Unusual corrosion threatens harbor
Pitting of steel plates worries experts
By John McCormick
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
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
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
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
Still, large mussel colonies did not develop in Duluth
until the late 1990s, well after it is believed the rapid
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
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
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.