DULUTH — Given the widespread corrosion that long has been well documented in the Twin Ports, it’s no surprise the hull of the William A. Irvin, Duluth’s museum ship, was found to be pockmarked when it entered drydock at Fraser Shipyards more than a month ago.

But the extent to which many of the rivets on its underside had been damaged stirred greater concern, said Chase Dewhirst, manager of marine civil engineering for AMI Consulting Engineers PA.

“It came as a surprise, but we’re working on a solution, and we’re confident that the Irvin will be back,” said Chelly Townsend, referring to the museum ship’s expected return to the local tourism scene in 2020. As executive director of the Duluth Entertainment Convention Center, which owns and manages the Irvin, Townsend emphatically maintains the 81-year-old laker will not miss another season, despite being sidelined for the past two.

The Irvin will be sandblasted, painted and coated with a two-part epoxy, which has proven to be an effective barrier to corrosion throughout the port.

“It’s the same coating we’ve been applying to dock walls,” Dewhirst said.

Steel submerged in the Duluth-Superior Harbor has been found to corrode more than twice as fast as would typically be expected in a freshwater setting, said Jayson Hron, director of communications and marketing for the Duluth Seaway Port Authority.

“Some estimates suggest this corrosion consumes approximately 50,000 pounds of dock wall steel annually in our harbor,” he said, noting that the problem was first identified in the late 1990s.

“Researchers connect it partly to the presence of metal-rich ore bodies around Lake Superior, which causes copper enrichment in shoreline sediments,” Hron said.

A multi-agency investigation into the phenomenon found that waterborne iron-oxidizing microbes attach to steel, forming tubercles. Those tubercles can be opened by the scouring power of winter ice. In turn, the ruptured tubercles attract dissolved copper, spurring a chemical reaction that can pit and weaken the surrounding steel. This is believed to be the cause of aggressive localized corrosion.

“The outcome of this effort provided engineers a basis for using existing high-performance coatings and application procedures to protect steel components and extend the lifespan of harbor structures,” Hron said.

The corrosion has been most evident along harbor docks. Marine traffic, which remains on the move, has been less susceptible. But the Irvin, which has remained docked and mostly stationary at Minnesota Slip for more than three decades was a virtual sitting duck for the aggressive corrosion.

Dewhirst said the damage the Irvin has sustained would be of far greater concern if it was still a working vessel instead of one on stationary display.

“This vessel’s not going to see the same types of forces. It does not go out on Lake Superior. It’s not going to be carrying loads. It’s not going to see all the wind forces. It’s going to be in a protected slip. So, the standards for this vessel are completely different than for active vessels,” he said.

“There is a possibility that we won’t do anything to the rivets,” said Dewhirst, explaining that one the Irvin returns to Minnesota Slip and resumes operation as a museum, its hull could be closely monitored on an ongoing basis and any problem rivets could then be replaced either from inside or outside the hull.

Dewhirst said that only a small portion of the hull has been thoroughly inspected for rivet damage. But of the 95,000 rivets in the hull of the Irvin, he estimates that maybe 5-10% of those below the water line have been corroded by half or more of their original dimensions.

Stray voltage may also have played a role in the accelerated corrosion of the rivets, Dewhirst said. He suspects the Irvin’s electrical system may have been grounded to its hull and explained: “The rivets and the steel shell plate are two different metals. So, when you have stray current and similar metals, the rivets start to become the sacrificial anode.”

New ground fault circuit interrupters that have been installed in the Irvin should help prevent that from happening in the future.

Dewhirst said he continues to evaluate the vessel and whether it requires additional work beyond new paint and epoxy coatings, but he expects the Irvin to return to its old mooring at Minnesota Slip by October.