2021 Volume 60 Issue 1 Pages 130-134
Strength–improved α Cu–Zn–Sn, which is an alloy of α brass (Cu–Zn) and about 1 wt% of Sn, is one of promising materials for electronic connectors. Although α Cu–Zn–Sn is not as susceptible to stress corrosion cracking (SCC) as α brass, further improvement of the susceptibility has been desired. Recently, it was reported that the time to fracture by SCC of α Cu–Zn–Sn in an ammonia gas atmosphere was extended about 10 times by adding 2 wt% of Si and 0.05 to 0.10 wt% of P. However, the mechanism by which the addition of Si and P improves SCC susceptibility has not been clarified. In this study, we focus on the effect of phosphorus and investigate why the addition of P improves the SCC susceptibility of α Cu–Zn–Sn. The polarization measurement and the SCC test at a constant potential are carried out in a sodium nitrite solution, which is known as a SCC environment for α–brass. Followings are investigated： changes in the polarization characteristics of α Cu–Zn–Sn–P with 0 to 0.20 wt% of P and in the time to fracture at a constant potential by adding several concentrations of disodium hydrogen phosphate (DSHP) to the sodium nitrite solution. The results of these electrochemical measurements suggested that the improved SCC susceptibility of α Cu–Zn–Sn–P was caused by the formation of a protective film consisting of phosphate products at a crack tip and subsequently the prevention of crack growth.
