Arwind Kumar Dubey - 4 May 2017
In this short blog the characteristics of microbiological induced corrosion are outlined. The major factors are discussed which lead to its occurrence as well as units/equipment which are affected by MIC. Finally, a few steps are mentioned as to how to monitor and reduce its presence.
Micro biologically induced corrosion (MIC) is a form of corrosion that is caused by living organisms such as bacteria and algae, etc. It is often associated with the presence of organic substances, and is usually observed as localized pitting under deposits. The MIC damage mechanism is often characterized by cup-shaped pits within pits occurring in carbon steel as well as on other metal surfaces.
Major factors which affect MIC
Microbiological induced corrosion is usually found in services where water is always, or at least sometimes, present, especially where stagnant and low flow conditions exist, which help to promote the growth of microorganisms. It should be noted that microorganisms can also survive and grow under very harsh conditions as well. For example, not only where there is a lack of oxygen, but also where there is a lack of light and where is dark. They are additionally to be found where high salinity prevails, or where a very low right through to a very high pH range exists, and also at temperatures from –17°C to 113°C. Different organisms thrive on different nutrients including inorganic substances, and organic substances. Hydrocarbons or hydrogen sulfide leakage lead to a massive increase in bio fouling and corrosion.
Fig. 1. Localized corrosion due to MIC on a leaked tube surface.
Units or equipment that are affected by MIC
MIC is most often found in storage tanks, heat exchangers, and pipes that contains stagnant or low flow water. It also exists in equipment in which hydrotest water has not been fully removed or in equipment has been left exposed to the elements. Further, it is to be found in product storage tanks and water-cooled heat exchangers in units where the cooling water has not been properly treated.
How to monitor MIC
There are a number of ways in which MIC can be monitored:
(a) Water which has a foul smell can possibly be affected by MIC.
(b) Special bioprobes have been designed to monitor for evidence of microorganism and these are able to detect possible MIC damage.
(c) Fouling and potential MIC damage can also be discovered by the increase in the loss of duty within a heat exchanger.
(d) The effectiveness of treatment is monitored by measuring the microbe count.
Fig. 2. Morphology of MIC on a leaked tube surface.
Control of MIC
(a) Systems which contain water should be treated with biocides and if necessary bio-dispersant should also be used in the system.
(b) Biocides can check for MIC but cannot eliminate the microbes so continued treatment is always necessary.
(c) Flow velocities should always be maintained above minimum levels. Low flow or stagnant zones need to be avoided.
(d) Hydrotest water should be empted for the equipment as soon as possible to avoid MIC.
(e) Pigging and chemical cleaning with biocide is required in order to completely remove organic deposits and organisms.
Fig. 3. A SEM micrograph showing the presence of microbes on a leaked tube surface.
The reference for these comments are based on:
(1) API 571 damage mechanisms affecting fixed equipment in a refinery.
(2) PK Butene-1 Pump Around Coolers Tubes Leakages Analysis.