A reaction study of sulfur vapor with silver and silver–indium solid solution as a tarnishing test method

It is well known that pure silver (Ag) or sterling silver can easily get tarnished under ordinary atmosphere by sulfur-containing gases. Over past several decades, the industries have tried to formulate and produce silver-based materials that do not get tarnished for various applications including electronics, optics, jewelry, and silverware. Recently, our group has grown silver and silver–indium solid solution ingots and studied their anti-tarnishing property. Since there are no standard tarnishing test methods for Ag-based alloys, we have designed and established an accelerated tarnishing experiment using sulfur vapor as the reacting agent. Sulfur vapor, rather than other sulfur-containing gases such as H₂S, is chosen because sulfur vapor reacts with silver more easily to form silver sulfide (Ag₂S). After ingot growth, disk samples were cut from silver and silver–indium solid solution ingots and examined with X-ray diffraction (XRD) and scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDX). Commercially available Argentium silver was also studied for comparison. Disk samples were placed into the sulfur vapor reacting chamber at 120 °C for 15, 30, 45, and 60 min, respectively. The thickness of Ag₂S film grown on the samples was measured. The results showed that Ag₂S growth rate depends on indium concentration. For example, with 19 at.% In, Ag₂S growth rate on silver–indium solid solution is only 4 % of that on pure silver and 7 % of that on Argentium silver. The excellent anti-tarnishing property of silver–indium solid solution should open up new applications in electronics, jewelry, joining, optics, silverware, and welding.