Weathering steels, such as ASTM A588, A242, A606-4 and Cor-Ten® exhibit superior corrosion resistance over regular carbon steel as a result of the development of a protective oxide film on the metals surface which slows down further corrosion. Their 50,000 minimum yield strength characteristic allows cost reduction through the ability to design lighter sections into structures. These steels were designed primarily to be used in unpainted applications where a reduction in maintenance costs, such as painting, were desired. Since their development, weathering steels are now being used in a variety of applications, including bridges, rail cars, transmission towers, highway poles and shipbuilding.
Cor-Ten® is actually a brand name for corrosion resistant products that were developed by United States Steel. Corten has subsequently been licensed to be produced by other steel producers, such as Nippon Steel in Japan. There are basically two types of Cor-Ten that are most prevalent, Cor-Ten A and Cor-Ten B. The comparison of Cor-Ten to the ASTM grades is loosely stated as Cor-Ten A is equivalent to ASTM A242 and Cor-Ten B is equivalent to ASTM A588 Grade A. Cor-Ten A and B both meet and/or exceed the requirements of ASTM A606 Type 4.
In November 1997, ASTM modified and enhanced the corrosion resistant specifications by emphasizing an “Atmospheric Corrosion Index” that can be calculated to compare the relative useful life of different grades and heat lots of corrosion resistant steels. It also required that each ASTM grade meet a minimum Atmospheric Corrosion Index of 6.0 which effectively enhanced the quality of the weathering steels being produced. Cor-Ten products normally carry a nominal Atmospheric Corrosion Index of 8.0.
Prior to the specification change in 1997, as referred to above, the ASTM standard simply stated that the “atmospheric corrosion resistance is substantially better than that of carbon steels, with or without copper added”. The “substantially better” was replaced by a quantifiable formula for calculating the relative corrosion resistance of weathering steels and the specification was changed to force the producer to meet a minimum 6.0 Atmospheric Corrosion Factor. It should be noted that some producing mills still produce material to the 1991 specification, thereby avoiding the required minimum corrosion index. The buyer is advised to order to the 2004 A606-4 revision if they are seeking a quality corrosion resistant product.
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