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

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 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, oa2. X-ray diffraction data indicated that compound A consisted of two fcc phases (aI and a2) in addition to a bcc PdCuxZnx_ phase. The phases of compound A were too small to be distinguishable by SEM. Alloys I-IV could not be solidsolution-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.

Influence of casting procedures on the corrosion resistance of clinical dental alloys containing palladium☆

“…Sarkar et al [22] studied an alloy similar in composition to Pors on 4; they recorded a corrosion potential, at 24 h, around 120 mV/SCE even though the electrolyte composition and its pH were diVerent from those in the present study. Taking into consideration that there may be an imprecision concerning the reproducibility of the results at §50 mV/SCE [21], this value could be compared to these of the present study. In the study mentioned the authors [22] found that a palladium-silver alloy is more susceptible to corrosion than palladium alone but less susceptible than silver alone.…”

“…For each alloy, the open-circuit potential vs. time curve for 24h was recorded, and then the value of 24-h OCP was determined and compared with that of other alloys used in dentistry [19][20][21] and especially with alloys from the same categories [5,19,22,23] in the same test procedure. The cyclic polarization test was run 5 min after Wnishing the open-circuit potential measurement, at OCP minus 20 mV, with a sweep rate of 1 mV/s within a scanning range from ¡800 mV/SCE to 1200 mV/SCE and back to ¡800 mV.…”

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

“…The appearance of the pre-polished surface of alloy lV tarnished under in viva conditions differs from that emerging after in vitro testing in 2% Na2S (pS = 1.6) solutions (Niemi and Hero, 1985), although alloy IV in both types of tests displayed the lowest tarnish resistance. The corrosion attacks during in vitro testing created numerous, small dark spots, presumably Ag2S, in the Ag-rich, single-phased areas, whereas the Pd-CuZn particles remained bright (Fig.…”

“…The assumption that the Pd-Cu-Zn-rich particles provide the sites for the cathodic oxygen reduction is based on previous static polarization measurements (Niemi and Hero, 1985). Alloy V, which is a compositional duplicate of these particles, was observed to have lower currents by anodic polarization and higher by cathodic polarization than were the Ag-rich, single-phased alloys.…”

“…Moreover, the corrosion attacks may vary considerably from one patient to another. The aim of the present work was to test in viva the tarnish resistance of multiphase and single-phase Ag-Pd-Cu-Zn alloys that previously have been studied in vitro (Niemi and Hero, 1985). …”