When Marie Zhuikov pulled her dock out of Wilson Lake for the winter, she found the steel members of the structure encrusted with the same kind of tubercles that have caused so much corrosion damage throughout the Duluth harbor basin. As a senior science communicator for the Wisconsin Sea Grant, Zhuikov knows a thing or two about the subject and showed photos of the dock located near Cotton to colleagues who confirmed her suspicions.
Chad Scott, a principal partner at AMI Consulting Engineers, wasn't surprised. Following dive inspections of several local marine facilities in the 1990s, Scott was among the first to sound the alert that steel in the harbor was being attacked by some type of aggressive freshwater corrosion.
That corrosion has been well documented in recent years and attributed to iron-reducing bacteria.
About five or six years ago, Scott placed steel coupons in the St. Louis River above the dam in Cloquet and in Floodwood to determine if the corrosion issue extended upstream.
"Basically what we're seeing is a similar or same type of corrosion that we saw in the harbor. It's just not quite as aggressive in the river system. But it is there," he said.
Scott said he and others have concluded that the bacteria responsible for the corrosion are naturally occurring.
"They come down the river system, then they basically, in a sense, kind of concentrate here in the harbor, where we have warm water for longer periods of time and we have a lot of steel. So, we're seeing more concentrated damage," he said.
Randall Hicks, a University of Minnesota Duluth professor emeritus, agreed that the phenomenon is natural, "because we have pretty strong evidence now that what we're seeing in the harbor is the outcome indirectly of the activities of some bacterial communities."
"These are native bacteria. You find them in a lot of places, different iron-oxidizing bacteria and sulfate-reducing bacteria. They're both pretty ubiquitous types of bacteria. But they're more commonly found in certain environments where you have slightly different water chemistry," he said.
"For instance, when springs kind of bubble up, they often have very low oxygen, so that iron is actually dissolved, and as soon as it hits kind of the interface with oxygen in a stream, a lot of that iron will oxidize and actually precipitate. And right at that interface, you'll find a lot of different bacteria that have a requirement for iron, including iron-oxidizing bacteria," Hicks explained.
Since the passage of the Clean Water Act in 1972, and the subsequent reduction of pollutants in the St. Louis River, Scott said the rate of corrosion in the Twin Ports has accelerated. He noted that steel structures from the 1950s have sustained similar degrees of corrosion to those installed in the 1970s, leading him to believe that the bacteria responsible for biocorrosion truly began to thrive only in a cleaner environment.
The corrosion has taken a costly toll on unprotected steel in the port. Scott noted that the cost to replace a failing dock wall can run between $3,500 and $4,000 per foot.
Hicks was exploring different methods to ward off biocorrosion, when Mikael Elias, a University of Minnesota associate biochemistry professor on the Twin Cities campus, reached out to him.
Elias had been working with lactonase enzymes that have a unique ability to confuse bacterial activity.
"Enzymes are proteins that can really disrupt bacterial communication," Elias said, explaining that this can keep bacteria from organizing to form biofilms.
"And biofilms are key to two processes that occur in marine biology: biocorrosion and biofouling," he said.
Elias described biofilms as a foundation for the growth of the types of tubercles that have caused such extensive corrosion in the Twin Ports.
"Biofilm is a matrix of sugar and DNA that bacteria make and that allows them to stick onto surfaces. It's a molecular glue, if you wish," he said.
Hicks agreed to experiment with some of Elias' lactonase enzymes as a coating additive, and the collaboration has proven productive.
"We tried different types of additives and coatings that had been suggested to be anti-corrosive, and the lactonase enzyme that we added to these coatings was the best at reducing the amount of corrosion. It consistently reduced it about 50% compared to a coating without that enzyme," Hicks said, referring to the results of a two-year test.
Elias noted that the enzyme also is organic and non-toxic, unlike the cuprous oxide used as a biocide in many other commercial coatings.
The coating shows such promise that the University of Minnesota is pursuing a patent for it, and Elias said there has been widespread industry interest, as research and field trials continue.