The problem
Steel 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 harbour on all types of steel piling buttressing the docks. Most of the steel is riddled with small pits, scooped out in diameters of 1/4 to one 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 harbour, 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 US$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 harbour to steel that is being weakened by corrosion.
Experts investigate
To 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 US 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 harbour 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 causes
The harbour’s water chemistry might have changed in ways that promoted corrosion. Highway de-icing salts may have added significant amounts of chloride to the harbour. Also, as reduced pollution improved water quality, higher amounts of dissolved oxygen in the harbour 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 harbour 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 harbour has experienced many changes during the past 35 years, such as the rate of ship traffic, types of cargo ships and various harbour modifications. The panel noted that without more detailed studies, it is difficult to know which changes in harbour use may have affected steel corrosion.
Short-term recommendations
The panel recommended corrosion rates at a number of sites in the harbour be measured to establish a baseline for future reference.
Water chemistry analyses should be made for at least two years at a number of representative sites and depths. Corroded steel should be tested for the presence of MIC activity. Tests to determine the presence and source of any stray currents should be performed. In areas where safety issues or economic losses are of high concern, structure conditions should be assessed.
Long-term recommendations
A coordinated maintenance management strategy should be developed to provide a systematic, proactive means of tracking current and projected conditions. An ongoing monitoring programme should be established for water chemistry and corrosion rate measurements. A standard replacement design should be developed using both coatings and cathodic protection.
The panel of experts strongly recommended that other Great Lakes ports and harbours be studied and their managers made aware of this issue. Because this type of rapid corrosion isn’t common in freshwater harbours, harbour managers may not be looking for the problem.
Corrosion study update
Funding: Multiple sources of study funding continue to be pursued by the steering committee. To date, the State of Minnesota has appropriated $100,000 and in November 2005 the US Army Corps of Engineers were approved $300,000 (FY 06 budget) for the initiation of studies to determine the causes of the corrosion. The steering committee is seeking similar levels of federal funding for FY07, approaching the State of Wisconsin for a grant similar to the Minnesota appropriation as well as financial support from owners of dock facilities throughout the harbour (goal of $50,000).
