Duluth, Minn. USA – November 24, 2009 ----- Long-awaited research findings just published identify one possible mechanism responsible for accelerated steel corrosion in the Duluth-Superior harbor. The peer-reviewed paper published in CORROSION, The Journal of Science and Engineering, outlines a study led by Brenda Little, Senior Scientist, Marine Molecular Processes, Naval Research Laboratory, Stennis Space Center. The study was supported by the U.S. Army Corps of Engineers, Detroit District, and the Duluth Seaway Port Authority.
Entitled “Factors Contributing to Corrosion of Steel Pilings in Duluth-Superior Harbor” (by R. Ray, J. Lee and B. Little), this research demonstrates a specific sequence of biological, chemical and physical events responsible for localized corrosion of carbon steel piling – including the role ice scour plays in accelerating that process.
Corrosion has been eating away at steel pilings in this harbor for over 30 years, consuming an estimated 50,000 plus pounds of steel a year. Once identified in the late 1990s, scientists, engineers and local dock owners (led by the Port Authority) immediately began researching potential causes and ways to either stop it or protect remaining steel structures.
Little’s findings confirm that ongoing repair and mitigation studies in the local harbor are on the right track. The study also may give engineers a basis for developing resistant steel alloys for new construction and coatings that lengthen the lifespan of steel structures already in place. Research will eventually lead to methods of determining the likelihood of similar corrosion developing in other freshwater harbors around the globe.
“Now that we understand an exact mechanism, we can narrow down our focus of how to best mitigate the problem in research and pilot projects” noted Jim Sharrow, Port Authority Facilities Manager. “Results will help steer subsequent studies by Brenda (Little) and by UMD researcher Randall Hicks. We are especially pleased to know that the Corps has received an additional $300,000 targeted for corrosion research in FY2010.”
In layman’s terms, conclusions of this study describe the process by which specific iron-oxidizing bacteria attach to carbon steel, creating a “nodule” of biomass and corrosion products. Conditions beneath those nodules (i.e. “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 which, in turn, causes the steel in those pitted areas to corrode at a faster rate…creating an almost “Swiss cheese” effect in nearly 14 miles of steel pilings and dock walls along the Duluth-Superior waterfront.
Scientists describe this process more definitively in the study’s abstract:
Field observations and laboratory testing were used to conclude that aggressive localized corrosion of carbon steel pilings in Duluth-Superior Harbor, Minnesota and Wisconsin, is caused by the following sequence of biological, chemical and physical events. Iron-oxidizing bacteria colonize the carbon steel sheet pilings and produce tubercles, made up of intact and/or partly degraded remains of bacterial cells mixed with amorphous hydrous ferric oxides. The reducing conditions beneath the tubercles cause copper dissolved in the water to precipitate. A galvanic couple is established between the copper layer and the iron substratum. Ice scouring breaks the tubercles. Exposure of the copper-covered iron to oxygen causes the galvanic current to increase. The result is aggressive localized corrosion.
Locally, a team of experts has been working collaboratively since 2004 to define the methodology for studying – and mitigating – the accelerated corrosion problem in the Duluth-Superior harbor. Several pilot projects are well underway, looking into applications and procedures that will save existing steel structures and protect new ones from corrosion.
• Jim Sharrow, Duluth Seaway Port Authority
• Dave Bowman, U.S. Army Corps of Engineers (Detroit)
• Steven Brossart, U.S. Army Corps of Engineers (Duluth)
• Gene Clark, Wisconsin Sea Grant
• Chad Scott, AMI Consulting Engineers
• Dale Bergeron, Minnesota Sea Grant
• Randall Hicks, University of Minnesota Duluth
• Ted Smith, Marine Tech