`Electric Shock' Halts Steel Bar Corrosion
BOSTON — FOR many old bridges across the nation, a kind of "electric shock treatment" is helping to stave off corrosion caused by salt.
Engineers in Oregon, for example, are employing electric current in a $3.5 million project to pull corrosive chloride ions away from the steel reinforcing bars that support the Cape Creek bridge near Florence, Ore. To replace the bridge would cost $17 million, says Walt Eager, principal engineer with the state's transportation department. Also, the bridge is a historic one, designed 60 years ago to look like a Roman aqueduct.
In the two-year project, now almost complete, a thin coating of molten zinc was sprayed on the underside of the bridge and the pier columns, covering 100,000 square feet.
A positive electrical charge of one or two volts (no danger to humans) will be flowed to the zinc, and a negative charge to the reinforcing bars. In some places, the engineers welded connections between reinforcing bars to make sure that the negative charge will stretch through the whole steel framework. This sets up an electric field in which the negatively charged chloride ions that would corrode the reinforcing bars are instead drawn to the positively charged zinc and then dissipated as gas.
The electric current thus stops corrosion in its tracks, prolonging the life of the bridge indefinitely, Mr. Eager says. The zinc coating should last for at least 15 years. He says it may be possible to turn off the electric current for periods of up to six months without allowing corrosion. That could double the useful life of the zinc coating.
"Cathodic protection," as this approach is known, "has its limitations," says Michael Swanson, chief engineer of the Massachusetts Highway Department. Although much less expensive than replacing a bridge, it is not a cheap fix, he says. And a ready supply of electric current is needed.
But Mr. Swanson says the method is proving useful in several cases in the Bay State. One Natick highway overpass will likely be fixed with cathodic protection for $175,000 this year, he says.
"We've been promoting the use of cathodic protection for quite a while now," says Stanley Gordon, chief bridge engineer for federal highways at the United States Department of Transportation.
It's "a wonderful way of actually stopping the active corrosion from occurring," Mr. Gordon says.
But he says the approach hasn't really caught on yet. A number of states, including California, are experimenting with the technique.
Cathodic protection can be done using other conductive materials in addition to zinc, including a carbon-loaded paint. Eager says another material, titanium mesh, doesn't wear down through oxidation as the zinc does, but the Oregon project used zinc to better preserve the historic architecture of the bridge.
Today, new bridges are generally made with galvanized or epoxy-coated reinforcing bars, to avoid the corrosion problem.
Gordon says that 45,000 bridges out of 577,000 in the US need repair or replacement.