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MAINTENANCE : Corrosion Does Not Have to Lead to Rust

September 18, 1993|From Associated Press

Metal rusts. Even if it's painted, it can still rust and look lousy. But just because metal rusts doesn't mean you have to accept it. You can fight back and win the battle against corrosion.

Corrosion is not necessarily synonymous with rust. Rust is the brown stuff that forms as a result of corrosion on steel. The simple forms of corrosion found around the house are rusting steel, galvanic corrosion and corrosion on uncoated aluminum.

Here's a simplified version of the complex electro-chemical process of how rust forms on painted metal:

Moisture penetrates through pores in the paint film or through a scratch. The oxygen in the water combines with the iron in the steel, and a small amount of iron is dissolved, forming a solution. There is an imbalance of electrons between the solution and the surrounding steel, and this leads to a flow of electrons, namely current. As long as the current flows, the metal will deteriorate (corrode) and rust will form.

Because rust has more volume than the steel, it expands and helps form a blister under the paint. The blister will cause the paint to fail, exposing the steel to more moisture and accelerating the corrosion process.

Here are four steps that can greatly slow the decay of steel. Note that the steps are inter-related and that paint can play a role in each step:

* Keep it clean. Dirt holds moisture, and moisture effects steel.

* Keep it dry. No moisture, no rust. This puts a premium on repairing blistered paint and using the right paint for the job.

* Insulate it. Not only does paint keep the metal dry, but also almost all paints act as insulators. By stopping current flow, paint prevents corrosion.

* Galvanize it. Galvanized steel is coated with zinc. The steel is dipped in molten zinc (known as hot dipped), or it is plated by being immersed in an electro-chemical bath. Homeowners can't be galvanizers, of course, but they can use galvanized hardware. They can also use a primer with 84% zinc by weight.

Zinc is a special material as far as steel is concerned. In corrosive conditions, it loses electrons at a faster rate than steel does. If you cover the steel with zinc, the zinc will corrode, but the steel remains unscathed until the zinc is gone.

A related means of protection is to position a zinc bar--called an anode--near the steel. This is the method used to protect steel swimming pools. Likewise, magnesium anodes are used to protect hot water tanks.

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