Potentiodynamic polarization analysis of silver-palladium alloys in chloride solutions
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I.
INTRODUCTION
THE increasing price of precious metals
such as Au, Pd, and so forth, has created uncertainties in the use of these metals for such diverse applications as dental restorations, jewelry, and electronics. It is to be expected that nonprecious alloys and semiprecious alloys would, to a large extent, replace precious metals and their alloys in many applications. The substitution of a 60 Pd-40Ag alloy for pure Pd as an electrical contact material on the wire spring family of relays has been reported to have resulted in substantial cost reduction without any loss in system performance and reliability, l Similar cost-motivated trend has recently brought about interest in Ag-Pd alloys for ceramic and crown and bridge applications as restorative dental materials. This trend toward increased interest in Ag-Pd alloys has made it desirable to characterize the electrochemical and environmental interactions of the Ag-Pd alloys encountered in specific applications. Since the Ag-Pd binary system is a continuous series of solid solutions between a transition noble metal Pd and a nontransition noble metal Ag, there is also a unique fundamental research interest in such a study. However, published documentation of the behavior of Ag-Pd alloys on exposure to environmental and electrochemical interactions appears to be far from adequate. In particular, electrochemical characterization of Ag-Pd alloys appears to be somewhat limited. For example, Ishizaki's report on the electrochemical characterization of Ag-Pd alloys in artificial saliva was only in the range of 0 to 30 pct Pd content. 2 More recently, the potentiostatic anodic polarization in Ringer's solution has revealed active-passive behavior of Ag-Pd alloys over a wide range of compositions. 3 Some commercial Ag-Pd alloys were also characterized by the potentiokinetic hysteresis technique in a 1 pct NaCI solution. 4 The objectives of the present investigation were broad based and were twofold: 1. To determine the electrochemical response of Ag-Pd alloys on anodic polarization in chloride media over a preselected range of anodic overpotentials, chloride ion concentrations, and nearly neutral pH values.
2. To optimize the composition ranges of the Ag-Pd alloys where the anodic polarization behavior of the alloy would indicate electrochemical stability for specific applications, thus allowing material selection based on economy and performance assurance.
II.
99.999 pet pure Pd and Ag were used in this investigation. Alloys of Ag and Pd were prepared by vacuum induction melting. Pure Ag, Ag-20 wt pet Pd, Ag-40 wt pet Pd, Pd-40 wt pet Ag, Pd-20 wt pet Ag, and pure Pd were studied. 7 mm diameter • 5 mm thick specimens were prepared by the centrifugal investment casting technique. The specimens were water quenched with the investment. All the specimens were subjected to a homogenization treatment for 48 hours at a temperature of 150 ~ below the solidus temperature of the respective compositions. The specimens were then mounted metallographically, polished through 6
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