Microbiologically Influenced Corrosion
SIB-96-148

Microbiologically influenced corrosion (MIC) is the initiation or acceleration of corrosion due to the interaction between microbial activity and corrosion processes. MIC was first identified over 100 years ago and was described mechanistically as early as 1934. However, MIC was not considered seriously as a practical form of degradation of modern industrial systems until the mid-1970s when the involvement of microbes in the rapid, through-wall pitting of stainless steel water tanks exposed to surface water was positively identified as the cause of the otherwise puzzling attack. Since that time, MIC has received considerable attention in power generation, oil production, chemical processing, transportation, and the pulp and paper industries. During the 1980s and continuing into the 1990s, the electric power industry, particularly the nuclear industry, has devoted increased attention to MIC and other forms of degradation that affect the reliability, and operating and maintenance costs of the balance of plant systems. The economic impact of failures in such systems in large nuclear units can be extremely costly; on the order of $1,000,000 per day. Costs for chemicals and delivery systems for water treatments to prevent MIC and biofouling can approach $1,000,000 per year. Several plants have been forced to undertake replacement or to make extensive repairs of their service water systems at a cost of $30,000,000 per plant. Regulators, such as the United States Nuclear Regulatory Commission, industry organizations like the Electric Power Research Institute (EPRI), or professional organizations (e.g., NACE-International) have devoted increased attention to MIC.

Carbon steels may experience random pitting, general corrosion, or severe degradation in flow as a result of MIC. Tubercles (comprised of corrosion products, microbes, the sticky exopolymer associated with both living and dead cells, and debris) often form on carbon steel pipes and other components. The tubercles create a hydraulic resistance to cooling water flow as well as sites for additional microbial activity. Tubercles can grow together, eventually becoming a severe impediment to cooling water flow. Pitting is also often observed beneath tubercles as mechanical and chemical conditions are established that encourage localized corrosion effects. Once tubercles have formed, microbial activity at the metal surface (i.e., beneath the tubercle) is effectively insulated from the bulk fluid, its level of aeration, and most water treatments. A wide variety of bacteria, from the aerobic acid formers to the more well known anaerobic sulfate reducing bacteria, can contribute to tuberculation and pitting of carbon steels.

Stainless steels are also subject to localized attack as a result of MIC. Occlusion of pipes is generally not observed, however, tubercles (much smaller than those associated with carbon steels) are often present. Rusty streaks appear on piping or tanks, often running both up and down the surfaces.

As shown in the scanning electron micrograph to the right, MIC of stainless steel weld metal often produces through-wall pits as essentially all of the austenite is removed from the duplex microstructure, leaving behind a skeleton of delta ferrite.

MIC of stainless steels most often produces pitting at welds, generally in the weld metal itself. Closed pits, typified by tiny entrance and exit holes, with a cavernous subsurface defect, are often observed as illustrated below.

Copper-based alloys are also subject to MIC despite copper's reputation as a material toxic to most organisms. Failures in these materials are generally manifested by pitting, erosion-corrosion, and occasionally by stress corrosion cracking.

The "diverse and redundant" design philosophy applied to nuclear power plants leaves a large number of standby and redundant systems, some of them safety-related, in extended periods of wet layup. This situation permits microbial consortia to become established in these systems, makes treatment difficult, and puts these systems at particular risk to degradation from MIC. Further, any large plant that takes years to construct, where systems may be left full of water used for hydrostatic testing for long periods of time, may exhibit susceptibility to MIC.

Structural Integrity Associates (SI) has been involved in all aspects of MIC, including technology transfer (books and software), diagnosis, consulting, and monitoring, since 1986. George Licina of SI is the author of both of the Electric Power Research Institute's (EPRI) reference documents on MIC, "Sourcebook for Microbiologically Influenced Corrosion in Nuclear Power Plants", and "Detection and Control of MIC - An Extension of the Sourcebook". The information in these books led to the development of MICPro, a pc-based predictive advisor on MIC that uses the knowledge in the Sourcebooks and an expert systems approach to provide users with an easy-to-use tool for predicting the susceptibility of systems and components to MIC and corrosion.

Mr. Licina has also performed "hands-on" investigations of raw water corrosion and potential MIC situations under EPRI and utility sponsorship. One such project involved the evaluation of corrosion of existing and candidate replacement materials (including weldments) for the service water system of Duke Power Company's Catawba Nuclear Station. For this project, electrochemical methods were used as a supplement to the corrosion coupons that were installed and monitored by Duke Power. Others have included root cause failure analysis, on-line monitoring of biolfilm activity, open circuit potentials, corrosion of coupons, and pipe spools.

Monitoring is particularly important for early detection and effective control of MIC. Biofilms can form on metal surfaces very rapidly. Mitigation measures such as water treatment are far more likely to be effective during the early stages of biofilm formation. Once a mature biofilm has become established, the slime layer produced by the microorganisms along with corrosion products and suspended and dissolved solids from the water, make the biofilm extremely resistant to the effects of biocides or other water treatments. An early warning of the onset of biofilm is essential for effective control. A sensor that also provides an indication of the effectiveness of treatments provides the user with a powerful tool to control biofilm. If biofilm is controlled, MIC is controlled.

SI has developed such a sensor, the BIoGEORGE™ probe, that provides the capability for on-line monitoring of biofilm activity. The probe has been shown to be an effective tool for tracking biofilm activity in a variety of cooling water chemistries, including both hard and soft fresh waters, brackish water, seawater, and in highly unpredictable and difficult-to-monitor gas field produced water.

BIoGEORGE probes have been installed at TVA's Browns Ferry Nuclear Plant (October 1991 and October 1993), at Philadelphia Electric Company's Limerick Station (September 1992), the United States Miliary Academy as part of an Army Corps of Engineers monitoring project (June 1993), and Alabama Power Company for demonstrations of monitoring of corrosion and fouling in their fossil and nuclear facilities. A complete corrosion monitoring project, including a BIoGEORGE probe, was performed for Pennsylvania Power & Light Company at their Susquehanna Steam Electric Station (June, 1994). Three BIoGEORGE probes were installed at Pacific Gas & Electric Company's Pittsburg Power Plant in 1994. Two probes were also installed at Southern California Gas Company's Montebello Station for tracking biofilm activity in gas field produced waters.

SI has also applied its expertise in nondestructive examination to inspection for MIC. The closed tunneling pits that are often produced in stainless steels by MIC attack are difficult to detect by methods other than radiography and are virtually impossible to size by standard volumetric methods. SI's Focused Array Transducer System (FATS) has been demonstrated to be an effective method for characterizing MIC in stainless steel weld metal.

If you would like to obtain additional information regarding the BIoGEORGE probe, MIC, or SI's capabilities in this area, please contact SI.

BIoGEORGE is a trademark of Structural Integrity Associates. Patent rights to the probe, U.S. Patent Numbers 5,246,560 and 5,356,521, are owned by EPRI. The probe is marketed and manufactured by Structural Integrity Associates under a license from EPRI.