In addition to deposit-forming microorganisms and iron-depositing, sulfate-reducing and nitrifying bacteria, there are two other mechanisms that cause MIC: denitrifying bacteria and acid producers.

Table 3. Microbiologically influenced corrosion is caused by one or more mechanisms.

In this column, I'll continue my look at microbiologically influenced corrosion (MIC) in cooling tower or chilled water systems by continuing to look at the mechanisms that cause MIC (table 3). For a thorough description of deposit-forming microorganisms as well as iron-depositing, sulfate-reducing and nitrifying bacteria, see "Water Works" in the May 2004 issue or read it here online [link at bottom of page].

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.

Table 4. MIC affects different materials in different ways. For example, MIC on mild steel often leaves concentric rings that are visible after cleaning the surface.

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.

Figure 1. This cleaned mild steel corrosion coupon (top) 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 1 x 4 x 0.0625", but only 2" are shown. In another example, this cleaned mild steel pipe (bottom) 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 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- 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.

Figure 2. This 304 stainless steel condenser tube, which has been cleaned, was in a cooler with water on the shell side. The pit is characteristic of sulfate-reducing bacteria.

Microbiological organisms in cooling tower systems are categorized into three groups: fungi, algae and bacteria. Bacteria are responsible for much of the MIC within cooling water systems. Fungi can infect and destroy cooling tower lumber. 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 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.