Duluth-Superior Harbor Corrosion
The problemSteel sheet piling in the Duluth-Superior Harbor is corroding at an accelerated rate. Based on observations of both older and new sheet pile installations, the increased rate of corrosion appears to have begun in the late 1970s. Underwater inspections have revealed that the corrosion is widespread throughout the harbor on all types of steel piling buttressing the docks. Most of the steel is riddled with small pits, scooped out in diameters of ¼ to 1 inch, primarily in the first four to six feet below the waterline and tapering off around 10 feet. Some of the steel beams supporting the dock structures have holes the size of footballs (these have already been or are in the process of being repaired). Thirteen miles of steel sheet piling and structures are corroding around the harbor, and if the problem isn’t addressed, the structural integrity of docks and loading facilities could be compromised and the failing steel would have to be replaced at a cost of $1,500 or more per lineal foot. The Duluth Seaway Port Authority estimates there could be 90 to 100 million dollars of possible repairs in the harbor to steel that is being weakened by corrosion.
Experts investigateTo provide a systematic focus for research and mitigation, a steering committee was formed by the Wisconsin and Minnesota Sea Grant programs, the Duluth Seaway Port Authority, the U.S. Army Corps of Engineers, and the University of Minnesota-Duluth and its Natural Resources Research Institute. The committee recommended that the corrosion problem be reviewed by an independent group of experts. In September 2004, a panel of five experts in corrosion, microbiology, and chemistry visited the harbor to examine the corrosion. They spent one day visiting areas of the port where accelerated corrosion was particularly apparent, and the next day they met to discuss their observations and review information gathered by the steering committee. Definitive conclusions about the causes and appropriate actions to mitigate the corrosion will require data gathered through formal measurement, testing, and engineering analysis. The experts narrowed down the initial list of 12 possible causes (all are described in the panel’s final report) to a few likely causes and made both long- and short-term recommendations.
Possible causesThe harbor’s water chemistry might have changed in ways that promoted corrosion. Highway de-icing salts may have added significant amounts of chloride to the harbor. Also, as reduced pollution improved water quality, higher amounts of dissolved oxygen in the harbor could have boosted corrosion rates. Microorganisms like bacteria or fungi could be eating away at the steel, a phenomenon known as "microbiologically influenced corrosion" (MIC). One type of MIC, accelerated low water corrosion, is reported to be a growing problem in European ports. Prior to the expert panel meeting, it was thought that stray current from a high-voltage direct-current line could be speeding up corrosion. The high voltage DC power line actually terminates several miles north of the harbor and the observed corrosion sites. Based upon preliminary testing, the experts considered stray currents to be an unlikely cause of the accelerated corrosion, but one that needs to be formally ruled out. The harbor has experienced many changes during the past 35 years, such as the rate of ship traffic, types of cargo ships and various harbor modifications. The panel noted that without more detailed studies, it is difficult to know which changes in harbor use may have affected steel corrosion.
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