A new study of rusting steel supports in the Duluth-Superior harbor points to dissolved copper in the water as a contributing factor.
Duluth shipping officials raised the alarm a few years ago about accelerating corrosion eating away an estimated 50,000 of tons of steel a year. Steel supports several miles of harbor walls and shipping docks.
The study cites a combination of factors including bacteria, dissolved copper and scouring by ice. Jim Sharrow, with the Duluth Port Authority, said the damage has been evident where ice forms.
"Sure enough, the corrosion damage is all in the upper area of the water column, in the top three to four feet, which is the depth of the ice typically in our harbor in the wintertime," Sharrow said.
He said the same sequence has been causing rust in other Lake Superior harbors. Sharrow said the corrosion might be more prevalent now that water in places like the Duluth harbor is cleaner than it was decades ago, which might help the bacteria survive.
Meanwhile, studies continue on how to hold corrosion at bay. Several promising coatings are now being tested on steel panels in harbor water. Others look at water chemistry and at reasons the corrosion has accelerated over the past 30 years.