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Galvanizing Into Action Against Rust
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# : |
15683 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
2 / 1989 |
2,405 Words |
| Author
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Franke E. Goodwin and Adolph L. Ponikvar
Frank E. Goodwin is vice president of material science for the
International Lead Zinc Research Organization, Inc. (ILZRO).
Adolph L. Ponikvar is a technical writer and consultant for
ILZRO. |
Rust, the product of the oxidation of iron and hence the corrosion of steel, is an unpleasant but unavoidable fact of life for a world whose backbone is still primarily made of steel. Although growth in plastics applications has been phenomenal in the last two decades, the world's buildings, automobiles, and bridges still depend heavily upon steel. World steel production dwarfs the combined production of copper and aluminum--the next two most widely used metals.
Steel's popularity as an engineering material comes from its relatively low cost, high strength, and workability. But unlike higher-priced copper and aluminum, steel disintegrates relatively quickly when left unprotected in outdoor environments. Therefore, the use of steel in engineering applications is economical only if its protection from corrosion can be assured, because the failure to provide this protection results in enormous costs. In any industrialized country, the annual cost of corrosion is estimated to be between 2 and 3 percent of its gross national product. In the United States alone, this means a cost of $80-120 billion annually.
How To Prevent Rust
There are three principal methods used to protect steel from rusting on an engineering scale. Paints are one family of organic coatings, and can be applied to steel sheets destined for appliances, metal buildings, and various automobile parts.
A second type of protection is achieved when a reactive chemical, such as phosphate, is sprayed onto steel. The steel reacts with the applied chemical to form a protective layer. The third method is to coat steel with a metal more resistant to corrosion than steel, such as copper, aluminum, or zinc. Of these, a zinc coating, in a process commonly termed galvanizing, is by far the most widely used because it does a better job and costs less. Sixteen billion pounds of steel products were galvanized in the United States in 1986, a figure that has grown even further since then. The biggest growth has occurred in galvanized sheets for automotive, appliance, and construction applications.
Galvanized coatings on structures such as bridges and electric transmission towers have a life expectancy of 25-50 years. Although galvanizing is more expensive in initial construction, it can pay for itself when the lifetimes of alternative coatings are considered. Bridges that are only painted can show rust after 5-10 years, requiring repainting at these time intervals. The
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