The formation of self-assembling membrane of hexadecane-thiol on silver to prevent the tarnishing

Artefacts in silver suffer from tarnishing when exposed to atmospheres polluted by sulphide. We have found the optimum conditions to form an efficient, invisible and protective film against the tarnishing appearance on silver in 0.5 M NaCl solution containing 10 mM Na2S. This solution corresponds to a highly aggressive medium not only by the coupling of two aggressive agents, but also by a high concentration of S2− (320 ppm). The silver surface was first degreased carefully by successive dippings in three organic solvents, ethanol, acetone and hexane, followed by a slight surface activation in sulphuric acid solution. Then, a silver specimen was immersed in an isopropanol solution with 0.15 M C16H33SH during 1 h at 30 °C. The kinetics of tarnishing was tracked by reflectance, quartz-micro balance measurements, and electrochemical impedance spectroscopy. The protection of hexadecane-thiol reaches 90% in terms of reflectance, after 1 h of corrosion test, that is, no alteration by visual inspection. It was deduced that the thiol film has a double structure, an inner self-assembling membrane of one or two monolayers and an outer-layer with some tenths micrometers. This surface film limits markedly the diffusion of dissolved oxygen to the electrode surface, thus slows down the rate of silver sulphide (Ag2S) formation.