Mechanism of Tarnishing of Silver Alloys Under Sulphide

Abstract

Silver and silver alloys is subject of corrosion in environments that contain sulphide (H2S). Thus, become tarnished and a blackish thin film is formed on the metallic surface. This problem is very important in the field of corrosion and conservation. Currently, there is growing attention in the study about tarnishing process in silver alloys, which allows supporting decision-making in the conservation of silver artefacts. However, previous works focused on the corrosion mechanisms and surface treatments of pure silver, more than on silver alloys. This work investigates the corrosion mechanism on the tarnishing of silver alloys in a 0.07% v/v ammonium sulphide solution as an accelerator of the tarnishing process for 48 h. The Electrochemical Impedance Spectroscopy (EIS) and Scanning Electron Microscopy (SEM) are used as analysis techniques. The impedance measurements reveal that the copper content at the alloy modifies the kinetic of silver tarnishing. On the basis of experimental results, a reaction model for the tarnishing process of silver alloys in sulphide media is proposed. The reaction mechanism is depicted as that of silver modified by the copper content.