacteria — within hours. This approach provides a direct, quantitative indicator of microbial activity at the sampling location.

Field qPCR data are integrated with local chemistry (pH, sulfide, nitrate, carbonate speciation) and metallurgical analysis to complete the MLOE framework. By combining molecular microbiology with corrosion science, MICBUSTERS enables evidence-based interpretation rather than assumption-driven conclusions.

• Rapid on-site quantification of MIC-relevant microbial groups using portable qPCR instrumentation.
• Correlation with corrosion morphology, deposit composition, and electrochemical data.
• Immediate feedback on biofilm dynamics and treatment effectiveness.
• Integration into long-term monitoring and mitigation programs.

This field-based molecular approach transforms MIC investigation from qualitative observation to quantitative measurement — supporting faster, data-driven decisions in asset integrity management.

To explore field qPCR diagnostics for your assets, contact the MICBUSTERS team.

Source

Based on Knisz et al. (2023), Microbiologically influenced corrosion—more than just microorganisms, FEMS Microbiology Reviews, and field experience within industrial and offshore systems.