Harbor Corrosion: It's the Pits
Alfred Beitelman with the U.S. Army Corps of Engineers measures electrical activity in water near corroding pilings in the Duluth Superior Harbor. Beitelman was part of a team brought in to look at the causes of steel corrosion in the harbor. Photo: Gene Clark, Wisconsin Sea Grant
Judging from the number of salt-eaten cars on streets, it's clear Minnesotans are familiar with corrosion. Unlike autos, the steel pilings in the Duluth Superior Harbor are corroding with unfamiliar rapidity. Even a team of corrosion experts can't quite explain what's eating the near-surface submerged steel in the harbor, although they have ideas on how to help pinpoint the cause.
"An as yet unexplained aggressive water chemistry specific to the lower St. Louis and Superior bays may be pitting the steel," said Rudy Buchheit, professor of materials science and engineering at The Ohio State University, "but we haven't ruled out other factors or combinations of factors." Buchheit was speaking on behalf of five experts summoned to Duluth in September by Sea Grant and partners to investigate the mystery.
Bouys used to mark navigation channels display the extensive pitting common to steel found in parts of the Duluth Superior Harbor.
Over the last 30 years, corrosion has accelerated to rates resembling those of saltwater environments in parts of the port. With over 12 miles of pilings, there's a lot to lose here. Underwater inspections reveal that steel put in before 1970 corroded within the norms of fresh water. Then something changed; younger steel began looking worn, pitting fast, deep, and in disproportion to age.
In instances of major corrosion, the sheet pilings are as holey as old underwear. More commonly, pilings evolve from factory-smooth to the texture of soda cans squashed on a gravelly road, pitted to 3/8 inches.
The perforations and pits are most severe in the first four feet below waterline; they seem to end at the point where zebra mussel colonies begin, below the scouring power of harbor ice (10 feet). Although zebra mussels are associated with corrosion elsewhere, the interplay is at their contact point, not above it. Should zebra mussels have a relationship with corrosion in the Duluth Superior Harbor it is unclear and couldn't date much further back than 1989 when mussels were first detected.
There is not only a gradient of pitting from the surface down, but also from the harbor entry inland. Corrosion seems normal eight miles up the St. Louis River, above the Oliver Bridge.
The panel discussed several causal theories, including water chemistry and corrosive microorganisms. Brenda Little of the Naval Research Laboratory, a specialist who focuses on microbial-induced corrosion, thought testing for bacteria and fungi might be worthwhile but that the root of the harbor's corrosion lies elsewhere. The panel also addressed whether an influx of dissolved oxygen from Lake Superior borne up river on seiches could be corrosive.
"In fresh and salt water, the dissolved oxygen content is a significant factor, if not the significant factor, in determining the corrosion rate of steel," said Buchheit. Still, he and the majority of the panel believe there is more to this story than an oxygen glut. The panel noted that corrosion is not as severe outside the harbor, where oxygen is even more abundant.
Charles Marsh and Alfred Beitelman from the U.S. Army Corps of Engineers and James Bushman from Bushman and Associates contributed their expertise. Bushman helped rule out the shocking possibility that the harbor corrosion was electrically induced while Marsh and Beitelman helped move the discussion from causes to solutions.
"Paint is the answer," said Beitelman. "But can you get it on? And, if you can get it on, can it withstand the ice?"
These pilings and chains from the Duluth Superior Harbor clearly show the worrisome corrosion.
Jim Sharrow from the Duluth Seaway Port Authority shuddered at what he called the "100-Million-Dollar Solution," the cost of replacing all the steel. The consensus was that the harbor should be doctored with "condition-based maintenance" where off-the-shelf repairs follow regular inspections. This approach would require diligence but could provide the greatest risk-to-reward ratio and flexibility.
The experts were very clear on one point. "Never tie new steel to old steel," said Bushman. "If you put new steel in water and tie it to older steel, you'll find it wants to corrode like mad to become just like the older steel. There are no magical fixes."
The Minnesota and Wisconsin Sea Grant Programs, the University of Minnesota Duluth and its Natural Resources Research Institute, the U.S. Army Corps of Engineers, and the Duluth Seaway Port Authority sponsored the expert panel meetings. A report is pending. For more information about corrosion in the Duluth Superior Harbor contact Jeff Gunderson, firstname.lastname@example.org or (218) 726-8715.
By Sharon Moen