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Great Lakes Article:

Unusual corrosion threatens harbor
Pitting of steel plates worries experts
By John McCormick
Chicago Tribune
Posted December 1, 2005


DULUTH, Minn. An underwater mystery has engineers and scientists discussing how to slow rapid corrosion that threatens the long-term structural integrity of the largest port on the Great Lakes.

The corrosion occurring two to 10 times faster than expected affects 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 about its cause include stray electrical currents, road salt runoff and zebra mussels. Harbor pollution also may 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."

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

Buchheit said some steel plates that are a half-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 required.

Some possible causes were ruled out, while additional study was suggested for others. Top theories identified by that report include environmentally influenced changes in water chemistry, microbe activity, dissolved chlorides from deicing salts and rising levels of dissolved oxygen.

Also under consideration are zebra mussels, which could protect some steel while concentrating corrosion at other water depths.

Still, large mussel colonies did not develop in Duluth 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 of Engineers. "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 for a harbor that moves an average of about 40 million metric tons of coal, iron ore, grain and other cargo each year.

The House has approved $300,000 for the Corps of Engineers 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. Minnesota also pledged $100,000 to match a portion of the federal funding.

Corps of Engineers officials say they are not aware of such extensive corrosion in any other Great Lakes ports, although they recommended further investigation of structures elsewhere.

The increased rate of corrosion is believed to have started in the late 1970s, when several significant changes were made to the harbor and surrounding areas.

A new wastewater treatment plant went online in 1978, consolidating the area's industrial and municipal discharges and potentially increasing the harbor's dissolved oxygen while concentrating the discharge of industrial chemicals.

The Corps of Engineers report also lists the start of filtering for asbestos in the 1970s at Duluth's water treatment plant, a process that added aluminum sulfate to the water and is known to have caused corrosion in the water lines.

"It is probably some combination of factors that are unique to this harbor," said Jim Sharrow, manager for the Duluth Seaway Port Authority.

Sharrow called the situation "very serious," citing cases in which steel that normally would have been expected to last 100 years has deteriorated in a quarter of that time.

"I have worked at facilities all across the Great Lakes, and I have never seen anything like this," said Chad Scott, an engineer and professional diver who was the first to notice patches of significant corrosion in the harbor in the late 1990s.

Scott, who works for a Duluth engineering firm, has researched the problem as a volunteer. "It is catching people a little off-guard because we have always been told freshwater ports shouldn't have this problem," he said.

Already, at least one business has spent about $1 million to reinforce its work site, fearing corrosion had weakened a 1,200-foot coal loading dock.

"You start losing the strength," said Fred Shusterich, president of Midwest Energy Resources, which loads about 20 million tons of coal yearly on boats for shipment across the Great Lakes. "We don't want to wait until it's too late."

Scott said he expects it will take about five years to research and fully understand the corrosion.

 

 

 

 


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