Mechanism for Improvement in Susceptibility to Stress Corrosion Cracking of a Tin–Bearing α–Brass by Adding Si and P

Abstract

A tin–bearing αBrass, αCu–Zn–Sn alloy, which is an αCu–Zn alloy with about 1 mass％ Sn added to increase the strength, shows good spring properties, but has high susceptibility to stress corrosion cracking (SCC)．The co–authors have found that the addition of Si and P to the αCu–Zn–Sn alloys significantly improves its SCC susceptibility. The objective of this study is to elucidate a mechanism by which the susceptibility is improved by the addition of Si and P. αCu–Zn–Sn alloy specimens with different concentrations of Si and/or P were prepared. The change over time in the anodic current density of the specimen bent to apply tensile stresses on the surface was measured in 0.1 M NaNO₂ solution under a constant potential. After the measurement, the components of the films formed on the specimen was analyzed by Raman spectroscopy. The concentration depth profile and the surface distribution of elements for the formed films were measured by XPS and EPMA respectively. The composition at the cross–section of the film sliced by FIB milling technique was analyzed by EDS. The results suggested that the addition of Si and P enhanced the protective properties of the surface film for the αCu–Zn–Sn alloy, suppressed the occurrence of local corrosion leading to the initiation of SCC, and prompted the repairing of the locally broken–down film by depositing as a silicon oxide or a phosphate film respectively. It was considered that low SCC susceptibility of the αCu–Zn–Sn alloys with Si and P is brought by the synergistic effect of these added elements.