Tarnish and corrosion of a commercial dental Ag-Pd-Cu-Au casting alloy.

The correlation between microstructure, tarnish, and corrosion behavior of a commercial dental Ag-Pd-Cu-Au casting alloy after various heat treatments was studied. The attacked structural components were identified by comparing SEM micrographs with light optical micrographs taken after exposure of the alloy to a 2% Na2S solution and after dynamic polarization in de-aerated artificial saliva. The characteristic dynamic polarization curves for the alloy after the heat treatments were established. The results were verified by analogous testing of two experimental alloys with compositions similar to those of the analyzed components in the commercial alloy annealed at 600 degrees C for seven wk. The matrix of the commercial alloy was decomposed, according to the heat treatments, into varying proportions of a tarnish- and corrosion-resistant Cu- and Pd-rich component (A) and a nonresistant Ag-rich component (B). The presence of A considerably increased the tarnish rate of B, due to galvanic coupling. Age-hardening of the alloy increased the proportion of the tarnish- and corrosion-prone component B.