Trouble in the harbor
Lake Superior has been called a a fresh water sea by many, but it turns out the great lake may have more in common with the sea than once thought.
A hefty amount of steel corrosion has been affecting large sections of sheet pilling along the lake’s harbors. The rate of corrosion is similar to that found in saltwater ports and is almost unheard of in freshwater environments.
Gene Clark, a coastal engineer for Wisconsin Sea Grant, has teamed up with commercial diver Chad Scott of AMI Consulting Engineers P.A.to research the curious condition of the 14 miles of steel sheet pile within the Duluth-Superior harbor.
Scott’s previous experience along the East Coast has earned him an eye for corrosion in salt and salt brackish water– a mixture of salt and fresh water commonly found in estuaries– where finding this type of steel corrosion is almost routine on his inspections.
As a structural marine engineer, Chad Scott said, “we learn early on to identify different corrosion.”
However, Scott was just as surprised as anyone to see the deep pitting of the steel piles within the 0–10 foot water levels throughout the freshwater harbor.
Scott initially reported the findings as an “aggressive corrosion type” but was written off by other experts as a form of electrochemical corrosion. About 6,000 feet of steel piling had effects from the corrosion, but only within the top waters along the harbor line — very uncommon for electrochemical corrosion, which is able to corrode metals from one foot of water to hundreds of feet below.
Clark and Scott found themselves stumbling onto a puzzle that a large portion of people within the science realm had not heard of. At a cost of $1,500 per lineal foot of replacement fees — solutions and preventions were vital to the harbor industry.
They assembled some experts and got down to business with a two day workshop among leading corrosion experts. Comparing steel sheet piles that were over a century old to newer ones placed in the harbor around the 1960s, they discovered the pitting to be almost identical.
Whatever was causing the corrosion in Lake Superior’s water had only been corroding the metal since the 1970s, leaving some experts to suspect the Clean Water Act’s impact on the great lakes environment.
Judging from the number of salt-eaten cars on streets, it's clear Minnesotans are familiar with corrosion. Unlike autos…www.seagrant.umn.edu
After sending some samples in, Clark and Scott learned that the leading factor causing the corrosion was a complex chain of events created by microorganisms.
“The problem is the most interesting one I’ve ever worked on,” Clark said.
Lake Superior’s cleaned up water, thanks to the Clean Water Act, may be serving as the perfect setting for native iron-oxidizing bacteria to thrive.
The iron-oxidizing microorganisms were able to attach themselves onto the carbon steel creating a biomass. Tubercles underneath the bio masses caused dissolved copper to secure themselves to the iron. As the seasons changed and the Duluth-Superior harbor found itself once again rubbing elbows with sheets of ice, the bio masses and tubercles were breaking off from the friction. The copper coated iron was then exposed to oxygen working as a catalyst to the harbor’s steel corrosion.
Erika Washburn, who works with Clark at Sea Grant said, “one of the neat things to come from this is the coordination and collaboration between Sea Grant and the private sector.”
That collaboration didn’t stop once the source of the problem was found. Clark and Scott exhausted themselves and their funds in trying to find preventative solutions for the harbor’s corrosion.
Lake Superior’s harbor takes on its fair share of abuse throughout the seasons. With ice sheets measuring as much as eight feet thick along the harbor walls, a simple protective coating of the steel piles was anything but simple.
“The coating needed to hold up to the environment and be applied in less than perfect conditions,” Scott said.
After a slew of different epoxies and hybrid coatings were tested, Scott and his diving team found a hand full of preventative coatings to get the job done. The majority of these coatings varied from $40–50 per square foot, a much more financially sound outcome than the $1,500 it would cost to replace the same amount of steel piling all together.
The research for this project began in 1998 and in 2013 the years of teamwork and problem solving paid off. The team recently received a NOAA award for its research adaptations to save the Duluth-Superior harbor.