A review of calculation methods for atmospheric corrosion ratings of high-strength, low-alloy weathering steels, such as ASTM A242 and A588, shows that a wide range of "rating numbers" relative to carbon steel (or copper-bearing steel) can be obtained from the same data set by using different methods. Therefore, such rating numbers cannot be used for comparison purposes unless the exact method of calculation is specified. (ASTM specifications do not specify the method.) More importantly, however, an analysis of the rating numbers as compared to actual corrosion losses in various environments leads to the conclulusion that the rating numbers have little practical significance. Moreover, misinterpretation of the rating numbers can lead to erroneous conclusions with respect to the durability of weathering steels in a given environment.

It is recommended, therefore, that the use of corrosion rating numbers for weathering steels be phased out, and that a more meaningful method of rating the performance of these steels be sought.

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