Steel Corrosion

Corrosion Doctors Web pages related to steel

Iron and steel, the most commonly used metals, corrode in many media including most outdoor atmospheres. Usually they are selected not for their corrosion resistance but for such properties as strength, ease of fabrication, and cost. These differences show up in the rate of metal lost due to rusting. All steels and low-alloy steels rust in moist atmospheres. In some circumstances, the addition of 0.3% copper to carbon steel can reduce the rate of rusting by one quarter or even by one half. The elements copper, phosphorus, chromium and nickel have all been shown to improve resistance to atmospheric corrosion. Formation of a dense, tightly adhering rust scale is a factor in lowering the rate of attack. The improvement may be sufficient to encourage use without protection, and can also extend paint life by decreasing the amount of corrosion underneath the paint. The rate of rusting will usually be higher in the first year of atmospheric exposure than in subsequent years, and will increase significantly with the degree of pollution and moisture in the air.

Ordinary steels are essentially alloys of iron and carbon with small additions of elements such as manganese and silicon added to provide the requisite mechanical properties. The steels are manufactured from a mixture of pig iron and scrap, which is treated in the molten state to remove excess carbon and other impurities. The steel may be continuously cast into strands or cast into individual ingots. The final product is then produced by rolling, drawing or forging. During hot rolling and forging the steel surface is oxidized by air and the scale produced, usually termed millscale. In air, the presence of millscale on the steel may reduce the corrosion rate over comparatively short periods, but over longer periods the rate tends to rise. In water, severe pitting of the steel may occur if large amounts of millscale are present on the surface.

Many pages of the Corrosion Doctors site discuss specific issues related to the corrosion behavior of steels. The following are references to some of these pages: