Bugging Water Systems for Corrosion Control
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There are numerous methods for preventing and controlling corrosion in power plant service water systems, including chemical flushing, organic coating (paint), and the most intriguing way of all to me--the introduction of bacteria. As if this weren't interesting enough, these little critters are called aerobic bacteria. I picture a gym class filled with tiny bacteria pumping iron and training to enter those pipes filled with evil corrosion. Sometimes life is stranger than science fiction!
EPRI (the science and technology organization for the energy industry) estimates that corrosion costs the U.S. electric power industry $5-10 billion each year, accounting for half the forced outages in steam generating plants. Additionally, they estimate that "corrosion increases the cost of electricity more than it increases the cost of any other U.S. product, adding over 10% to its price."
Putting Biofilm to Work
The problem has always been that metal surfaces (e.g., in pipes), when exposed to most process waters, are colonized by a variety of microbial species that develop in microcolonies before merging to form a biofilm--a very thin coating, or film that forms over the metal surfaces. This biofilm creates a barrier between the metal surface and the water, and has a lot to do with the corrosion rate of the metal surface. For example, anaerobic sulfate-reducing bacteria (SRB--oxygen-producing bacteria) in the biofilm leads to elevated oxygen levels, thus speeding corrosion. According to Barry Syrett, manager of Corrosion Science and Technology at EPRI, "biofilms are usually damaging, causing pitting corrosion even in resistant metals such as stainless steel and aluminum."
Recent tests on corrosion behavior studied in a continuous flow system by the University of California, Irvine (UCI) and University of Nevada, Reno (UNR) have shown that introducing aerobic (oxygen-consuming) bacteria can initially cause as much as a 35-fold decrease in the corrosion rate of mild steel, and significant decreases in aluminum (10-fold) and copper (5-fold), by consuming the oxygen that would otherwise oxidize the metal. Funded by EPRI, these tests conclude that the presence of aerobic bacteria initially causes the biofilm to absorb oxygen, which then becomes depleted, slowing the rate of corrosion. The poorly aerated surface, however, encourages the growth of anaerobic SRB, which ultimately reduces your gains. Says Syrett, "the work being done is an effort to genetically engineer aerobic bacteria in order to fight corrosion by reducing access of oxygen to the metal surface, and releasing chemicals that kill SRB or that inhibit corrosion in some other way."
RoboBacteria
Putting biofilms to work in the fight against corrosion is the dream of researcher Thomas Wood of UCI, who believes that the bacteria can be engineered to be protective of metal surfaces. One of his goals is to genetically engineer a natural metal colonizer to secrete a protective polymer to prevent corrosion. In the EPRI study, one genus of bacillus was chosen based on its ability to efficiently secrete protective polymers. According to Wood, "The most likely scenario is that scientists will take a sample of bacteria already thriving in the biofilm at a specific site, give [the bacteria] the genes to manufacture antimicrobials, and reintroduce [the bacteria] to the site."
An additional advantage, that has no parallel in competing corrosion control methods, is the engineered biofilm's self-repair feature. It was discovered that reformation of the biofilm, if damaged by abrasion or some other mechanical process, is especially quick in the first two hours, regenerating itself completely within 20 hours. This ability to repair itself is advantageous because of the biofilm's potential corrosion protection attributes.
The benefits of bugging your water system with engineered aerobic bacteria are numerous and far-reaching. The result of this method, reduction of corrosion, leads to fewer outages, lower maintenance costs (parts and labor), extended equipment life, and more time for your maintenance crew to focus on other issues.
An equal opportunity bug, this corrosion control method is equally applicable to large energy company plants and small independent power producers, as well as regenerators, and other industrial applications.
Field-testing of this engineered bacteria is currently being performed in the 6-million gallon chilled water system operated by UCI's Central Generating Facility. This will allow a study of more realistic conditions in which a variety of bacteria co-exist and interact. Future laboratory work will include a more detailed examination of bacteria that can release substances that control the growth of SRB or control corrosion in some other way. EPRI's plans are to further field test the corrosion protection capabilities of the most promising biofilms at industrial or power plant sites this year. "If the research is successful, full scale demonstrations could start in 2001," says Dr. Syrett.
N/A="steve">Steve Hoffman is president of <%=company%>, a California-based firm that specializes in writing for the energy industry. His column appears every Monday on poweronline.
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