Rapid Corrosion Threatens Biggest Port on Great Lakes

A mystery below the harbor surface here has engineers and scientists debating how to slow abnormally rapid corrosion that threatens the long-term structural integrity of the largest port on the Great Lakes.

The corrosion is happening at a rate thought to be two to 10 times faster than expected.

The rust is damaging about 13 miles of steel plates that line the harbor, metal that provides support for bridges, iron ore loading docks and other vital structures.

Theories abound about the cause, such as stray electrical currents, road salt runoff and zebra mussels.

Lower levels of harbor pollution may also be speeding the corrosion process because there is more dissolved oxygen in the water.

“They are suffering some severe corrosion,” said Rudolph Buchheit, a material science and engineering professor at Ohio State University who is studying the problem.

“But it is not at all agreed upon why this is happening.”

Although salt water is known to attack untreated steel, experts say the rate of rusting in the freshwater found in the Duluth-Superior Harbor is unlike anything they have ever seen on the Great Lakes.

Buchheit said some steel plates that are half an inch thick or more have been perforated in less than 10 years.

“To make it through that thick a material in just a few years is a significant corrosion rate,” he said.

After assembling a panel of experts, the U.S. Army Corps of Engineers issued a report this year that said the problem could cost more than $100 million to correct if major replacement of the steel pilings is ultimately required.

Some possible causes were ruled out, and additional study was suggested for others.

Top theories identified by that report include environmentally influenced changes in water chemistry, microbe activity, dissolved chlorides from de-icing salts and rising levels of dissolved oxygen.

Also under consideration are zebra mussels, which could protect some steel but concentrate corrosion at other depths.

However, large mussel colonies did not develop here until the late 1990s, well after it is believed the rapid corrosion began.

The corrosion has pitted steel throughout the harbor, primarily near the water line and tapering off about 10 feet below the surface.

The individual pits are often about the width of a dime, and in some places their abundance is weakening the steel, which is used to reinforce the land and make deep-water channels possible.

“Steel sheet piling is very expensive,” said David Bowman, a project manager for the corps. “When you put it in, you expect it to last at least 50 years.”

Officials say there is no imminent threat to bridges, docks or other structures, but that could change in a decade or less if no solution is found.

The harbor moves an average of about 45 million tons of coal, iron ore, grain and other cargo each year.

The House has approved $300,000 for the corps and others to study the problem.

The Senate has yet to approve the money, although local officials say they are hopeful it will be secured this year.

The state of Minnesota has pledged $100,000 to match a portion of the federal funding.

Corps officials say they are not aware of such extensive corrosion in any other Great Lakes ports, although the panel of experts recommended further investigation of structures elsewhere for possible undetected problems.