Effect of 1 Percent Copper Addition on Atmospheric Corrosion of Rolled Zinc After 20 Years' Exposure

Abstract

The atmospheric corrosion characteristics of a commercial rolled zinc alloy containing 1 percent copper and an unalloyed grade have been evaluated after 20 years' exposure under ASTM Committee B-3 (now G 01.04.04) 1957 test program on atmospheric corrosion of nonferrous metals. Corrosion damage was assessed by measurement of loss in weight, loss in mechanical properties, and depth of pitting. The 20-year corrosion rates are analyzed with respect to previous 2- and 7-year results. Comparisons are also made with the behavior of three unalloyed grades of rolled zinc investigated in the 20-year 1931 program by Committee B-3 using weight loss and tension test methods. Weight loss method appears to be the most reliable way of determining the corrosion rates of zinc. Corrosion rates found for zinc in the 1931 and 1957 test programs are in good agreement. The corrosion rate of zinc is not significantly affected by variations in compositions investigated. Local galvanic action is probably responsible for the pitting which occurs in the copper-containing zinc alloy. Pitting is most severe in the marine atmosphere in direct contact with salt spray. The pit-depth/total-penetration ratio tends to decrease with increasing total penetration. The net change in tensile strength after 20 years' exposure at all four test sites is equal to or less than that expected due to weight loss. Tensile elongation is found to be more sensitive to corrosion effects than tensile strength, although both alloys still possess considerable ductility after 20 years. A better way of compensating for aging in zinc is needed if tension test results are to be useful in evaluating corrosion damage.