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.
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.
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.
Typical MICMicrobiologically 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.