MIC Basics

Microorganisms pervade our environment and readily "invade" industrial systems wherever conditions permit. These agents flourish in a wide range of habitats and show a surprising ability to colonize water rich surfaces wherever nutrients and physical conditions allow. Microbial growth occurs over the whole range of temperatures commonly found in water systems, pressure is rarely a deterrent and limited access to nitrogen and phosphorus is offset by a surprising ability to sequester, concentrate and retain even trace levels of these essential nutrients.

Many engineers continue to be surprised that such small organisms can lead to spectacular failures of large engineering systems. The microorganisms of interest in Microbiologically Influenced Corrosion (MIC) are mostly bacteria, fungi, algae and protozans.

Bacteria are generally small, with lengths of typically under 10 µm. Collectively, they tend to live and grow under wide ranges of temperature, pH and oxygen concentration. Carbon molecules represent an important nutrient source for bacteria.

Fungi can be separated into yeasts and molds. Corrosion damage to aircraft fuel tanks is one of the well-known problems associated with fungi. Fungi tend to produce corrosive products as part of their metabolisms; it is these by-products that are responsible for corrosive attack. Furthermore, fungi can trap other materials leading to fouling and associated corrosion problems.

Protozans are predators of bacteria and algae and therefore potentially mitigate microbial corrosion problems.

MIC is responsible for the degradation of a wide range of materials. Bacteria can exist in several different metabolic states. Those that are actively respiring, consuming nutrients, and proliferating are said to be in a "growth" stage. Those that are simply existing, not growing because of unfavorable conditions, are said to be in a "resting" state.

Some strains, when faced with unacceptable surroundings, form spores that can survive extremes of temperature and long periods without moisture or nutrients, yet produce actively growing cells quickly when conditions again become acceptable.

The latter two states may appear, to the casual observer, to be like death, but the organisms are far from dead. Cells that actually die are usually consumed rapidly by other organisms or enzymes. When looking at an environmental sample under a microscope, therefore, it should be assumed that most or all of the cell forms observed were alive or capable of life at the time the sample was taken.
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