Accelerated Freshwater Harbor CorrosionGene Clark, Coastal Engineering Specialist www.seagrant.wisc.edu/corrosion

The problemSteel sheet piling in the Duluth-Superior Harbor is corroding at an accelerated rate. This rate of corro-sion is similar to that commonly observed in saltwater ports but not seen in freshwater environments. In addition, corrosion to this extent has not been docu-mented in other Great Lakes ports and harbors. Based on observations of both older and new sheet pile in-stallations, 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. Most of the steel is covered with small pits, scooped out in diameters of six to 25 mm, primarily in the first one to two meters below the wa-terline and tapering off around three to three-and-a half meters. For most structures there is only minor or insignificant corrosion loss deeper than three-and-a-half meters, all the way down to the mud line. Some of the steel dock structures have holes as large as 15- 30 cm or more, and several of these structures have already been or are in the process of being repaired or replaced.

There is light marine growth evident within the pits with zebra mussels (recent infestation since 1998) tending to cover the steel below three-and-a-half me-ters. However, the maturity of the corrosion indicates that its initiation predates the zebra mussel infesta-tion.

Over 20 kilometers 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 120 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 miti-gation, 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 ex-amine the corrosion. The panel noted that definitive conclusions about the causes and appropriate actions to mitigate the corrosion will require data gathered through formal measurement, testing, and engineer-ing analysis. The experts listed the most likely causes of the corrosion and made study recommendations.

Possible causes � The harbor’s water chemistry might have changed in ways that promoted corrosion. Highway de-ic-ing salts may have added significant amounts of chloride to the harbor. Also, as reduced pollution improved water quality, higher amounts of dis-solved 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

Underwater inspections have revealed that accelerated corrosion is widespread throughout the Duluth-Superior Harbor on all types of steel piling buttressing the docks. The Duluth Seaway Port Authority estimates there could be 120 million dol-lars of possible repairs in the harbor to steel that is being weakened by corrosion.

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Updated April 2010

that stray current from a high-voltage directcur-rent line could be speeding up corrosion. The high voltage DC power line actually terminates several miles north of the harbor and the ob-served corrosion sites. Based upon preliminary testing, the experts considered stray currents to be an unlikely cause of the accelerated corrosion, but one that needed 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 modifica-tions. The panel noted that without more detailed studies, it is difficult to know which changes in harbor use may have affected steel corrosion.

Short-term Pre-Project Funding Studies CompletedPrior to the actual allocation of project funding, two short-term investigations were completed. The first in-vestigation was a coating test to observe commercial products available for protecting steel sheet pile struc-tures from corrosion. The coatings have been in place for almost five years and were removed in fall 2006.

The second investigation involved the visual in-spection of the chains and harbor channel marker buoy anchors placed and removed by the U.S. Coast Guard each year throughout the harbor. These were visually inspected for evidence of pitting and corrosion as they were removed in early winter 2005. Observations of the anchor chains indicate a gradual lessening of cor-rosion as one moves upstream from the lower harbor basin. This trend is consistent with the general obser-vations of more severe corrosion in the lower harbor structures than those that are further upstream.

Formal Studies InitiatedMultiple sources of study funding continue to be pur-sued. Initial project funding began in 2005 when the State of Minnesota appropriated $100,000 and the U.S. Army Corps of Engineers approved $300,000 (FY 06 budget) for the initiation of studies to determine the causes of the corrosion. Additionally, the University of Minnesota Center for Urban and Regional Affairs (CURA) and Minnesota Sea Grant began supporting research into specific biological influences that may be contributing to the corrosion process.

2006 Field Investigation ResultsThe reports for the 2006 project field investigations are available as PDF documents in the “Documents” section of the project Web site. Both reports contain data summaries, structure condition photographs, and the results of these first phases of the Duluth-Superior corrosion studies..

