Table 1. MIC is caused by microbiological organisms that produce byproducts such as acids, alkalis or reducing agents, which are corrosive to metals.

What is MIC? It stands for microbiologically influenced corrosion -- yes, corrosion in water systems due to microbiological organisms. These microbes do not "eat" metals such as mild steel, stainless steel, copper alloys or galvanized steels. Rather, they produce byproducts that are corrosive to these metals. They are acids, alkalis or reducing agents such as hydrogen sulfides, ammonia and sulfuric and organic acids (table 1). These wastes or byproducts also can produce gelatinous deposits, often identified as biomasses, that prevent corrosion inhibitors from reaching the metal surfaces. Commonly, this is referred to as underdeposit corrosion.

So, do you have MIC in your cooling tower or chilled water system? Can you identify these microbes? First find out, then deal with them -- the microbes -- to protect your systems from corrosion.

Table 2. 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.

What Causes MIC?

As we all know, cooling towers are air scrubbers; they use air to reduce water temperature. Any airborne bacteria or fungi will be cleaned out of the air and deposited into the cooling tower water and system. Air contains dust particles that can, and often do, contain various bacteria, fungus and algae spores (table 2).

The cooling water also may contain all of these various microbiological organisms -- even when treated by microbiocides -- depending upon whether it is untreated raw water, treated raw water or potable water. If the system has an ineffective biocide treatment, or even an effective program, these organisms may enter and settle into an environment in which they can flourish. MIC microorganisms have been identified in many cooling tower systems that have well-maintained biocide treatment programs. MIC is due primarily to bacteria.

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

MIC organisms require an environment that enables their growth. These requirements include moisture, nutrients and an ideal temperature, usually 40 to 120oF (4 to 49oC). They can live under deposits in flowing cooling water. They can live in the presence, as well as the absence, of oxygen, ammonia, acid or alkali. They can “hibernate” at temperatures below 40oF. Usually, temperatures of 140 to 160oF (60 to 71oC) will kill most MIC microorganisms.

Thus, in a cooling water system, there is almost always the combination of moisture, nutrients and temperature ideal for the growth and multiplication of the organisms. Most often, the presence of deposits provides an ideal environment to shield microorganisms from toxic microbiocides.

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.

Typical MIC

Microbiologically influenced corrosion is caused by one or more mechanisms. I'll begin describing those mechanisms in this issue and continue with the July/August issue.

Deposit-Forming Microorganisms. Simply by forming slime deposits, an oxygen-deficient zone will be established with the anode (where the corrosion attack occurs) under the deposit.

Iron-Depositing Bacteria. These convert soluble iron (ferrous) ions to insoluble iron (ferric) ions that form deposits and thus increase corrosion under the deposits.

Sulfate-Reducing Bacteria. These convert sulfate ions to elemental sulfur and often to sulfides (hydrogen sulfide), which attacks most metals. The bacteria are anaerobes that do not live in the presence of free oxygen.

Nitrifying Bacteria. These convert ammonia or nitrites to nitrates (often, nitric acid), resulting in a pH of 2 to 4 and a localized acid environment that will attack most metals.