Microbiological influenced corrosion (MIC) has gotten much attention as one of the unique forms of corrosion. It occurs as a result of the life processes of several types of microorganisms or it can occur due to the simple formation and growth of colonies of these organisms on wetted metal surfaces. Different microorganisms can produce a variety of effects that lead to accelerated corrosion. These include formation of black iron sulfide deposits that create corrosion cells on carbon steel; production of acids and the resulting lowered pH; formation of hydrogen sulfide and corrosion in oil and natural gas applications; driving the electrode potential of stainless steels more anodic and out of their passive state into the pitting range or by generating hydrogen so that high strength steels are made more susceptible to hydrogen embrittlement or SCC. These organisms often form slime deposits on metallic surfaces called biofilms. The presence of the films alone – without added other effects – can cause crevice corrosion.    

Most alloys are subject to MIC although titanium alloys offer resistance. Carbon steels and cast irons are the materials where MIC is most often encountered. These materials typically suffer general corrosion while stainless steels and aluminum alloys experience pitting and crevice attack. If MIC is confirmed there is little or no value in switching from carbon steel to stainless steel to gain more resistance.

The microbes involved with MIC need three conditions to initiate and reproduce on a surface. These are water, some type of nutrient, e.g., a source of hydrocarbons, nitrogen or phosphorus, and some source of energy. The latter can be supplied by sunlight or by the redox (oxidation and reduction) chemical reactions that occur in all corrosion. Some microbes are anaerobic such as the very common sulfate reducing bacteria (SRB) and need an oxygen-free environment to live and grow while others require oxygen to live. MIC microbes can survive over a wide range of temperatures – from subzero up to maximum of about 200 degrees F. They cannot live at very high temperatures. Submerged colonies of microbes attach to metal surfaces and grow quickest when exposed to stagnant or low velocity liquids. They can survive over a wide range of pH values.

MIC may occur in several types of applications. Industrial water-handling systems are probably the most common. Others include the exterior of underground pipelines, in fire sprinkler systems and on marine piers and other structures exposed to seawater. In the past MIC has been a significant problem on aluminum in the lower sections of commercial aircraft below galleys and restrooms when adequate constraints on liquid spills have been overlooked. Stainless steels are particularly susceptible to MIC at welds.

Control methods for MIC include using biocides to kill the microbes and adding chemical corrosion inhibitors to retard corrosion. Coatings as well as design changes, e.g., assuring there are no internal areas in a piping system where there is incomplete drainage or other stagnant flow conditions exist, can be useful. Specific and diverse knowledge is needed to effectively analyze and combat MIC problems. The individuals that do the work need to be competent in the process chemistry of the given system, laboratory methods to identify and quantify specific microorganisms, effects of different corrosion inhibitors combined with biocides and methods to assess and assure the continuing success of different treatment schemes.  Once implemented it is essential that trained personnel continue to do sampling and monitor the effectiveness of the given control method. Adequate control of MIC is definitely not something that the user can “install and forget about”.

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