Microbial Corrosion - Why cleaning and biocide treatment do not solve your problem.

Corrosion is one of the most widespread and costly forms of material damage in industrial operations. It is an electrochemical process in which metals react with their environment and slowly return to a more stable form, such as oxides or sulfides. While corrosion is often associated with purely chemical reactions involving oxygen, moisture, and salts, in many cases microorganisms play a decisive role in accelerating and sustaining it. This form is known as microbiologically influenced corrosion (MIC).  

In MIC, microorganisms such as sulfate-reducing bacteria (SRB), iron-oxidizing bacteria, acid-producing bacteria, and methanogenic archaea settle on metal surfaces and form biofilms. These biofilms act like protective layers for the microbes but harmful zones for the metal beneath. Within these microenvironments, local pH values, oxygen levels, and redox conditions change drastically. For example, SRB produce hydrogen sulfide (H₂S), which reacts with metals like iron to form iron sulfide (FeS) - a compound that weakens the metal structure and accelerates corrosion  

Even after aggressive cleaning or treatment, several factors cause MIC to return:  

Why corrosion “comes back” after cleaning  

1. Biofilms are never fully removed  

Mechanical or chemical cleaning often removes the visible layer, but:  

• Microbes can persist in microscopic cracks, pits and weld defects.  

• Cells can retreat into pores, gaskets, dead legs, crevices and rough internal surfaces. 

• The biofilm matrix itself can remain partially intact, providing a scaffold for recolonization. One single surviving cell is enough to restart the process once water and nutrients are present again.   

  
  

  

  biofilm composed of two corrosive microorganisms one methanogen and one sulfate-reducing bacterium

2. Dormant cells and spores  

Many microorganisms can enter a dormant state:  

• Some form endospores (e.g. certain Firmicutes) that resist heat, chemicals and desiccation.  

• Others persist in low-activity states with minimal metabolism but remain viable.  

When conditions become favorable again (appropriate moisture, nutrients, temperature, and specific electron donors/acceptors), these dormant populations reactivate and rebuild biofilms quickly.  

3. Continuous re-inoculation  

Even if a surface is temporarily disinfected, industrial systems are rarely sterile:  

• Fresh water, process fluids, and ambient air can introduce new microbes.  

• Biofilms upstream can slough off and seed downstream surfaces.  

• Leaks, condensation and contaminated substances continuously reintroduce organisms.  

Without ongoing control measures, recolonization is almost guaranteed.  

4. Stable environmental conditions  

If the underlying conditions that favor MIC are not altered, the system behaves like a “corrosion greenhouse” where microorganisms thrive happily:  

• Stagnant or low-flow zones  

• Periodic wetting and drying  

• Warm temperatures (often 20–40 °C, ideal for many relevant microbes)  

• Presence of nutrients (carbon sources, nitrogen, phosphorus, sulfate, etc.)   

Cleaning acts as a temporary reset, but the environment still selects for microbial growth and biofilm formation.   

deposit accumulation in a low flow zone of steel pipeline

Though the important question is: How bad can it get and what are its consequences? 

The severity of MIC ranges from cosmetic damage to catastrophic failure.  

1. Pitting and localized attack  

MIC is notorious for pitting corrosion - deep, narrow holes. Pits can penetrate a wall section while the surrounding metal still appears sound. It deceives you into thinking your structure is safe and secure. Localized thinning is especially dangerous because it is difficult to detect by casual inspection.  

 pitting corrosion on steel

 2. System performance and safety  

 MIC can cause:  

• Leaks in pipelines carrying water, fuels, chemicals or gases  

• Structural weakening of tanks, vessels and infrastructure  

• Blockages due to biofilm buildup and corrosion deposits  

• Contamination (e.g. fuel contamination, imbalances in water systems, product spoilage)  

• Safety incidents, such as flammable or toxic releases, fires, or explosions  

Several documented pipeline failures and tank leaks worldwide have been traced to MIC as a significant contributing factor.  

 heavy metal sufide deposit in surface gas infrastructure

3. Economic cost  

The global cost of corrosion is estimated to be US$2.5 trillion, which is equivalent to 3.4% of the global GDP (2013) when including maintenance, downtime, replacement, and failures. It is believed to contribute a significant fraction of these costs in sectors such as oil and gas, hydrogen, maritime, power generation, water utilities and chemical processing.  

Beyond direct repair and replacement, MIC adds hidden costs:  

the hidden cost iceberg of corrosion

In other words: “bad” can be very bad - up to multimillion-euro losses or more per incident in critical infrastructure.  

How to prevent MIC in a sustainable way  

Long-term MIC control is not about occasional “emergency” treatments. It requires a preventive, integrated management approach:  

• Risk assessment: identify high-risk systems and components based on environment, materials, and operating history.  

• Defined monitoring program: regular sampling, corrosion measurement, material coupon tests and inspection.  

• Control strategy with feedback: adjust biocide and inhibitor programs based on monitoring results, not guesswork.  

• System design and retrofit: reduce stagnation, improve drainage, and facilitate cleaning in future maintenance cycles.  

Such an approach reduces not only the likelihood of severe damage and downtime, but also the overuse of chemicals, aligning MIC control with environmental and regulatory requirements.  

Many MIC control strategies fail in the long term because they address symptoms rather than root causes. Corrosion that returns after cleaning is often a sign that microbes are part of the story. The result can be severe, localized damage with high economic and safety consequences.   

However, MIC is not an unsolvable mystery. By recognizing its biological nature, investing in proper monitoring, and combining chemical, mechanical, operational and design measures, it is possible to move from crisis response to proactive control. The goal is not just to clean what is visible, but to reshape the environment so that biofilms struggle to form and corrosion has fewer chances to return.