Microbiologically influenced corrosion (MIC) and chemical corrosion are fundamentally different processes. Chemical corrosion is driven by electrochemical reactions between metal and its environment, while MIC is caused by microorganisms that accelerate or initiate corrosion through their metabolic activity. The distinction matters because MIC often progresses faster, strikes in unexpected locations, and resists conventional corrosion control measures. This article unpacks the key differences, the environments where MIC thrives, and how to protect your steel pipes and systems against it.
What actually causes microbiologically influenced corrosion?
Microbiologically influenced corrosion is caused by microorganisms, including bacteria, fungi, and archaea, that colonise metal surfaces and produce metabolic byproducts that attack the metal directly or disrupt its protective oxide layer. The most common culprits are sulfate-reducing bacteria (SRB), which produce hydrogen sulfide as a waste product, and acid-producing bacteria, which generate organic acids that eat into metal surfaces.
These microorganisms typically form a biofilm, a thin, structured community of cells attached to a surface and protected by a self-produced slime layer. Biofilms are significant because they create localised chemical environments that are completely different from the surrounding fluid. Inside a biofilm, oxygen levels can drop to near zero, pH can shift dramatically, and aggressive compounds can build up to concentrations far higher than in the bulk water.
MIC does not require a particularly hostile environment to get started. Stagnant or slow-moving water, moderate temperatures, and the presence of nutrients like carbon, nitrogen, and phosphorus are often enough. This is why it appears in cooling water systems, ballast tanks, buried pipelines, and offshore structures with equal frequency.
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Ask your steel question →How does MIC damage metal differently than chemical corrosion does?
MIC damages metal through highly localised attack, producing deep pits and perforations rather than the uniform surface thinning typical of purely chemical corrosion. While standard chemical corrosion spreads relatively evenly across exposed surfaces, microbial corrosion concentrates under biofilm colonies, creating aggressive micro-environments that bore through metal far faster than the surrounding area degrades.
This localised nature makes MIC particularly dangerous. A pipe wall can appear visually sound from the outside while a pit driven by microbial activity has already eaten through 80% of the wall thickness directly beneath a biofilm patch. Conventional corrosion inspection methods, such as visual checks or generalised thickness measurements, can easily miss this kind of damage.
The chemistry of the attack also differs. In standard electrochemical corrosion, metal loses electrons to an oxidising agent in the surrounding environment. In MIC, the microorganisms themselves act as catalysts or electron acceptors, accelerating anodic dissolution of the metal at rates that can be orders of magnitude higher than abiotic corrosion alone. Sulfate-reducing bacteria, for example, consume hydrogen at the metal surface, removing a product that would otherwise slow the corrosion reaction and allowing it to continue unchecked.
Which environments and materials are most vulnerable to MIC?
MIC is most prevalent in environments where water is present, flow is limited, and nutrients are available for microbial growth. Marine and offshore environments are among the highest-risk settings, but cooling water systems, fire suppression pipework, ballast tanks, sewage infrastructure, and buried carbon steel pipelines are all highly susceptible. Any system that experiences periods of stagnation is at elevated risk.
High-risk environments
Seawater is a particularly fertile environment for MIC because it carries a diverse population of microorganisms and supplies sulfates that fuel sulfate-reducing bacteria. Ballast water tanks on vessels, seawater cooling loops, and submerged structural steel on offshore platforms are all classic locations for microbial corrosion. Freshwater systems are not immune either, especially where biofilm-forming organisms like iron-oxidising bacteria can establish themselves in low-flow zones.
Vulnerable materials
Carbon steel and low-alloy steel are the most commonly affected materials because they lack the chromium content that gives stainless steels their passive protective layer. That said, stainless steel is not immune. Certain bacteria produce chlorides or reduce the local pH to levels that break down the passive film, making even austenitic stainless steels vulnerable under biofilm colonies. Copper alloys can also be attacked by ammonia-producing microorganisms, which is relevant for any system using brass or bronze components alongside steel pipework.
