Steel Corrosion in the Duluth-Superior Harbor
This week’s River Talks headed to Wisconsin for an enlightening discussion on a topic that affects every port throughout Lake Superior: Corrosion of steel in the Duluth-Superior harbor.
Gene Clark of Wisc. Sea Grant and Chad Scott of AMI Consulting Engineers presented on how they have worked together over the past years to solve the mystery of steel corrosion in the Duluth-Superior Harbor.
Some time ago scientists discovered that docks throughout almost all the Lake Superior harbors were corroding. This corrosion was microbiologically induced, and causing holes the size of soft balls to appear in the steel support beams of docks.
A similar problem — splash zone corrosion — occurs in coastal environments. Coastal conditions do not exist in Lake Superior, so, they needed to discover what could be causing the corrosion.
Dive teams traveled to 44 sites around Lake Superior. Their goal was to test the pits in steel beams throughout the lake. They discovered both iron oxidizing and iron reducing bacteria in the pits of the steel.
Dr. Brenda Little, the Navy’s senior scientist for Marine Molecular Processes in the Naval Research Laboratory’s (NRL’s) Oceanography Division; looked at these pits under an electron microscope. She found copper plated bacteria inside of the pits. Through a series of chemical reactions between the naturally occurring iron oxidizing bacteria and the copper plated bacteria inside of the pits; an anaerobic reaction that eats away at the steel occurs.
The next step was to develop preventative methods that stop or slow down the corrosion. Their next problem concerned the environment. In most places where corrosion to this extent occurs there is no ice. The melting and freezing of the massive amounts of ice throughout the lake that scrape the beams each year which could impact how effective some preventative methods would be. The best solution they decided, was to find an epoxy that could stand up to the environmental conditions presented by the harbor.
These couldn’t be just any epoxy’s, they had to hold up to the environment in the Duluth-Superior harbors and be easily applied in the less than ideal conditions that often occur there. After testing over 50 types of epoxy, a hybrid epoxy was determined to be the best solution. Durable and tough, yet maintaining some flexibility over the course of many tests, they were the best choice.
After many more months of work and discovering how to best cover the different types of beams that exist throughout the harbor, they have now covered over 6,000 ft. of port throughout the lake. With the ability to continue to add more epoxy and panels to the beams, the lives of the beams throughout the harbor can extend indefinitely.
Even though the corrosion problem is solved for the time being, questions remain about why the rate of corrosion increased at an exponential rate in recent years. The beams were originally installed in the 1900s, so lasting as long as they did is a feat in itself. They found that in the 1970s, the rate of corrosion substantially increased. They know this because the beams installed in the 70s have the same level of corrosion and pitting as the beams from the 1900s.
If this change in chemistry is still occurring, then what is to say that the new beams will even last 50 more years. What they found is that in the 1970s the Clean Water Act passed. This act resulted in the cleaning of the harbor to a point where the iron oxidizing bacteria could thrive. Since this bacterium is what causes the pitting in steel, the rate of pitting accelerated as the number of bacteria increased.
The mystery of steel corrosion is solved for now, but there is still much work to do in finding even better preventative methods for the future. New epoxy’s are being tested and research is still being done to find new and better ways to improve our Duluth-Superior harbors.