Corroding harbor structures at the Duluth-Superior Port posed an expensive mystery. Wisconsin Sea Grant's coastal engineer, Gene Clark, working with Minnesota Sea Grant and other partners, discovered both the cause and the solution.
Sea Grant Research
What's Eating Freshwater Ports?
Research Leads to Answer and to National Award
By Moira Harrington
It’s a mystery to set a coastal engineer’s heart aflame. Why are the steel piers, cofferdams, pilings and walls that form the bones of the largest port on the Great Lakes—the Duluth-Superior Port—corroding at an accelerated rate? The plot thickens when you consider that the corrosion levels were significantly higher than typically seen in fresh water and that other Great Lakes ports had not yet seen evidence of this in their own structures.
In fact, this mystery has set Gene Clark’s ticker racing as he teamed with colleagues to get to the bottom of this premature, extensive and costly infrastructure failure mechanism. Pits and holes, some as large as a softball, dot the harbor structures. The first assessments identified 13 miles of corroded steel sheet piling and structures requiring replacement, estimated at $1,500 per lineal foot or $120 million in all.
The Duluth-Superior Port annually moves more tonnage than any other Great Lakes port. The projected value is $12.6 billion, supplying 73,719 jobs and representing $3.2 billion in personal income (figures from a 2011 study by Martin Associates). Something had to be done!
“I recall when I first learned about the problem, back in 2004,” said Clark, Sea Grant’s coastal engineer. “I was at a port meeting sitting next to Jeff Gunderson, then the assistant director at Minnesota Sea Grant. At the end of the engineer’s presentation, Jeff and I both looked at each other and asked, ‘Did you know about this?’ Neither one of us did but both knew our two Sea Grant programs could help with the science needed to solve the problem.”
Fast forward to September, when the two Sea Grant programs celebrated national recognition for field and lab work that found solutions. The effort was named the winner of the 2014 National Sea Grant Association Research to Application Award in a competition open to 33 Sea Grant programs that attracted entries from nine states.
What were the antecedents to the award? Over several years, Wisconsin Sea Grant and the U.S. Army Corps of Engineers focused on repair, rehabilitation and corrosion-prevention techniques—coating small steel plates, known as coupons, with a variety of protective materials. Control coupons were left untreated. All coupons were placed in trays and submerged to assess harbor corrosion water-chemistry conditions, test the effect of ice abrasion and impact, and evaluate coatings to protect the steel. They also tested various wraps and jackets for piling protection.
Success. Now, these wraps and jackets protect steel pilings while the coatings and outer steel plates protect the piers and walls. Coatings such as epoxies, paints and tars have been applied to more than 26,000 lineal feet of steel walls, saving an estimated $39 million. In 2014 alone, more than seven new projects or repairs of existing corroded walls were initiated using the technology and lessons learned.
Through the years and concurrent with field testing, Minnesota Sea Grant and U.S. Navy researchers searched for corrosion triggers. Bacteria and fungi were prime suspects. Results demonstrated that corroding steel structures were covered by complex microbial biofilms that contained bacteria responsible for steel corrosion in other environments.
Through DNA analysis, researchers pinpointed the process by which specific iron-oxidizing bacteria attach to carbon steel, creating tubercles of biomass and corrosion products. Conditions beneath those tubercles cause copper dissolved in harbor water to precipitate and adhere to the iron. When ice chunks scrape against those pilings each winter, the tubercles break, exposing the copper-covered iron to oxygen. This causes the steel in those pitted areas to corrode at a faster rate.
Clark noted, “One of the nagging questions we had during the early stages of the studies was why is the steel corroding faster now than in the past? It turns out that the corrosion has accelerated since the 1970s, which is when we really started to clean up our harbor thanks to the Clean Water Act. The bacteria and the copper were always present, but perhaps we created the ‘perfect storm’ by providing for a cleaner environment in which the bacteria thrive.”
Other Lake Superior freshwater ports have begun to see structural deterioration. Deeply pitted steel has been found in Two Harbors, Minn.; Ontonagon and Houghton, Mich.; Ashland and Bayfield, Wis.; and Thunder Bay, Ontario. Thanks to past work, Clark now knows how to advise those port managers.
Mystery solved. Money saved. Jobs protected. Award won. Plus, there’s a coastal engineer with a now-normalized heart rate, unless you count his pleasure with the national accolades.
“The award is certainly a surprise and greatly appreciated. And this successful team effort of applying sound science to a very difficult problem and coming up with solutions that will save our Lake Superior ports millions of dollars is our greater reward,” said Clark.