Microbiological organisms (or, microorganisms) in cooling tower systems are categorized into three groups:

• Bacteria
  Bacteria are responsible for much of the MIC within cooling water systems.
• Fungi
  Fungi can infect and destroy cooling tower lumber.
• Algae
  Algae are microscopic plants or plant-like organisms containing chlorophyll. Algae range in color from colorless through green and blue to red. They may float free or attach themselves en masse to any available surface.

Bacteria are unicellular organisms that possess no well-defined nucleus and are devoid of the green pigment chlorophyll. Oxygen can be toxic to some organisms, so they seek out areas within a cooling tower system that shield them. This characteristic results in bacteria being found under algae growths, bacteria slime, fouling deposits and cooling tower basin sludge.

Therefore, MIC takes on a completely different attitude when dealing with cooling water systems. They can be present in many different areas and environments within the same system.

All common metals are thought prone to MIC. In water systems, it is more likely to occur on copper and copper alloys and cast iron and mild steel, although it has also been reported on stainless steels and aluminium.

MIC affects different materials in different ways. As for examples:

• Mild steel: Pitting and shallow localized attack
• Galvanized steel: Pitting and general corrosion
• Stainless steel:  Severe localized corrosion and pitting

MIC on mild steel often leaves characteristic concentric rings that are visible after cleaning the surface (following figure). The concentric rings indicate the growth of a sulfide-producing (sulfate-reducing) bacteria colony that has produced hydrogen sulfide.

Fig: This cleaned mild steel corrosion coupon (left) is an excellent example of sulfate-reducing bacteria colonies growing and producing concentric rings of corrosion. The start of the corrosion is the deep pit. As the colony grows, the rings of corrosion occur. This coupon is 1x4x0.0625", but only 2" are shown. In another example, this cleaned mild steel pipe (right) was installed in a cooling water system. The pit and concentric rings are typical of sulfate-reducing bacteria. The MIC area is marked with a crayon

MIC on stainless steel leaves a number of characteristic signs after the surface is cleaned. Pitting can be much localized. Right figure shows a MIC-induced perforation that occurred in just six weeks. Another typical corrosion characteristic is a deep penetration of the stainless steel that is similar in appearance to the tunneling caused by termites in wood.

MIC on copper alloys can be of several types. Pitting can occur with sulfide-producing bacteria forming copper sulfide corrosion. Also, denitrifying or ammonia-producing bacteria cause localized corrosion. A blue/black area at the site of the MIC identifies this type of corrosion.

MIC on aluminum alloys is characterized either by localized pitting or general metal removal. If the microorganisms are acid-producing or alkaline-producing, then a general metal or localized attack occurs. Sulfate-reducing bacteria or other anaerobic microbes attack the oxide protection film on aluminum and cause localized and/or pitting corrosion. MIC on galvanized steel is similar to that of aluminum alloys, often exposing the mild steel when the galvanizing is gone.