Development of Corrosion resistant Ag-Pd-Cu-Au alloys for patient with metal hypersensitivity

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The corrosion behaviors of eighteen Ag-Pd-Cu-Au alloys with different compositions were examined in 0.9 % NaCl and 0.1 % Na_2S solutions to develop a high corrosion resistant and biocompatible (allergy free) dental casting alloy. The effects of small amount of Ga on the castability, mechanical strength, and corrosion resistance of the Ag-Pd-Cu-Au alloy were also examined. The corrosion resistance was evaluated by electrochemical techniques and the analysis of released metal ions by atomic absorption spectrophotometry. The surface of the alloy was analyzed by X-ray photoelectron spectroscopy (IPS). The corrosion resistance of the Ag-Pd-Cu-Au alloy both in 0.9 % NaCl and 0. 1% Na2S solutions increased with an increase in Pd content. It was found that the Pd-25Ag-18Cu-12Au alloy with 45 mass% Pd was passive in 0.1 % Na_2S solution with the formation of Pd-rich sulfide film and did not tarnish unlike commercial Ag-Pd-Cu-Au alloys which tarnished with the formation of crystalline corrosion product of Ag_2S. The amount of released metal ions from the Pd-25Ag-18Cu-12Au alloy in 0.9 % NaCl solution was approximately 10 % of that from the commercial Ag-Pd-Cu-Au alloys. The addition of small amount of Ga (less than 5 mass%) lowed the liquids point of the Pd-25Ag-18Cu-12Au alloy and improved its castability without decreasing the resistance to corrosion and tarnish. The small amount of Ga also increased the value of Vickers hardness number. These results suggested that high level of resistance to corrosion and tarnish and excellent biocompatibility of the Pd-25Ag-18Cu-12Au alloy make this alloy suitable as a material for dental casting restoration.