Study closes in on causes of corrosion in Duluth-Superior Harbor

What do the Twin Ports waterfront locations of Oliver Bridge, Midwest Energy Resources Co. and the former Cutler-Magner Co. lime plant have in common?...

Steel corrosion in the harbor
Corrosion turns the cellular structure of steel into something resembling Swiss cheese. Photo courtesy of AMI Consulting Engineers

What do the Twin Ports waterfront locations of Oliver Bridge, Midwest Energy Resources Co. and the former Cutler-Magner Co. lime plant have in common?

All three sites have corrosion eating their steel pilings in the Duluth-Superior Harbor or St. Louis River, according to a recently published journal study from researchers who have examined the spots for two years.

"This is a newly discovered combination of factors that lead to corrosion," said Jim Sharrow, facilities manager for the Duluth Seaway Port Authority.

Brenda Little, lead scientist and an author of the study, described this corrosion as the result of bacteria in the water meeting steel pilings to form a tubercle or small lump on the steel. The lump limits oxygen and allows copper traces in the water to meet the steel. Contact between the two metals results in the steel dissolving, she said in a presentation last week to the Harbor Technical Advisory Committee in Superior.

"At all three locations you see the tubercles, and they are in all cases related to corrosion," Little said. "Whatever is happening is happening at all three locations."


The sampling of decaying steel could be an indication of a larger corrosion problem in the harbor. With about 70,000 feet of steel submerged in Lake Superior, corrosion could be an estimated $300 million problem to fix, Sharrow said.

"We characterize that as the problem or the risk -- the risk of doing nothing is the failure of all of the steel in the harbor," Sharrow said.

The new corrosion study answered some unknowns, but many questions remain.

Further research between Little's team at the Naval Research Laboratory at the Stennis Space Center in Mississippi and Randall Hicks of the University of Minnesota Duluth will include a finer microscopic look at how the tubercles or lumps work.

"They will use some specialized equipment that goes in millimeter by millimeter and unpeels these cells and looks at the conditions," Sharrow said. "They will see exactly what is present through the cell down to the surface of the steel and looking at cells of different ages."

Another research goal will be to measure the corrosion's scope. Tests are planned to gauge the level of steel corrosion near Duluth in Lake Superior, as well as port towns such as Two Harbors and Ashland.

To find the best way to fight the corrosion, eight different coatings were applied at Cenex Harvest States near the Blatnik Bridge in Superior.

"They are keeping close track of the actual performance of these coatings," Sharrow said, adding that other facilities also have coated their docks.


Sharrow said the theory is that corrosion began in the 1970s when the shipping industry stopped dumping into the harbor.

"What we believe is that the water in the harbor was what I'll describe as toxic," Sharrow said. "It was toxic to the bacteria and they couldn't get colonized."

When dumping ceased, Sharrow said, more oxygen entered the water, creating the environment suitable for bacteria.

"It started with the cleanup," Sharrow said. "If you look at 50-year-old steel and you look at some 100-year-old steel in the harbor, it looks surprisingly similar. If this had been going on for 100 years, the 100-year-old steel wouldn't exist anymore. We believe if you look at steel that is less than 40 years old the corrosion is proportional to its age."

Research suggests that corrosion in the harbor has been exacerbated by ice scraping up against the impacted steel, Little said. The ice scouring breaks the tubercle, exposing the iron to the copper and causing another peak in corrosion, she said, "and this cycle is repeated over and over again."

But corrosion isn't exclusive to the Twin Ports, Little said, and her research didn't explore whether levels of copper were especially high in Lake Superior.

"Corrosion of steel pilings is an international problem," she said.

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