Messenger - Vol. 2, No. 2, Page 12 Winter 1993 On Campus Corrosion culprit Marine corrosion costs the U.S. some $25 billion each year in rusted-out ships' hulls, weakened offshore oil platforms and pitted pipes at coastal power plants. Sometimes, the culprit causing the corrosion is the microscopic life that befouls metals in seawater. Stephen Dexter, professor of applied ocean science in the College of Marine Studies, investigates the impact that these bacteria have on corrosion and how best to control the growth of bacterial film on metals. Even stainless steel, which is often corrosion-resistant, is susceptible when covered by a bacterial film, Dexter says, because the microorganisms can change the chemistry of the seawater at the metal surface. Dexter has found that the chemistry of the very thin layer of water trapped against the metal surface by the bacterial film can be different from that of the surrounding water. "You can have a normal pH of about 8 outside the surface, but within the film and at the metal surface, the pH can be anywhere from 10 (very alkaline) down to 2 (very acidic), and either pH could help corrosion," he says. Studying the pH changes, organic compounds and heavy metal concentrations produced by the bacteria in a biofilm, Dexter theorizes that the bacterial film accelerates the reaction of dissolved oxygen at the metal surface. "This is important because the oxygen reaction often controls the rate of marine corrosion," he says. His corrosion research group is also looking at how bacterially-produced enzymes can interact with metal oxides and hydrogen peroxide to accelerate the oxygen reduction reaction on the metal surface. Of course, not all types of bacteria have the ability to speed up corrosion. "Sometimes, bacteria don't influence corrosion at all and may stop or slow the process," Dexter says. He also has found that sunlight and the salinity of the water affect the corrosion capabilities of the bacteria. For example, biofilms that grow in the dark under freshwater conditions have a higher tendency to be corrosive than films with photosynthetic algae that grow in the sunlight. Because chloride in salty seawater is known to accelerate the corrosion process, Dexter now wants to be able to specify the maximum level of salinity that certain metal alloys can tolerate before the onset of corrosion. "What I want to determine is at what point do the bacteria in combination with the chloride cause metal to corrode?" he says. "In other words, at what salinity level does the combined effect of chloride and bacteria get you in trouble?" His answers will be useful for coastal power plants, which use alloy tubing for their condensors, and for off-shore oil structures. Eventually, the results of his research also should filter down to commercial fishing and recreational marine operators. Supported largely by Sea Grant funding from the U.S. Department of Commerce, Dexter's research has also received financial support from the Du Pont Co., the Electric Power Research Institute and the Office of Naval Research. -Cornelia Weil