Steel pilings in some Lake Superior harbors are showing extraordinary amounts of corrosion, which may be partly caused by iron-damaging bacteria. —Minnesota Sea Grant photo

Local scientists and organizations are working to solve an economic and environmental dilemma: steel pilings in North Shore harbors corroding as quickly as they would in saltwater.

“It’s amazing to see four-year-old steel pitted and damaged like it’s been in the water for a century,” said Dale Bergeron, Minnesota Sea Grant’s maritime extension educator.

Several years ago, a Sea Grant partnership with the U.S. Army Corps of Engineers, corrosion experts, and others produced hypotheses that pointed to water chemistry changes based in dissolved oxygen and chlorides as the corrosion culprit.

“It’s amazing to see four-year-old steel pitted and damaged like it’s been in the water for a century”

Those hypotheses have yet to be proven, but some facts have been determined:

• many harbors in Western Lake Superior seem to be corroding at unusually fast rates;
• at least two forms of iron-associated bacteria live on the harbor’s steel;
• river water speeds corrosion;
• corrosion slows in colder temps;
• corrosion has a vertical range.

In October 2007, Bergeron and Jim Sharrow, facilities manager of the Duluth Seaway Port Authority, began hunting for Lake Superior evidence of accelerated corrosion.

Coupled with South Shore information collected by Gene Clark of Wisconsin Sea Grant, their findings indicate that freshwater corrosion has accelerated Duluth-Superior, Two Harbors, Thunder Bay, Bayfield, Houghton, and Marquette, some of Western Lake Superior’s busiest harbors.

“The more we look, the more we see,” said Clark.

Minnesota Sea Grant researcher Randall Hicks says orange nodules covering corroded harbor steel contain at least two bacteria that may influence corrosion: the iron-oxidizing Siderooxidans lithoautotrophicus and the iron-reducing Rhodoferax ferrireducens.

These results don’t prove that bacteria are accelerating the corrosion, but hold them at least partially responsible. Hick’s work indicates that corroding steel structures are covered by complex microbial biofilms, which contain bacteria that damage steel in other environments.

Experts also note that rapid corrosion occurs only on steel directly contacting water and seems worst on pilings facing the sun.

Duluth-Superior Harbor water levels have fluctuated almost four feet (1.2 meters) in the 40 years since corrosion is thought to have sped up. Steel seems in good shape along most waterlines, but from just below the waterline to about three feet down it’s consistently distressed, and sometimes fully perforated or riddled with ice-cream-scoop-style pits. Corrosion transitions from deep to shallow pitting, which peters out at about 10 feet, below the scour power of winter ice, and where zebra mussels blanket the steel.

Research has been accelerated by the possibility of $100 million worth of repairs in the Duluth-Superior Harbor, and evidence of accelerating corrosion and imminent repairs in other Lake Superior ports.