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

摘要

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 H_2S, is chosen because sulfur vapor reacts with silver more easily to form silver sulfide (Ag_2S). 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℃ for 15, 30, 45, and 60 min, respectively. The thickness of Ag_2S film grown on the samples was measured. The results showed that Ag_2S growth rate depends on indium concentration. For example, with 19 at.% In, Ag_2S 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.