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Microbiologically Influenenced Corrosion (MIC)
1) What is MIC?
“MIC” is an acronym for microbiologically influenced corrosion, a mode of corrosion incorporating microbes that react and cause the corrosion or influence other corrosion processes of metallic materials.
2) What causes MIC?
MIC is caused by bacterial microbes in combination with four other environmental conditions: metals, nutrients, water and oxygen. These MIC-causing bacteria are commonly found throughout all types of water supplies. If the water in your sprinkler system tests positive for these bacteria, it doesn’t mean that you have MIC. All five environmental conditions must be present in a sprinkler system to cause the rapid growth of the MIC-related bacteria.
3) Why should you be concerned about MIC?
As MIC bacteria grows, consumption of the metal pipe occurs and can result in the formation of tubercles. During this process, pitting of the interior lining of the sprinkler pipe may occur. Regardless, the hydraulic characteristics of the sprinkler pipe is negatively impacted by the tubercles. Last, the tubercles and loose scale can cause sprinkler heads or valves to become blocked or not to function properly.
4) How is MIC detected?
Building maintenance personnel may suspect that MIC is present because of pinhole leaks in the pipes, increased amount of debris in the pipes, sprinkler heads being plugged, or due to the color or smell of the water. The least intrusive method to ascertain whether bacteria associated with MIC are present is for the liquid within the pipe to be tested. The best way to determine if you have MIC in your pipes is to take out sections and have them evaluated by a microbiologist or metallurgical engineer. This process also allows for other types of corrosion to be found.
How MIC Tubercles Form
5) Why should you test for MIC?
NFPA 25 requires an obstruction inspection every 5 years or whenever there is evidence of rust, foreign debris or pinhole leaks. Further, it requires that if any tubercles or slimes are observed, they shall be tested for indications of MIC (NFPA25-2002 Section 13.2.1.2).
NFPA 13 requires the building owner to give the installing contractor an Owner’s Certificate if they know that they have MIC in their system.
6) What can be done if MIC is found in your sprinkler system?
FM Global recommends the following actions if you discover MIC at your facility:
Step #1- Diagnosis of the corrosion and of the condition of piping,
Step #2 - Assessment of possible alternatives,
Step #3 - Cleaning of the piping,
Step #4 - Treatment of the local water with disinfectant and biocides,
Step #5 - Recharge of the system and go through the acceptance test.
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