Stress-Corrosion Cracking of Copper Alloy C27000 Ferrules in Storage and in Service in Chemical Plants

A substantial number of copper alloy C27000 (yellow brass, 65Cu-35Zn) ferrules for electrical fuses cracked while in storage and while in service in paper mills and other chemical processing plants. The ferrules, made by three different manufacturers, were of several sizes. One commonly used ferrule was 3.5 cm long by 7.5 cm in diam and was drawn from 0.5 mm (0.020 in.) thick strip. Investigation (visual inspection, metallographic examination, and a mercurous nitrate test, which is an accelerated test used to detect residual stress in copper and copper alloys) of both ferrules from fuses in service and storage in different types of plants, and ferrules from newly manufactured fuses, supported the conclusion that the ferrules failed by SCC resulting from residual stresses induced during forming and the ambient atmospheres in the chemical plants. The atmosphere in the paper mills was the most detrimental, and the higher incidence of cracking of ferrules there was apparently related to a higher concentration of ammonia in conjunction with high humidity. Recommendations included specifying that the fuses meet the requirements of ASTM B 154.