XPS and electrochemical characterization of tarnish films on dental alloys

Three gold resin casting alloys ranging in nobility between 30 and 74 wt.% have been analysed by XPS, SEM plus energy-dispersive spectroscopy (EDS) as well as by L∗ a ∗ b ∗ colorimetry before and after in vitro tarnishing in 0.1 M Na2 S solution with a rotating wheel apparatus. The lower the nobility of the alloy, the higher the tarnishing tendency: the alloys appear darker with increased redness and yellowness, as determined by colorimetric analysis. On the three alloys, corrosion products of almost the same chemical composition are detected in different amounts. The products are mainly Cu (53–57 wt.%), S (23–29 wt.%) and Ag (~4 wt.%), as determined by EDS. X-ray photoelectron spectroscopy analysis carried out in the first tarnishing steps indicates more sulphur enrichment on the surfaces of the alloys with the lower nobility. Auger maps of the tarnished surfaces show evidence of the onset of a selective reaction of sulphur with Cu and Ag in the first steps. The electrochemical results are in accordance with the previously exposed data: the polarization curves recorded on alloys with increasing Ag and Cu amounts show higher current density values indicating a worse performance.