Steel corrosion in Duluth’s harbor
The Duluth Harbor, while being ridden with nasty, cold temperatures for most of the year, is one of Minnesota’s most aesthetic attractions. It’s home to several lighthouses, several piers, and the iconic Aerial Lift Bridge. Above the water’s surface the harbor is nothing short of beautiful, but below the water’s surface is, unfortunately, a completely different story. For nearly 50 years the harbor’s steel has been playing victim to serious, aggressive corrosion. Aside from the general deterioration of the harbor’s necessary steel docks, the cost of fixing this issue raises a significant problem as well.
In 1998, structural and marine engineer Chad Scott began to investigative the issue via diving and spending time underwater in the frigid temperatures of Lake Superior.. Chad, along with a board of experts, originally hypothesized that the corrosion could be a product of several factors. The list consisted of the following: changed water chemistry as a result of de-icing salt runoff, bacteria and/or fungi eating away at the steel, and they even considered stray currents from the distant DC power line. Due to the extensive list of possible factors, the team’s goal was to isolate the leading cause.
After nine years of investigative research and testing, the panel of experts, with help from the University of Minnesota Duluth and the Naval Research Laboratory, found what they were looking for. It was unveiled that the main perpetrator of corrosion in the harbor was a naturally occurring, iron-oxidizing bacteria. Essentially, these bacteria latch onto the carbon steel beneath the waters surface, creating a growth that attracts the naturally occurring copper in the water. This creates a cooper/iron combination that, when scraped off and exposed to oxygen, catalyzes corrosion.
Upon revealing the culprit, the problem then became the plan of action. The team set out to find the most cost effective, efficient solution to the problem.
“To replace the steel it’d cost $3,500 per lineal foot,” said project engineer Chad Scott.
Being the corrosion has been spreading rapidly since the 1970’s, the effected area is large, making the complete replacement of the corroded steel simply not an option. Although adhesive coatings and composite covers may seem like an easy answer to the problem, according to Gene Clark, a University of Wisconsin Sea Grant Institute researcher, most of the product solutions on the market don’t account for environments where ice is present.
Through an extensive process of trial-and-error, the researchers found two main solutions to combat the steel corrosion, one being the use of hybrid coatings/wraps and the other being the application of fiberglass jackets. Both of these options are relatively cost effective and durable, int urn creating an affordable option that does it’s job. The coatings and wraps are primarily used on the the pipe piles, as they’re malleable, allowing for the application process to be smooth.
In contrast, due to their obscure shape, the underwater h-piling is a different beast. Currently, the researchers are using custom-fit, fiberglass jackets to protect the h-piling. Although the jackets are form fitting, there is still dead space that exists between the steel and fiberglass, that, without treatment, would allow for the corrosion to continue. In response to this issue, the expert team now injects an epoxy into that space to fill the void.
“Once the epoxy is fully bonded from fiberglass to steel, it’s permanent,” said Chad.
Scott also mentioned that research and development on the corrosion project continues to progress, but the information is confidential.