Metals company aims to save rusty bridges with zinc 'jackets'

Posted: Thursday, April 20, 2000

INDIANAPOLIS (AP) -- An Indianapolis metals company is turning the destructive power of saltwater into an environmentally friendly way to protect bridges in the Alaskan wilderness.

Alltrista Corp., also a maker of home canning products and plastic goods, has created a system to fight corrosion that will soon be in place on two bridges in Ketchikan, Alaska, within the Tongass National Forest. The company will provide 243 zinc ''jackets'' to protect bridge piles and beam supports that are decaying because of moisture penetration.

The $7.6 million project aims to stem corrosion by wrapping bridge supports in a zinc mesh that absorbs corrosion. The system uses saltwater and the natural properties of zinc to create a rust-preventing electrochemical brew.

The Ketchikan venture is Alltrista's largest installation of its Lifejacket system, which has been in development for about 10 years.

It is designed to preserve concrete bridge supports that have begun to decay because their steel reinforcements are corroding.

The corrosion starts when moisture penetrates aging, porous concrete, causing rust to form on the steel inside. As the steel corrodes, it expands, causing cracks in the concrete that allow more water inside.

''The general tendency has been to paint it or hide it and hope the problem goes away,'' said Douglas Leng, an inventor of the system and director of North American sales for Alltrista Zinc Products.

But Alltrista's system takes the ''evil culprit'' -- saltwater -- and uses it to prevent corrosion through an electrical process, Leng said.

The principle is similar to how a flashlight battery works, by manipulating a mild electrical current. Because zinc is more easily corroded than steel, it begins to rust faster. As it rusts, it creates a low current that attracts corrosive elements, protecting the steel from further damage.

The design is called a ''sacrificial anode'' because the zinc in essence ''sacrifices'' itself for the steel, Leng said. A similar process is used to make galvanized steel and to fight corrosion on the hulls of many oceangoing vessels

Alltrista's system provides a way to extend the life of bridges by up to 30 years without extensive reconstruction that could harm wildlife, Leng said.

That's especially valuable in Alaska's tidal zone, which is both environmentally sensitive and harsh on concrete structures, said Jack Tinnea, an engineer hired by Alaska's state government to help with the project. The region has tidal ranges up to 30 feet twice a day.

The method is also less disruptive to popular natural areas such as southern Alaska, where cruise ships make frequent stops and the economy depends on tourism. Rebuilding a bridge could mean closing roads for months, Tinnea said.

With 17 million acres, Tongass is the largest national forest in the United States. Among its wildlife are wolves, rare hawks and a large concentration of bald eagles and brown bears.

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