Inspectioneering Journal

99 Diseases of Pressure Equipment: Microbiologically Influenced Corrosion (MIC)

By John Reynolds, Principal Consultant at Intertek. This article appears in the May/June 2005 issue of Inspectioneering Journal.

Microbiologically Influenced Corrosion (MIC) is caused by biological growth, i.e. organic slime (typically bacteria, algae, and fungi) in water under low flow or stagnant conditions. The industry experiences it in cooling water systems, piping, vessels and storage tank bottoms where the conditions are ripe for it. MIC manifests itself as pits, often under deposits, tubercles, or slime. These pits sometimes have the unique characteristic (which often makes MIC readily identifiable) of subsurface “caverns” that are larger than the pitted opening to the surface. The MIC “bug” can affect most any type of construction material, but is most often a problem with carbon steel and 300 series stainless steel, partially because those are the two most common construction materials. Not infrequently MIC strikes when equipment that has been hydrotested is not completely drained and dried and kept dry before installation or use. An effective specification, procedure and management system is necessary for hydrotesting in order to make sure that those involved know that the MIC “bug” is waiting to bite the uninformed.

One of the reasons MIC is so insidious is that it can “drill through” a pipe or tube in a matter of days or weeks, if equipment is left with stagnant, untreated water in it. Imagine your unhappy surprise when a piece of equipment that was hydrotested 6 weeks ago is now ready to be put in service and in so doing, it sprays liquid out like a shower nozzle. Aluminum, 300 series SS’s, brass and carbon steels are especially vulnerable to MIC. As you might expect, the proper treatment for preventing MIC in water systems is the application of biocides that kill the organic compounds. Now you know, if you didn’t already, the primary reason why chlorine or chlorinated compounds are added to CWT’s. The MIC bugs thrive when their CWT’s are inadvertently contaminated with hydrocarbons, or when someone temporarily stops the chlorination without knowing the potential consequence of their actions. Firewater systems that are not adequately treated with biocides often give rise to MIC, especially when firewater is used for hydrotesting and the equipment is not adequately drained and dried.

Are all the right people at your facility aware that through-wall pitting penetration by MIC can happen very rapidly under stagnant water conditions in storage tanks, and in equipment left with standing water (with biological contamination) after hydrotesting and during temporary shut downs?

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