Effect of palladium on the tarnishing of Cu-Ag-Au alloys

Tarnishing of ternary Cu-Ag-Au alloys was studied by the Tucillo-Nielson method. Both single-phase and two-phase alloys were investigated and it was found that susceptibility to tarnishing increases with silver content only for the single-phase alloys, i.e. alloys with high gold content. But for the two-phase alloys, susceptibility to tarnishing is a function of both the silver and copper content. The role of palladium in preventing tarnishing was also investigated using alloys which had 6 at.% Pd and Cu, Ag, Au in the same ratios as in the ternary alloys. It was found that palladium is effective only in the single-phase alloys where the mode of tarnishing is by silver sulphide formation.