Corrosion resistance

Long considered a serious drawback for steels; corrosion not only reduces steel through loss of material but is often unaesthetic. But much is being done to improve steel's corrosion resistance and, in at least one application, to promote the process of surface corrosion.

Rust can be good for steel!

Known worldwide as weathering steel, this development evolved from a 1933 discovery that steel made from a copper-bearing ore had greater than expected corrosion resistance. In most climatic conditions, the various modern grades of weathering steel develop an adherent layer of corrosion that serves as a partial barrier to further corrosion. Once this occurs, and if the adherence prevents flaking of the corrosion layer, the corrosion process proceeds at a rate that is not structurally detrimental to the capacity of the structure.

First used for railroad cars carrying coal, and now widely applied for fabrication of cargo/freight containers, the evolutionary versions of this steel today are very important to the unique application of bridge construction. Maintaining paint on a bridge is costly and, over its lifespan, may well cost far more than the original bridge. Use of weathering steels provides a cost-effective solution; first by the initial savings in paint and, secondly, by the material and environmental savings of not having to repaint a difficult structure.

The use of weathering steels has grown in recent years and will continue to grow in the future. Recent developments have increased levels of strength and facilitated welding processes. It is anticipated that increasing worldwide environmental restrictions of the removal and replacement of paint will further boost the market for these steels in both bridge and building applications.

Resistance to corrosion is also addressed by other developments, principally protective corrosion-resistant barriers. Zinc, tin, lead, chromium and aluminum have served this function for years, but new developments have enhanced their effectiveness. One such improvement has been the addition of various quantities of aluminum to zinc for hot-dipped coatings, creating a product that is a synergistic improvement - the resulting corrosion resistance, for the same thickness of coating, is better than the sum of both protective materials. Developments in this field, in which other metallic barrier materials are being studied, continue worldwide. The extended life of automobiles, appliances and other capital goods will depend even more on improved corrosion resistance as the tendency toward higher strength, lighter and thinner steels continues.

Paint, both organic and inorganic, is another version of the corrosion-resistant barrier. These products have also undergone major improvements in recent years. One of the more significant is when paint, in multiple coats from primer to topcoat, is applied under controlled conditions to steel in coil form. These paint systems, which then have the adhesion and ductility needed for being formed into final products, provide a high quality finished surface that eliminates the environmental and workplace hazards of post-fabrication spraying or dipping. Use of these products will continue to rise, not only because of the better quality of the coated surface but because environmental restrictions will favour the application of paint in a totally controlled environment.

When an even more durable surface treatment is needed, an alternative to a hot-dipped coating is a heat-cured epoxy coating. Today, worldwide, this product is usually the material specified for environments typified by marine, bridge and highway applications where salt corrosion is anticipated. Its use is growing, particularly to extend the life of concrete in bridge surfaces.