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
     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
     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
     "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
                                        -Cornelia Weil