Tarnishing and structures of some annealed dental low-gold alloys.

Tarnishing of low-gold alloys in a Tuccillo-Nielsen test with 2% Na2S is avoided when a single-phase structure is present as compared with multi-phase structures, unless the Ag content of the single-phased alloys is high. In all multi-phase structures, only the Ag-rich phase is attacked. The possibility of annealing alloys to a single-phase structure decreases with the reduction of Au content and with increasing amounts of Pd and Zn, which tend to follow the Cu-rich phase. In static polarization testing of single-phase materials in weak sulphide solutions, the Ag-rich alloys display considerably higher anodic current densities (5-10 microA/cm2) than do the Cu-rich low-gold and high-gold alloys (1 microA/cm2). No significant differences between the alloys were observed with respect to cathodic currents.