Stress corrosion cracking behavior of pure copper in ammonia solution and groundwater containing ammonium ion

Taniguchi, Naoki ; Kawasaki, Manabu; Naito, Morimasa 

Since the propagation rate of stress corrosion cracking (SCC) is generally larger than that of other corrosion mode without cracking, it is difficult to avoid the penetration due to SCC by designing the corrosion allowance. Therefore, it is important to clarify the possibility of SCC initiation or conditions where SCC is possible to be occurred. It has been known that copper and copper alloys are susceptible to SCC in ammonia environment depending on the conditions. In this study, the SCC susceptibility of oxygen free copper was investigated in ammonia solution and groundwater containing ammonium ion under oxidizing condition by slow strain rate technique. As the results, no SCC was observed both in 0.05M and 0.1M NHOH solution. In Horonobe groundwater containing ammonium ion, brittle fracture surface and cracks were observed at -144mV vs. SCE. The morphologies of the SCC were not only intergranular type but also transgranular type and transgranular cracks branched from intergranular crack. In these test conditions, corrosion products were strongly adhered to the specimen surface and inside of the cracks. This indicates that the SCC was caused by tarnish rupture mechanism. In buffer material saturated with Horonobe groundwater, mechanical properties such as maximum stress and fracture strain were comparable with those in silicon oil, and no distinct cracks due to SCC were detected on the specimens.