Study author to speak in Superior Dec. 2.   11/25/2009 by Richard Thomas

A perfect storm of biological, chemical and physical factors has accelerated steel corrosion of dock walls in the Duluth/Superior Harbor, according to long-awaited research recently published in a scholarly journal.

The peer-reviewed paper published in “Corrosion, The Journal of Science and Engineering,” outlines a study commissioned by the U.S. Army Corps of Engineers, Detroit District, and the Duluth Seaway Port Authority.

The corrosion has been eating away at steel pilings in the Duluth/Superior Harbor for more than 30 years, consuming an estimated 50,000 plus pounds of steel a year.

The study leader — Brenda Little senior scientist, marine molecular processes, at the Naval Research Laboratory, Stennis Space Center in Mississippi — was not available for comment.

She will address the Metropolitan Interstate Committee’s Harbor Technical Advisory Committee on Wednesday, Dec. 2, in the Superior Public Library meeting room. The public meeting begins at 9 a.m. and Little is scheduled to speak at about 9:35 a.m.

Her study is the first to identify this form of corrosion, which appears to be a problem in all Lake Superior harbors, said James Sharrow, Duluth Seaway Port Authority facilities manager.

It identifies likely culprits— bacteria, ferric oxides, dissolved copper— but is less definitive about the sources.

“That area hasn’t been studied,” Sharrow said. “We’re still trying to characterize the full range of process and microbes involved.”

The report suggests manmade events are possible culprits. It quotes the Journal of Great Lakes Research, which describes the Lake Superior watershed as “an ecosystem that was disturbed earlier by turn-of-the-century mining in a patchwork manner along the shoreway. Portions of the lake are in a slow recovery phase, whereas other areas are still impacted by the slow resuspension-deposition dynamics and continuing mining activities.”

The report cites “erosion of metal-rich ore bodies around Lake Superior as a source of Cu (copper) enrichment in shoreline sediments.”

The Lake Superior region is naturally rich in copper. But mining activity from as far away as the Iron Range has contributed to leaching of elemental copper by streams in the watershed and run-off into the lake, said Chad Scott of AMI Consulting Engineers in Duluth/Superior. He first identified the corrosion in the 1990s.

Scott also found accelerated corrosion in Thunder Bay, Two Harbors, Ashland, and Lac Labelle, MI (site of a former copper mine), all areas fed by streams or rivers. By contrast, Silver Bay, MN, which has no river draining into the lake (and is home to an iron ore processing operation), has no accelerated corrosion problem, he said.

The corrosion apparently began to accelerate in the 1970s. Congress passed the national Clean Water Act in 1972 and the Western Lake Superior Sanitary District became operational in 1978. Before then, the water “probably wasn’t clean enough to support the bacteria,” Sharrow said.

Scott said it is unclear whether the bacteria involved is invasive or native to the area. It was not detectable during the decades the bay was polluted, but may have rebounded since the cleanup, he said.

Iron-oxidizing bacteria attach to carbon steel, creating a “nodule” of biomass and corrosion products. Conditions beneath those nodules (i.e. “tubercles”) cause copper dissolved in harbor water to precipitate and adhere to the iron.

When ice chunks scrape against those pilings each winter, the tubercles break, exposing the copper-covered iron to oxygen which, in turn, causes the steel in those pitted areas to corrode at a faster rate…creating an almost “Swiss cheese” effect in nearly 14 miles of steel pilings and dock walls.

A local team of experts has worked collaboratively since 2004 to define the methodology for studying — and mitigating — the accelerated corrosion problem in the Duluth/Superior Harbor. Several pilot projects are well underway, testing chemical coatings, applications and other procedures that will save existing steel structures and protect new ones from corrosion.

Scott said “99.99 percent” of the coatings available have not been tested in heavy ice zones.

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