Microbiologically Influenced Corrosion (MIC)
MIC refers to corrosion that is influenced by the presence and activities of microorganisms and/or their metabolites (the products produced in their metabolism). Bacteria, fungi and other microorganisms can play a major part in soil corrosion. Spectacularly rapid corrosion failures have been observed in soil due to microbial action and it is becoming increasingly apparent that most metallic alloys are susceptible to some form of MIC. The mechanisms potentially involved in MIC are summarized as:
- Cathodic depolarization, whereby the cathodic rate limiting step is accelerated by micro-biological action.
- Formation of occluded surface cells , whereby microorganisms form "patchy" surface colonies. Sticky polymers attract and aggregate biological and non-biological species to produce crevices and concentration cells, the basis for accelerated attack.
- Fixing of anodic reaction sites, whereby microbiological surface colonies lead to the formation of corrosion pits, driven by microbial activity and associated with the location of these colonies.
- Underdeposit acid attack, whereby corrosive attack is accelerated by acidic final products of the MIC "community metabolism", principally short-chain fatty acids.
Certain microorganisms thrive under aerobic conditions, whereas others thrive in anaerobic conditions. Anaerobic conditions may be created in the micro-environmental regime, even if the bulk conditions are aerobic. The pH conditions and availability of nutrients also play a role in determining what type of microorganisms can thrive in a soil environment. Microorganisms associated with corrosion damage are classified as:
- Anaerobic bacteria that produce highly corrosive species as part of their metabolism.
- Aerobic bacteria that produce corrosive mineral acids.
- Fungi that may produce corrosive by products in their metabolism, such as organic acids.
- Apart from metals and alloys they can degrade organic coatings and wood.
- Slime formers, that may produce concentration corrosion cells on surfaces.