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

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% Na2S solutions. In both types of tests, a multiphase alloy 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 microgalvanic cells due to chemical inhomogeneities caused the observed susceptibility to tarnishing. Sulfur was detected by a microprobe 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 of exposure time.