Role of copper in tarnishing process of silver alloys in sulphide media
(Departamento de Ingeniería Metalúrgica, Facultad de Química, Universidad Nacional Autónoma de México, Ciudad de México, C.P. 04510, México)
Abstract: Silver and silver alloys usually tarnish, which causes some changes in their aesthetic appearance and electrical properties, due to their exposure to sulphide environments (H2S), and this is a problem in the field of corrosion and conservation of cultural heritage metallic artefacts. In this study, the role of copper content in the tarnishing process of 0.925, 0.800 and 0.720 silver alloys in a 0.07 vol.% ammonium sulphide solution for different immersion periods was analyzed by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The polarisation curves showed that the copper content and sulphide increased the corrosion current density and delayed the passivation of silver alloys. The impedance spectra collected at the open circuit potential (OCP) showed a single capacitive, incomplete and depressed loop, indicating that the charge transfer resistance decreased as the copper content increased in the alloys. In contrast, the double-layer capacitance increased as the copper content increased. The SEM-EDS analysis confirmed that the copper-rich phase in the silver alloys was selectively dissolved due to the preference of S to react with Cu, resulting in a localised attack, thus delaying the formation of a passive film. A marked localised attack was observed in alloys with lower copper content. The mechanism for the tarnishing of silver alloys in sulphide media was dependent on the nature of the alloy and the greater affinity of copper for sulphur. The dissolution of Ag and Cu to form corrosion products was proposed as the rate determining step.
Key words: silver; conservation and restoration; tarnishing; electrochemical impedance spectroscopy; silver alloys; corrosion mechanism