In the past, ASTM specifications for low-alloy weathering steels, such as Specifications A242/A242M, A588/A588M, A606 Type 4, A709/A709M Grade 50W, HPS 70W, and 100W, A852/A852M, and A871/A871M stated that the atmospheric corrosion resistance of these steels is “approximately two times that of carbon structural steel with copper.” A footnote in the specifications stated that “two times carbon structural steel with copper is equivalent to four times carbon structural steel without copper (Cu 0.02 maximum).” Because such statements relating the corrosion resistance of weathering steels to that of other steels are imprecise and, more importantly, lack significance to the user (1 and 2) , the present guide was prepared to describe more meaningful methods of estimating the atmospheric corrosion resistance of weathering steels.

The first method of this guide is intended for use in estimating the expected long-term atmospheric corrosion losses of specific grades of low-alloy steels in various environments, utilizing existing short-term atmospheric corrosion data for these grades of steel.

The second method of this guide is intended for use in estimating the relative atmospheric corrosion resistance of a specific heat of low-alloy steel, based on its chemical composition.

It is important to recognize that the methods presented here are based on calculations made from test data for flat, boldly exposed steel specimens. Atmospheric corrosion rates can be much higher when the weathering steel remains wet for prolonged periods of time, or is heavily contaminated with salt or other corrosive chemicals. Therefore, caution must be exercised in the application of these methods for prediction of long-term performance of actual structures.

1.1 This guide presents two methods for estimating the atmospheric corrosion resistance of low-alloy weathering steels, such as those described in Specifications A242/A242M, A588/A588M, A606 Type 4, A709/A709M grades 50W, HPS 70W, and 100W, A852/A852M, and A871/A871M. One method gives an estimate of the long-term thickness loss of a steel at a specific site based on results of short-term tests. The other gives an estimate of relative corrosion resistance based on chemical composition.

1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.