In addition to deposit-forming microorganisms and iron-depositing, sulfate-reducing and nitrifying bacteria, there are two other mechanisms that cause MIC.
Denitrifying Bacteria. These convert nitrates and nitrites to nitrogen and possibly ammonia, resulting in localized high pH of 9 to 11, thus attacking copper alloys. Denitrifying bacteria also are known as ammonia producers.
Acid Producers. These convert both organic and inorganic compounds into acids. Organic acids include acetic, formic and butyric. Inorganic acids include sulfuric, nitric and hydrogen sulfide.
What Does MIC Look Like?MIC affects different materials in different ways (table 4). MIC on mild steel often leaves characteristic concentric rings that are visible after cleaning the surface (figure 1). The concentric rings indicate the growth of a sulfide-producing (sulfate-reducing) bacteria colony that has produced hydrogen sulfide.
MIC on stainless steel leaves a number of characteristic signs after the surface is cleaned. Pitting can be very localized. Figure 2 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 aluminum alloys is characterized either by localized pitting or general metal removal. If the microorganisms are acid- 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.
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 sludges.
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.