Issue 17, November 2005

Microbiological Influenced Corrosion (MIC) is the corrosion or deterioration of a material, which is initiated and/or accelerated by the activities of micro-organisms.

Cetec has experienced the effect of MIC in chillers and associated pipework and fire system piping on numerous occasions, but MIC is also know to affect non-metallic materials as well, such as plastics and concrete.

MIC is estimated to be represented in the order of one-fifth of all corrosion cases. Furthermore, the cost of untreated or unrecognised MIC can be huge, in terms of loss of flow and in replacement of materials/systems.

Would you want to replace a piping system and put in a corrosion plan, only for the system to fail again due to the fact that MIC was undiagnosed in the first instance?  A costly and unnecessary mistake.

The formation of a biofilm, where micro-organisms accumulate on surface/s, is the starting step. The micro-organisms in the biofilm include fungi, algae, bacteria, viruses and protozoa. The biofouling occurs initially on a micro-scale and this allows it to start to form on small surface imperfections such as weldlines. Underneath the biofilm the chemical properties change compared to that of the bulk environment. These differences ultimately create an environment that can lead to varying degrees of MIC.

Numerous micro-organisms are thought to contribute to MIC. The most common ones can be categorised into sulphate reducing bacteria (SRB), iron oxidising bacteria and acid producing bacteria (APB).

SRBs are characterised by hydrogen sulphide odour and blackened water or black coloured deposits. Iron oxidising bacteria generally form in filamentous clumps and can be detected under microscope by their distinct appearance due the excreted products that grow. This corrosion by iron bacteria often forms tubercles. MIC is often implicated by "active metal" where tubercles are broken off and a shiny surface is revealed after washing (see the image on this page).

Microbiological testing of the corrosion product/s is also recommended to confirm MIC and the type of bacteria involved.

By controlling the temperature of the environment you can reduce corrosion enhancing bacteria. 20- 35⁰C is the most desireable temperature range for growth of most of the micro-organisms.

Do not allow dead-spots to form in your system, if possible. These stagnant environments provide the best environment for biofilms to form and bacteria to grow.

Generally a more alkaline environment is preferred in order to minimise MIC, but some of the bacteria prefer an alkaline environment, so diagnosis of the type of bacteria for the individual corrosion problem is required.

Finally do not rely on just culturing microbiological tests to monitor for MIC, as not all of these organisms will be detected in the testing. Regular visual (annual) assessment of known problem systems is recommended by appropriately experienced and qualified personnel.

If you want to know more about MIC you can contact us here.

Other related articles
Monitoring Corrosion in Cooling Tower Systems
Corrosion and Successful Building Operation
Don't Let Corrosion Eat Away At Your Profits

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