Structure, Corrosion, and Tarnishing of Ag-Pd-Cu Alloys

¹ Institute of Dentistry, University of Turku, Finland
² NIOM, Scandinavian Institute of Dental Materials, Forskningsveien 1, 0371 Oslo 3, Norway

The structure of Ag-Pd-Cu-Au-Zn alloys can be expected to have an influence on their corrosion and tarnish resistance. Four experimental Ag-based alloys (I-IV) with varying Pd/Ag ratios were studied by scanning electron microscopy (SEM), microprobe analyses, and x-ray diffraction analyses. Both as-cast and in the annealed state, they contain a Pd-Cu-Zn-enriched compound A and a Ag-rich matrix (compound B) with a fcc structure, ₂. X-ray diffraction data indicated that compound A consisted of two fcc phases (₁ and ₂) in addition to a bcc PdCu_xZn_{x - 1} phase. The phases of compound A were too small to be distinguishable by SEM. Alloys I-IV could not be solid-solution-annealed to a single phase.

Static anodic polarization testing was carried out in an artificial saliva containing Na₂S (pS= 5). It was found that an alloy made as a duplicate of the Pd-Cu-Zn-rich compound A for a given potential had current density clearly lower than that of the Ag-rich alloys. Furthermore, cathodic reduction of oxygen took place at a considerably higher rate on this Pd-Cu-Zn-rich duplicate alloy than on the Ag-based alloys. It is suggested that micro-galvanic cells are created between the Pd-Cu-Zn-rich compounds A and the Ag-rich matrix in alloys I-IV. Immersion tests in aqueous Na₂S solutions (pS = 1.6) showed tarnishing to take place only on the Ag-rich matrix. However, increasing the Pd content of an Ag-rich single-phase alloy increased its tarnish resistance.