Microbial Influenced Corrosion (MIC) | The Invisible Heat Exchanger Killer

Heat exchangers that don’t receive regular preventative maintenance risk costly and damaging effects of microbial influenced corrosion (MIC). 

‍Part of AHI Marine heat exchanger article series

What Is Microbial Influenced Corrosion?

MIC is corrosion affected by the presence or activity, or both, of microorganisms

as defined by the American Society For Testing and Materials (ASTM).

Microbial Influenced Corrosion & Marine Heat Exchangers

The environmental conditions of temperature, pH, oxygenation and continuous flow of raw water in marine heat exchangers create the perfect conditions in which millions of algae, bacteria and funghi can thrive if unchecked.

The variety of dissimilar metals in the heat exchanger unit are under continuous threat of corrosion that is accelerated by microbial growth.

The Role of Biofilm & MIC

Waterborne microbes deposit onto the surfaces of heat exchangers eventually creating a persistent biofilm layer causing changes to local environmental conditions and biofouling. 

Microbes act as biological catalysts accelerating standard corrosion processes. 

Harsh corrosive metabolites and electrochemical gradients caused by the microbes and biofilm cause lasting damage to heat exchangers if left untreated:

Physical Effects On Corrosion Process

• Biofilms act as a barrier that limits the penetration of corrosion inhibitors and prevents effective surface passivation.
• Their presence restricts oxygen diffusion creating localized environmental gradients.
• Previously mechanically cleaned areas are prone to accelerated re-colonisation

Electrochemical Effects on Corrosion Process

Biofilm areas change the local environmental conditions. Areas of build up known as cells create electrochemical gradients when interspersed between clean surface regions of unaffected environmental electrochemistry.

• Oxygen depletion beneath the biofilm produces anaerobic, anodic regions, while adjacent cleaner oxygen-rich areas act as cathodes, driving localized corrosion.
• These conditions promote galvanic-like corrosion mechanisms, including pitting and crevice corrosion.

Here corrosion involves electron flow through the metal from the anode to the cathode, where oxygen is the electron acceptor - under aerobic conditions.

Biochemical Contributions to MIC

Anaerobic microorganisms in the biofilm thrive in low oxygen environments and instead of using oxygen use sulfates and nitrates to derive chemical energy.

In the absence of their preferred reactants a less efficient fermentation-like redox reaction occurs producing various corrosive organic acids.

• Microorganisms within biofilms act as biological catalysts, accelerating existing electrochemical corrosion reactions.
• Metabolic activity generates corrosive by-products (e.g., organic acids, sulfides), which can directly attack metal surfaces or alter local pH.
• Anaerobic microbes, such as sulfate-reducing bacteria (SRB), thrive in oxygen-depleted zones and can (according to historic studies) contribute to corrosion through sulfide production and cathodic depolarization effects - though this process is under debate in the metallurgy community.

Mechanisms of Corrosion

• Oxygen Gradient Corrosion Localized corrosion influenced by presence of deposits of corrosion products. For example a biofilm or metal deposition by metal-oxidizing bacteria that is formed in a contiguous patch-like arrangement causing oxygen gradient cells. 
• Crevice Corrosion The accumulation of chloride and other aggressive anions in a crevice or pit accelerates corrosion.‍
• Electrical MIC Caused by electron transfer in and out of microorganism cells in the biofilm with corrosion resulting from direct and indirect contact with surface metals.
• ‍Metabolite MIC Metabolites released by microorganisms both in aerobic and anaerobic conditions have corrosive effects on heat exchanger metals.

What seems like clean innocuous water flow is in fact a hostile biochemical environment with complex chemical reactions that threaten the integrity of heat exchanger metals and cooling performance.

If raw water flow is the primary cooling mechanism marine heat exchangers will always be subjected to this dynamic biological environment.

MIC & Heat Exchanger Maintenance

The best way to maintain your heat exchanger is to prevent the build up of biofilm and schedule regular servicing with a professional marine engineer.

Prevention and Mitigation

• Biocide Treatment: Regular application of chlorine-based or non-oxidizing biocides to control algae and bacterial biofilm formation.
• Regular Cleaning: Mechanical cleaning (sponge balls, brushing) or chemical cleaning to remove biofilms before they harden.
• Surface Treatment: Using specialized anti-fouling or non-stick coating‍
• Flow Monitoring: Maintaining high turbulence and flow rates to inhibit biofilm.