Several studies have been conducted since about 2004 to try to sleuth out the causes of this corrosion. Scientists funded by MN and WI Sea Grant, are cooperating with the Seaway Port Authority, the Army Corps of Engineers, and the University of Minnesota Duluth on a long term effort to identify the causes of the corrosion. Corrosion in freshwater has received little attention relative to corrosion in seawater but has typically been found to occur at rates as much as 10 times lower than in saltwater. Recent studies have shown that some St. Louis River Estuary sites have low rates typical of freshwater, while others are showing accelerated corrosion, in some places similar to that of saltwater ports.

This detective work has led to the identification of a combination of physical, chemical, and biological factors that may be influencing corrosion rates. Termed MIB for microbially-influenced corrosion, it involves naturally occurring iron-oxidizing bacteria, which attach to carbon steel, forming nodules called ‘tubercles.’ Work by the Army Corps of Engineers has suggested that conditions beneath the tubercles cause copper dissolved in harbor water to adhere to the iron. When the inevitable winter ice arrives, chunks of ice scrape against the pilings causing the tubercles to break. This exposes the copper‐covered iron to oxygen, which in turn, causes the steel in the pitted areas to corrode faster – creating a Swiss cheese effect. Each year these tubercles are exposed to scraping by ice to varying degrees, depending on the severity of the winter.

Is accelerated corrosion in the SLRE harbor unique in the Great Lakes?

The SLRE is distinctive because the brown-stained waters of the St. Louis River Estuary (SLRE) are periodically intermixed with the green-blue waters of western Lake Superior as a result of seiche activity. Researchers have looked at historical water chemistry data to help explain the corrosion phenomenon, as well as a variety of other potential factors, but a detailed understanding remains elusive. Water quality has in fact greatly improved since the creation of the Western Lake Superior Sanitary District, and in particular, levels of chloride and sulfate have decreased, which would be expected to mean less corrosion. On the other hand, WLSSD’s improved wastewater treatment has helped dissolved oxygen levels rise back to normal levels from pre-WLSSD conditions - ; this could increase corrosion. Moreover, these water quality factors can vary greatly among sites in the lower estuary and likely have important effects on the many different types of microbes that colonize the tubercles. Given that these effects may be further affected by the increased temperatures and decreased ice cover observed in the past few decades, it is easy to see why one definitive answer has not been found. The SLRE corrosion issue has now prompted closer examinations of other Lake Superior ports as well as other Laurentian Great Lakes to see how unique the problem really is here.

The search is on for solutions

Whether or not the exact causes of the corrosion are understood, the search for short and long term solutions is underway. In the short term, catching corrosion early can help port managers avoid the need to replace expensive pilings. In the long term, research into preventative technologies, such as cathodic protection, in which a 'sacrificial metal' is attached to the steel piling, application of coatings, or use of ice protection barriers, may provide an answer to this economically important issue.

Learn More

• Accelerated Corrosion of Piers in the Duluth Harbor, UM Center for Transportation Studies, Principal Investigator:Randall Hicks, Professor, UMD-Sci/Engr Admin
• Harbor Corrosion Photo Gallery - Wisconsin Sea Grant
• Harbor Corrosion Study

The development of the port over the past century has brought dramatic changes to the shoreline. According to William Hearding’s survey of 1861, the estuary coastline that now serves as docks and slips for 1000 ft lakers and salties was once a series of mud flats and shallows, with floating vegetation mats that would drift in and out of the harbor. This part of the lower estuary was biologically rich, providing an important spawning habitat for fish and supporting a rich bird and wildlife community.

The lower estuary has changed dramatically, now converted to hundreds of acres of dock facilities and industrial sites. As part of the water access required for shipping and industry, the mud flats have been replaced with ‘hardened shorelines’ – the steel sheet pilings, bulkheads and rip-rap that now make up most of the industrialized coast.

In our study of environmental stress within the estuary, we sampled water quality, wetland vegetation and stream macroinvertebrates at the mouths of streams entering the estuary. The sampling was designed to collect information across the entire range of low to high environmental stress. The ‘high stress’ areas were most often the industrial areas of the harbor, where road and population densities are highest. While we were always able to collect water samples, in many shipping and industrialized areas wetland vegetation was simply absent. This was largely due to historic and recent modifications to the original shoreline [ 3]. The abrupt transition from land to deep water along the hardened shorelines has reduced suitable aquatic plant habitat, preventing the establishment of the emergent and floating-leaf wetland communities that once characterized the lower estuary.