Inspectioneering Journal

Characterizing Microbiologically Influenced Corrosion of Welded Alloys - Innovative NDT Techniques

By Susan W. Borenstein at Structural Integrity Associates, Inc., and George J. Licina at Structural Integrity Associates, Inc. This article appears in the September/October 1995 issue of Inspectioneering Journal.

Low-energy piping system failures in power-generating facilities are often the result of fouling and corrosion. These degradation mechanisms can affect the capacity of piping for fluid-carrying, the heat transfer rates of heat exchangers, and the structural integrity of piping systems. Steels are not immune to pitting, underdeposit corrosion, and microbiologically influenced corrosion (MIC). One advanced inspection technique, using an enhanced ultrasonic inspection system coupled with special imaging software, has demonstrated the capability to accurately image and size closed tunneling pits that often form in austenitic stainless steel weldments due to MIC.

Microorganisms have the capability to adhere to surfaces, grow, and form biofilms. Often biofilms create operational difficulties, such as when slimes are produced on heat exchanger tubes resulting in loss of heat transfer capabilities. Sometimes corrosion occurs beneath the biofilm. Microbiology influenced corrosion (MIC) is the term for the initiation or acceleration of corrosion by microorganisms or their metabolites, usually on metal surfaces (Borenstein, 1994).

Localized corrosion, such as pitting and crevice corrosion or underdeposit corrosion, can be damaging and difficult to monitor. Pitting occurs often on exposed surfaces. Crevice corrosion typically occurs in cracks or crevices between mating surfaces, such as gasketed flanged connections. Pitting and crevice corrosion often occur on film-protected (passive) metal surfaces, such as austenitic stainless steels. Underdeposit corrosion results when sediment or corrosion products produce a creviced environment. Some instances of MIC may be considered a form of crevice corrosion (or underdeposit corrosion) when the exopolymer film formed by the microorganisms creates a "living crevice."

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