2007 Project Investigation ResultsThe majority of field work completed in 2007 consist-ed of collecting existing coupons (small steel plates) from the trays installed in 2006 and the installation of eight new trays. A study to collect field data from both

bare steel and coated steel samples was also initiated. The bare steel samples will continue to be evaluat-ed for actual corrosion process information and the coated samples will investigate how several different types of protective coatings work under the actual harbor corrosion and ice conditions.

MIC laboratory investigations were initiated by the University of Minnesota Duluth and the Naval Research Laboratory. Initial results demonstrated that bacterial communities on the corroded steel in the most affected part of the harbor are different from bacterial communities on these structures at a less af-fected area just outside the harbor. A bacterium that oxides iron (from Fe2+ to Fe3+) was isolated from the corroding structures. While the preliminary research may indicate that MIC may be responsible for the ac-celerated corrosion, conclusive evidence has not yet been found. Additional research continues.

2008 Investigations$310,000 of new project funding was secured for the 2008 project investigations. Grants were received by the University of Wisconsin Sea Grant Institute (Wisconsin Coastal Management Program fund-ing) and the Federal Water Resources Development Act. Testing completed included (project reports are available as PDF documents in the “Documents” sec-tion of the project Web site):

� Existing Coupon Sample Collection: Investiga-tion focused on the collection of a representative set of coupons from the trays already in place throughout the harbor. Both uncoated and coated coupons were removed, inspected, and then re-placed. Several coupons were also removed and sent on for additional lab tests.

� In-Place Coating Tests-Ice Zone Focus (Ice Abra-sion and Impact): This study consisted of installing a total of 18 three-foot long steel samples (nine pairs of each coating test product) directly within the critical ice zone. Samples were placed with their tops approximately six inches above the ex-isting water level at a pier location along the high

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Coupon tray example with pulled coupon

volume shipping traffic channel. Test samples will be inspected yearly to observe their ability to withstand the high ice abrasion and impact which is common in the Duluth-Superior harbor.

� Existing In-Place Coating Inspections—Review of Existing Coatings In Place: This study docu-mented the coating history and included diver inspections of six existing coated sheet sites al-ready in place throughout the harbor. These coated steel structures were placed before the current corrosion study was initiated and evalu-ation of these locations will serve as companion tests to the new, more controlled coating trials.

� In-Place Coating Tests—New Coating Applied To Existing Sheets: This study began the full-scale testing of coating existing steel structures within the harbor. This work demonstrated the applica-bility of using cofferdams as a base case method for cleaning and coating existing steel already in place but not damaged beyond repair. Approxi-mately 300 feet of dock wall was successfully cleaned and coated with an additional 900 feet to be completed in 2009.

� DC Current Tests: A rigorous high voltage DC (HVDC) current test was completed with mea-surements taken at four separate locations in the harbor. This test investigated the possible effects of the high voltage DC power line that terminates in the region and showed no stray current effects from the HVDC system on the corrosion of the steel piling at the four harbor test locations.

� Continued MIC Testing: Microbiologically In-fluenced Corrosion (MIC) testing continued by sampling microbial biofilms that developed on the steel coupons and then extracting DNA from those biofilms. Preliminary comparisons of these bacterial communities was completed by DNA fingerprinting. Preliminary results have indicated that iron-oxidiz-

ing bacteria are present and tests will continue to determine whether or not these or other bacteria may be the cause of the accelerated corrosion.

� Alternative Technology Tests: This effort installed three alternative corrosion control methods (different than the standard coating methods). Included in these tests were the installations of commercially available products designed for jacketing pipe piles and H piles. The third system installed a cathodic protection system to study how effective cathodic protection could be for corrosion control within the harbor.

