Corrosion and Tarnish Behaviour of 925Ag75Cu and 925Ag40Cu35Zn Alloys in Synthetic Sweat and H2S Atmosphere

Abstract

Abstract The purpose of this paper is to investigate the corrosion and the tarnish behaviour of 925Ag75Cu and 925Ag40Cu35Zn alloys in synthetic sweat at 37 °C and H2S atmosphere at room temperature. The alloys were in two hardening states, viz., aging at 250 °C for 4 h and 70 % cold rolling + aging at 200 °C for 4 h, and tested by means of static corrosion, examined by XRD, SEM, EDS and XPS. The results show the corrosion and the tarnish in synthetic sweat at 37 °C of these two 925Ag alloys are induced by Cl− and OH− anions. The tarnish of 925Ag40Cu35Zn alloy is much slower than that of 925Ag75Cu alloy, and the white AgCl and the grayish black Ag2O produced by corrosion are difficult to deposit on the surface of 925Ag40Cu35Zn alloy resulting in grayish black corrosion film. The corrosion and the tarnish in H2S atmosphere at room temperature of these two 925Ag alloys are induced by S2− anion. The corrosion and the tarnish of 925Ag40Cu35Zn alloy are much slower than 925Ag75Cu alloy. This is because Zn in 925Ag40Cu35Zn alloy reacts earliest with H2S to form white ZnS, which significantly retards the reaction between Ag or/and Cu and H2S to form grayish black Ag2S and black Cu2S.