Experts recommended further studies to link the corrosion problem in the Duluth-Superior Harbor to its causes.
UW Sea Grant
Trouble Under the Boardwalk 4/21/05
Mysterious Corrosion Draws Experts to Duluth-Superior Harbor
By Kathleen Schmitt
Chad Scott was on a routine inspection dive in the Duluth-Superior Harbor a few years ago when he came face to face with a problem—one big enough to put his fist through.
“I’ve seen some corrosion here and there at other Great Lakes ports, but nothing like this,” he said. Scott saw that some of the beams supporting the dock structures “had holes the size of a softball.”
Subsequent inspections revealed that corrosion is widespread throughout the harbor on all types of steel piling buttressing the docks. Scott found that most of the steel is covered with small pits, scooped out in diameters of 1/4 to 1 inch, primarily in the first four to six feet below the waterline and tapering off around 10 feet.
Scott, a marine and structural engineer and a commercially certified diver, began working with the Duluth Seaway Port Authority to learn more about the problem. By comparing older and newer sheet pile installations, they determined that sometime in the 1970s the rate of corrosion in the harbor shifted into high gear.
Pitted against time
The accelerated corrosion could have significant safety and financial implications for the port, which handles the largest total cargo volume in the Great Lakes. Thirteen miles of steel sheet piling are corroding around the harbor, and if the problem isn’t addressed, the structural integrity of docks and loading facilities could be compromised and the failing steel would have to be replaced.
“This is potentially a very costly problem,” said James Sharrow, facilities manager of the Duluth Seaway Port Authority. “We have about 90 to 100 million dollars of possible repairs in our harbor to steel that’s being damaged by corrosion.”
The Port Authority requested federal and state funding for a $250,000 study of the problem. Meanwhile, Gene Clark, UW Sea Grant coastal engineering specialist, and Jeff Gunderson, Minnesota Sea Grant associate director, began assembling a steering committee to look for help in narrowing down the list of possible culprits.
Experts weigh in
Last September, five experts in corrosion, microbiology, and chemistry visited the harbor to look for the root of the corrosion problem. All of them were surprised by what they saw.
“In seawater areas we see sheet pile fall apart in 10–20 years all the time,” said James Bushman, president and principal corrosion engineer of Bushman & Associates, Inc., a consulting firm in Medina, Ohio. “But this is a freshwater harbor, and that’s normally a much less aggressive environment, but it doesn’t appear to be in this case.”
Bushman and the other experts concurred that the corrosion is unusual and needs to be formally studied. They also quickly ruled out many of the possible causes, leaving only a few to be examined in more detail.
Microorganisms like bacteria or fungi could be eating away at the steel, or stray current from a high-voltage direct-current line could be speeding up the corrosion. Another possibility is that the harbor’s water chemistry might have changed in ways that promoted corrosion. Highway de-icing salts may have added significant amounts of chloride to the harbor. Also, as water quality improved with tougher legislation like the 1972 Clean Water Act, higher amounts of dissolved oxygen in the harbor could have boosted corrosion rates.
In addition to testing the possible causes, the panel recommended measuring corrosion rates and assessing the condition of several structures in the harbors. Regardless of what is causing the accelerated corrosion, dock owners need to know if their steel is sound and at what point repairs are no longer viable and replacement is necessary. Knowing the rate of corrosion is critical to devising a set of best management practices for structures throughout the harbor.
Clark says none of the experts have seen this kind of corrosion elsewhere in the Great Lakes, but that doesn’t mean the problem doesn’t exist elsewhere. “Most people don’t expect this type of corrosion in a freshwater harbor, so they may not be looking for it,” he said. “That’s why the panel strongly recommended that other Great Lakes ports and harbors be studied and their managers made aware of this issue.”
The Wisconsin and Minnesota Sea Grant programs, the Duluth Seaway Port Authority, the U.S. Army Corps of Engineers, and the University of Minnesota-Duluth and its Natural Resources Research Institute sponsored the expert panel meeting. The panel’s report is available at www.seagrant.wisc.edu/coastalhazards/ or by contacting Gene Clark at (715) 394-8472 or firstname.lastname@example.org.