Surface enoblement by dissolution of Cu, Ag and Zn from single phase gold alloys

Abstract

Single phase alloys with 30 at.% Au and with different proportions of Ag, Cu and Zn as additional alloying elements have been investigated with regard to selective dissolution. Measurements of rest potentials and ESCA analyses show that Cu and Zn are preferentially dissolved in aqueous solutions of chloride and small concentrations of sulphides leaving behind a surface significantly enriched in Au with a thickness of approximately 1 nm. No significant selective dissolution of Ag was observed in the solutions investigated. Only alloys containing substantial amoungs of Ag tended to become tarnished in sulphur containing solutions. It is suggested that Ag 2S is more easily formed than sulphides of Cu and Zn because also other oxidative reactions to oxides/ hydroxides of these elements take place, which is not the case for Ag. These oxides/hydroxides of Cu and Zn dissolve into the solution creating an Au-surface enrichment also in the diluted sulphide-containing solutions.