Tarnishing in vivo of Ag-Pd-Cu-Zn Alloys

Abstract

Pre-polished specimens of four Ag-Pd-Cu-Zn alloys have been tested for tarnishing while embedded in full acrylic dentures for from six to eight months in vivo. The results have been compared with those from previous immersion tests using 2% Na ₂S Solutions. In both types of tests, a multiphase allo consisting of a Pd-Cu-Zn-rich compound in a Ag-rich matrix displayed less corrosion and tarnish resistance than did alloys with compositions similar to the Pd-Cu-Zn-rich compound or the Ag-rich matrix. It is suggested that the presence of local micro-galvanic cells due to chemical inhomogeneities caused the observed susceptibility, to tarnishing, Sulfur was detected by a microprebe in the tarnish layer developed in the oral cavity. The discoloration on this multiphase alloy covered the interdendritic, areas with short distances between the chemical inhomogeneities. This microstructural location was somewhat different from the many dark spots created predominantly on Ag-rich areas during in vitro testing. This difference. is most likely due to dissimilar conditions with regard to sulfide ion concentrations, the presence of protein films, and the length af exposure time,