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

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Abstract
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 PdlAg 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, α2. X-ray diffraction data indicated that compound A consisted of two fcc phases (α1 and α 2) in addition to a bcc PdCuxZnx-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.