Ru-Enriched Phase Displays Higher Corrosion Potential in High-Palladium Alloys

Dealloying has been proposed as a possible corrosion mechanism in high-palladium alloys, and the released palladium ions may be the cause of the so-called “palladium allergy” for some patients. Last year at the AADR annual meeting (Abstract No. 130) we reported that Scanning Kelvin Probe Force Microscopy/Atomic Force Microscopy (SKPFM/AFM) is a powerful tool to study the corrosion behavior of alloys by simultaneously acquiring the topographic image and the Volta Potential map of the surface. This potential map represents the in situ relative nobility among the multiple phases in the alloy. Purpose: Apply SKPFM/AFM to evaluate the relative corrosion tendencies of various phases in a high-palladium alloy. Methods: Freedom Plus (78Pd–8Cu–5Ga–6Sn–2Au–Ru, wt %, Heraeus Kulzer Jelenko) was selected for study. Oxidation and simulated porcelain-firing cycles were performed according to the recommendations of the manufacturer. Specimens were ground and polished with diamond paste having a final particle size of 1 micron in an alcohol solution (DP-Blue, Struers). The polished specimens were immediately examined by SKPFM/AFM (Nano III, Digital Instruments), and the areas of interest were observed with a scanning electron microscope (XL30 ESEM FEG, Philips). The chemical compositions of the phases of interest were analyzed by an x-ray energy-dispersive spectrometer (EDS) associated with the SEM. Results: The Ru enriched phase found in high-palladium alloys from the use of ruthenium as a grain-refining element was identified as being more noble than the palladium solid solution matrix. Our recent transmission electron microscopic studies (Dent Mater 2003;19:334-340) indicate that this phase may be nearly pure ruthenium. Other secondary precipitates that exist in the microstructure of this high-palladium alloy have minimal differences in Volta potential compared to the matrix. Conclusion: The Ru-enriched phase may cause galvanic corrosion in high-palladium alloys. Supported by NIDCR Grant DE10147.