What is Microbiologically Influenced Corrosion, and why is it a threat to industrial systems? 

When a metal structure corrodes, most people automatically think of poor water quality, oxygen exposure, or an aggressive chemical as the possible cause. Fewer people realize that microorganisms often play a role in corrosion processes as well. This phenomenon is known as microbiologically influenced corrosion, or MIC. 

MIC is not a single corrosion mechanism. Rather, it is a complex interplay of biological, chemical, and metallurgical processes. This is precisely why it can be difficult to identify and manage. MIC often receives attention only after a pipe leak, equipment failure, or unexpected operational disruption has already occurred. 

Do microorganisms really “eat” metal? 

Generally, microorganisms do not “eat” metal in the everyday sense of the word. In most cases, their biological activity alters the immediate environment at the metal surface, which can accelerate or modify electrochemical corrosion reactions. Some microorganisms, however, may participate more directly in electron transfer processes. Their metabolic products can promote corrosion reactions, while their activity may create localized chemical and electrochemical conditions that accelerate metal deterioration. 

The key players are usually not free-floating individual cells, but communities known as biofilms. 

Biofilm: more than a slimy layer 

A biofilm is a community of microorganisms embedded in a slimy polymeric matrix that they produce. Other organic and inorganic materials may also accumulate within it. Biofilms are extremely common in both natural and industrial environments because surface-associated growth is a natural mode of life for many microorganisms. Therefore, wherever microorganisms, moisture, and colonizable surfaces are present, the potential for biofilm formation should be considered. Biofilms can develop in pipelines, water tanks, heat exchangers, cooling systems, and even fire sprinkler systems. 

Biofilms are particularly important in MIC because the microenvironment within a biofilm can differ significantly from the surrounding fluid. For example, even when the fluid around a biofilm is rich in oxygen, oxygen levels in its deeper layers may be low or even completely depleted. This allows different groups of microorganisms to coexist and enables corrosion processes to develop that are not visible from the outside. 

A single bacterium is not the only culprit 

Sulfate-reducing bacteria are frequently discussed in connection with MIC. They are indeed important contributors, but the reality is far more complex. Many groups of microorganisms may be involved in corrosion, including: 

• sulfate-reducing microorganisms, 
• sulfur-oxidizing bacteria, 
• acid-producing bacteria, 
• iron-oxidizing bacteria, 
• iron-reducing bacteria, 
• manganese-oxidizing bacteria, 
• methanogenic archaea, 
• fungi, and other groups of microorganisms. 

Modern molecular methods indicate that corrosion sites typically contain complex microbial communities rather than a single species. Members of these communities can interact with one another and alter the environment at the metal surface, although not all of them necessarily play a direct role in corrosion. 

Where does MIC occur most often? 

MIC can develop in nearly any engineered system where water, persistent moisture, or another medium capable of supporting microbial life and electrochemical reactions is present at a metal surface. Systems that are particularly susceptible include: 

• drinking water and industrial water systems, 
• oil and gas pipelines, 
• cooling water systems, 
• water storage tanks, 
• facilities exposed to seawater, 
• marine and port facilities, 
• firewater and sprinkler systems, 
• power generation equipment. 

Common contributing factors include the presence of water, adequate nutrient availability, and areas within the system where stagnant or difficult-to-flush zones can develop. 

Why is MIC difficult to confirm? 

One of the most common misconceptions is that detecting microorganisms is sufficient to prove the presence of MIC. In reality, it is not that simple. 

The presence of microorganisms alone does not prove that they caused the damage. Likewise, a negative microbiological result does not automatically rule out MIC. Results may be influenced by the sampling location and timing, the type of sample, how it was stored and transported, and the analytical method used. Corrosion must always be interpreted in the context of the entire system. 

A well-founded investigation generally requires the combined evaluation of several types of information: 

• microbiological data, 
• water chemistry results, 
• materials testing and analysis, 
• operational information, 
• analysis of deposits and corrosion products, 
• the morphology and characteristics of the damage. 

This is why investigating MIC is fundamentally an interdisciplinary task. 

Early detection can reduce damage and intervention costs 

MIC poses significant economic and technical risks worldwide. The problem extends beyond repair costs. Potential consequences include lost production, unplanned shutdowns, environmental damage, and safety risks. 

A proactive, risk-based approach is generally the most effective way to manage the risk of MIC. This includes: 

• monitoring tailored to the risks of the system; 
• sampling at the right location, at the right time, and using the right method; 
• integrated interpretation of different types of data; 
• early identification of changes and unfavorable trends. 

Summary 

MIC is not purely a microbiological problem, nor is it simply a corrosion issue. Rather, it results from complex interactions among microorganisms, the environment, materials, and operating conditions. 

Understanding and managing MIC therefore requires systems-level thinking. The key to successful diagnosis and prevention is not a single measurement or laboratory test, but the coordinated evaluation of multiple sources of information. 

If a system experiences recurring corrosion damage, deposit formation, blockages, or unexplained operational disruptions, the potential role of microbial processes should be investigated early. Asking the right questions, using a targeted sampling strategy, and evaluating different test results together often provide far more insight than any single laboratory result viewed in isolation. 

BIOCORIX provides expert support for the systems-level investigation of biofilm- and MIC-related problems, the development of sampling strategies, and the interpretation of test results.