A Superior Mystery: Steel corrosion in the Great Lakes largest port
The atmosphere of the River Talk meeting at the Barkers Island Inn last Wednesday was relaxed as around thirty-five citizens from the Twin Ports chatted amongst themselves as they waited for the meeting to begin. Surrounded by several photographs of the massive shipping vessels, the very industry that arguably has the most at stake from the effects of advanced steel corrosion which was the topic of the meeting.
In 1998 a hardhat diver named Chad Scott discovered severe corrosion on some steel sheet piling underneath the US Coast Guard dock on Park Point in Duluth, Minnesota. The discovery led to a twelve-year investigation into what was causing this type of corrosion in freshwater.
The ports of Duluth, Minnesota and Superior, Wisconsin collectively known as the Twin Ports are the busiest ports on the Great Lakes. Per the Duluth Seaway Port Authority, the Twin Ports handle an average of 35 million tons of cargo per year as well as accommodating over 900 vessels. This makes the Twin Ports arguably one of the most important ports on the Great Lakes, and in the United States.
So when Chad Scott, a former hardhat diver from the east coast was examining the steel sheet piling on the US Coast Guard dock in 1998 he was very concerned to find softball-sized pits in the steel. A level of corrosion usually not seen in freshwater. This led to an investigation into the cause of this corrosion which was described in the meeting by Scott in the following way. “The problem was a global one in the Duluth, Superior area.”
Further examinations of underwater dock structures across the harbor showed advanced corrosion from a level of 0–10 feet underwater, with most of the pitting, according to an article written by Gene Clark a coastal engineer working for Wisconsin Sea Grant, being about 25 millimeters deep.
According to Clark, “The corrosion was initially written off as stray current corrosion not biological corrosion.” The latter according to Clark “is very, very rare in freshwater.” The investigation ran into a dead end after partnering with Minnesota Power and Light to investigate whether or not stray current corrosion from electric pipelines might be responsible for the advanced rates of corrosion.
In 2004 the investigation took another turn when a study was started by the Wisconsin Sea Grant and five national and international experts on corrosion. This study led to the examination of 44 sites across the Twin Ports including taking photographs, water samples, and measurements of the corrosion pits. Randall Hicks, an expert on biological corrosion used a type of DNA fingerprinting to pinpoint the types of biologicals partly responsible for the advanced state of corrosion, these were called tubercles.
Clark and the rest of the team also formed a hypothesis to explain the strange rates of corrosion at a certain depth below the surface. “Ice would scrape off the growths and in the summer they would come back.”
This was a turning point in the investigation described by Scott the following way. “We knew what the problem was and now we had to find a solution.” Several types of coating underwent testing and four were selected for use as a way to combat the corrosion of the steel sheet piles and H-Beams in the harbor. The coatings had to do two main things, hold up under extreme conditions and be able to be applied in all conditions.
An example of the extent of the problems in the harbor can be summed up by the Midwest Energy coal dock in Superior, Wisconsin. A quote from Scott explains the extent of the damage to the H-Beams in the dock. “They were so bad, so thin that some were snapping off.” According to the Midwest Energy website, the dock is the largest and busiest single vessel coal dock in the United States. Even a short delay to conduct an emergency repair on the corroded beams would lead to a major loss of money for the coal industry here in the Twin Ports.
The study conducted by the Wisconsin Sea Grant into the advanced corrosion in the Twin Ports led to the National Oceanic and Atmospheric Administration awarding them with a Research to Application award. The main takeaway from the meeting was that this problem was serious, but there is a cost effective and long lasting way to fight this type of corrosion in the Twin Ports.