Tarnish Protection of Silver Using a Hexadecanethiol Self-Assembled Monolayer and Descriptions of Accelerated Tarnish Tests

A four-step procedure was developed for depositing a hexadecanethiol self-assembled monolayer (SAM) onto the surface of silver in order to provide tarnish resistance. The four steps may be characterized as cleaning, etching, mono-layer self-assembly, and rinsing. A key observation in this work is that an optimal deposition time exists for a given concentration of the hexadecanethiol. For example, a 2 vol% solution (2 mL hexadecanethiol in 98 mL trichloroethylene) required 30 min to 60 min for optimum coating formation. The quality of the coatings was characterized using water drop contact angle measurements and electrochemical impedance spectroscopy (EIS). In addition, two tarnish tests were developed specifically for this project. One test was a lab bench vapor test that could tarnish silver, copper, or brass to a dark color within a few hours. A second test exposed the silver to a stream of a sulfide-containing foam and could tarnish silver to black within several minutes. The degree of tarnishing from these two tests was quantified by measuring the percent reflectance of the surface using visible light.