2009 Project InvestigationsWith a 2009 project budget of over $300,000 from new funding from the state of Wisconsin (grant to the City of Superior) and the U.S. Army Corps of Engineers, several new and continuing tests were completed dur-ing 2009:

� Continued Coupon Sample Collection: The col-lection, inspection and selected removal of coupons from the existing coupon trays in place throughout the harbor continued. Both uncoated and coated coupons were removed for inspec-tion and then re-installed. Another representative sample set was permanently removed for addi-tional MIC lab tests.

� In-Place Coating Tests-Ice Zone Focus (Ice abra-sion and Impact): The ice abrasion and impact test samples that were installed in late fall of 2008 were inspected and then returned for another year of ice abrasion and impact tests.

� In-Place Coating Tests—New Coating Applied to Existing Sheets: This study continued the 2008 ef-forts of full-scale testing of coating existing steel structures within the harbor. This work tested the applicability of using cofferdams as a base case method for cleaning and coating existing steel al-ready in place but not damaged beyond repair. A total of eight commercially available coatings were applied side-by-side (each covering an area approxi-mately 8 ft wide by 12 feet deep) using the cofferdam method of in-situ steel surface preparation.

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Samples of alternative pile protection

Installation of abrasion coupons

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� Additional Coupon Trays: Additional coupon trays may be added at new harbor locations such as up-stream in the Nemadji River, at the upstream dam in the St. Louis River and other selected locations.

� Continued Bulk Chemistry Tests: Additional bulk water chemistry samples and associated testing may be collected both at the Duluth/Superior harbor and also several other harbors to compare changes of water chemistry with those samples collected in 2006.

� Continued MIC Testing: Microbiologically Influ-enced Corrosion (MIC) testing will continue to investigate the role MIC may have in the acceler-ated corrosion process.

� Project GIS Work: The project Geospatial data-base will continue to be updated with both new project and other historical harbor data. Several methods for data visualization will be reviewed and prepared.

Corrosion at Other Great Lakes LocationsCorrosion issues continue to be observed visually at several other Great Lakes harbors and marinas, but these locations have yet to be inspected by divers. The most severe corrosion seen elsewhere tends to be observed in other Lake Superior facilities. Photos of some of these locations have been posted to the project Web site. While the causes for these struc-ture corrosion issues at other Great Lakes locations may or may not be directly related to the same type of accelerated corrosion seen at the Duluth-Superior harbor, the photos illustrate that study results will be extremely useful for rehabilitation projects elsewhere in the Great Lakes region.

� Continued MIC Testing: Microbiologically In-fluenced Corrosion (MIC) testing continued to investigate the role MIC may have in the ac-celerated corrosion process. In late 2009 it was determined by the Naval Research Laboratory in-vestigations that MIC did indeed play a role in at least one corrosion mechanism through the com-bination of iron oxidizing bacteria and copper in the water plating out in the pits creating a galvanic couple which caused the corrosion to accelerate.

� Project GIS Work: A project Geospatial data-base was prepared which included all project data measured to date and several data layers compared with the corrosion observations to in-vestigate potential water quality factors with the high corrosion rates.

2010 Project Investigations PlannedSeveral new and continuing tests are scheduled for 2010. Testing is expected to include:

� Continued Coupon Sample Collection: Coupons will continue to be collected from the existing trays and analyzed for effects from another year of harbor corrosion.

� In-Place Coating Tests-Ice Zone Focus (Ice abra-sion and Impact): The ice abrasion and impact test samples will be pulled, inspected and then returned for another year of ice abrasion and impact tests.

� Linear Polarization Tests: Additional linear polar-ization testing may be conducted for up to 12 to 14 locations (minimum of two cycles each) to es-timate instantaneous corrosion rates at different locations throughout the harbor.

� Alternative Technology Tests: These investiga-tions will explore seven additional alternative corrosion control methods. Anticipated meth-ods that would be tested may include additional wraps and jackets for both pipe and H pile struc-tures and certainly several types of plates (both steel and HDPE) for sheet pile structures.

Cofferdam coating trials

Corrosion observed at the Houghton County Marina, Michigan