Corroding steel affecting Duluth/Superior harbor

On Wednesday night, February 8th, about 40 or so locals gathered at the Barker’s Island Inn Resort and Conference Center in Superior, WI to hear about the problems the Duluth/Superior harbor is facing; steel corrosion. River talks, a Wisconsin Sea Grant series, gave the public an opportunity to hear about this issue. Structural and marine engineer Chad Scott, a Duluth native who started an underwater inspection unit for the Duluth/Superior harbor, discovered this problem in 1998. These inspections through the Duluth City Port Authority started in 2003 and continue to keep Scott and his team busy.

Scott has worked in salt water environments and has seen this same type of corrosion, but never in fresh water environments. Scott’s initial inspections at the U.S. Coast Guard Unit in Duluth back in 1998. “What we were finding [on the H-piling(steel beams)]… were holes the size of softballs,”, Scott said. There was also severe pitting on the sheet piling, seen in photo below, anywhere from the surface of the water down to about 10 feet below the water line. The most significant was down to the 4 foot line according to Scott’s findings.

Here is a photo of the pitting. Photo courtesy of MPR News.

“We discovered that the problem was a very global problem to the Duluth/Superior harbor. It was everywhere in the harbor,” , Scott said.

There are approximately fourteen miles of steel sheet piling and 640 H-piles in the harbor and it was all affected. With time and a little more understanding, Scott and his team were able to test the steel.

Gene Clark, Coastal Engineer Specialist for Wisconsin Sea Grant, teamed up with Scott and his team to discover the possible reasons for this steel corrosion. “… common solutions in salt water, but not in fresh water. Some of the steel can be a century old in the other Great Lakes and not be impacted,”, Clark said. Water chemistry, variations in dissolved oxygen caused by water temperature variations and volume of moving water, and excess salt content from road salt stored at the end of 21st Avenue West in Duluth were possible causes. They later ruled out dissolved oxygen and were still had open minds about other possibilities.

Scott and his team went to forty-four different sites in the harbor. They took measurements to find variations in the testing. Clark involved Brenda Little, international expert on microbial influenced corrosion, from the Stennis Naval Laboratory in Mississippi, and Dr. Randall Hicks from UMD. It’s safe to say this was a challenging study for even theses experts.

In 2013, thanks to Little, Hicks, and Sea Grant, answers to why the steel was corroding were found. There are high amounts of iron in the harbor which makes iron-oxidizing bacteria common. Thanks to chemistry, it adheres to the steel piles and sheet piles. When the ice comes in the winter, it scrapes off the bondage which exposes copper-covered iron to oxygen. This is the reason for the pitting shown above. This corrosion is increasing and that answer has not been found yet.

Looking forward, these repairs will cost a tremendous amount of money. $3,500 per linear foot and there are about 640 H-piles. So, who plans on paying for this? On the Wisconsin side, Wisconsin Department of Transportation helps fund the costs of these repairs. On the Minnesota side, the owners of the docks will have to evaluate if this is something they need to fix immediately and that is influenced heavily by Scott’s team.

Some of these repairs need to happen as soon as possible. Clark and Scott started to piece together some possible solutions. They looked at a variety of protective solutions for the structures that aren’t as impacted and for the replacement H-piles and sheet piling. Scott and his team made sure that each solution was environmentally safe.

Here is a close-up of corroding sheet piling. Photo courtesy of Sea Grant.

They tested a variety of coatings and protective covers. Ice was a big obstacle. With the dynamics of ice, freeze, thaw and freeze again, some solutions weren’t holding up. The water would seep into the tiny cracks and then freeze which would pry off the covering. Fiberglass and heavy duty rubber seem to do the trick. Scott and his dive team were able to install these protective solutions and are expected to last 50 years. Scott and his dive team have been able to protect over 6,000 feet of sheet piling with the protective materials so far.

Efforts to find corrosive resistant steel are in the works according to Scott, but are confidential. Scott and Clark have been hard at work on this issue for over a decade and have made some remarkable progress on not only finding the cause for this, but also preventing this from happening again.

You can find more in-depth information at Wisconsin Sea Grant and at “Factors Contributing to Corrosion” to see what else Clark and Scott discovered.