How can you tell if corrosion damage is caused by microbes or chemistry?
The clearest indicator of MIC rather than purely chemical corrosion is the pattern of damage. MIC typically produces irregular, deep pits clustered in areas of low flow or beneath visible deposits, while chemical corrosion tends to produce more uniform wall thinning or predictable attack at welds and stress points. The presence of a dark, sulphurous-smelling deposit or a slime layer on the corroded surface is a strong indicator of microbial involvement.
For a more definitive diagnosis, several approaches are used in practice:
- Microbiological sampling: Water or deposit samples are analysed for the presence and concentration of key organisms, particularly sulfate-reducing bacteria and acid-producing bacteria.
- Chemical analysis of deposits: The presence of iron sulfides, sulfur compounds, or organic acids in corrosion deposits points strongly toward microbial activity.
- Pit morphology: MIC pits often have an undercut or mushroom-shaped profile when viewed in cross-section, reflecting the way biofilm communities concentrate attack beneath the surface.
- Location pattern: Damage appearing at the six o’clock position in horizontal pipework, where water pools and sediment accumulates, is a common MIC signature.
It is worth noting that MIC and chemical corrosion frequently occur together. Microbial activity can initiate pitting that is then accelerated by electrochemical processes, and chemical attack can damage protective coatings in ways that make surfaces more hospitable to biofilm formation. A thorough investigation should consider both possibilities.
What are the most effective ways to prevent MIC in steel pipes and systems?
The most effective MIC prevention strategies combine physical, chemical, and operational measures to disrupt biofilm formation before it can establish. No single method is universally sufficient, but a layered approach targeting water quality, flow conditions, and surface protection consistently delivers the best results.
- Maintain flow velocity: Stagnant water is the primary enabler of biofilm growth. Keeping flow above minimum velocity thresholds, or flushing low-use sections regularly, removes the conditions microorganisms need to colonise surfaces.
- Biocide treatment: Oxidising biocides such as chlorine or non-oxidising alternatives are used in cooling systems and water injection lines to control microbial populations. Rotation between different biocide types prevents resistance from developing.
- Pigging and mechanical cleaning: Regular pipeline pigging physically removes biofilm deposits before they mature and become protective shelters for corrosive organisms.
- Protective coatings and linings: Internal coatings on carbon steel pipework reduce the surface area available for biofilm attachment and create a barrier between the metal and the biological environment.
- Material selection: In environments with a known high MIC risk, specifying higher-grade materials with greater resistance to localised attack can reduce long-term maintenance costs, even if initial procurement costs are higher.
- Monitoring programmes: Regular microbiological and chemical water testing, combined with ultrasonic thickness measurements at known risk points, allows early detection before damage becomes structural.
Prevention is always more cost-effective than remediation. A pipe failure in a marine or offshore setting carries consequences far beyond the replacement cost of the pipe itself, including downtime, environmental liability, and safety risk.
How Marine Steel helps you source the right materials for corrosive environments
Selecting the right pipe specification for an environment prone to MIC or chemical corrosion is not always straightforward, and getting it wrong is expensive. We work with clients across maritime, offshore, construction, and industrial sectors to make sure they get the materials that match their actual operating conditions, not just a standard catalogue choice.
Here is what we bring to the table:
- Broad stock availability across carbon steel, stainless steel, and non-ferrous metals including copper, brass, and bronze, covering a full range of pipe sizes up to 20 inches
- ASTM-certified pipes and fittings, including schedule 40 and schedule 80, with full documentation for clients who need it for compliance or classification purposes
- Technical advice included – you explain your situation once, and we help you work through the right specification, whether that means a different material grade, a different wall thickness, or a different coating standard
- One-stop supply covering pipes, flanges, fittings, and related metals so you are not chasing multiple suppliers for a single project
- Fast turnaround from Rotterdam and Houston, with over 15 years of experience serving clients who cannot afford to wait
If you are dealing with a corrosion challenge or specifying materials for a demanding environment, get in touch with our team. We are ready to help you find the right solution quickly.
