Development of Highly Corrosion Resistant Ag-Pd-Cu-Au Casting Alloy

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To develop a highly corrosion resistant Ag-Pd-Cu-Au alloy, the corrosion and tarnish behaviors of an experimental Ag-Pd-Cu-Au alloy with 45 mass % Pd were addressed in 0.1% Na2.S and 0.1% NaCI solutions. Corrosion and tarnish resistance of Ag-45Pd-l8Cu-l2Au alloy was evaluated in relation to the microstructure after various heat treatments by potentiodynaniic polarization and spectrocolorinietric measurements. The alloy surface after immersion in 0.1% Na2S solution was examined by X-ray photoelectron spectroscopy (XPS).This alloy has Pd-rich single-phase structure and the segregation observed in as-cast alloy did not markedly affect the corrosion behavior. The potentiodynamic polarization and the spectrocolorimetric measurements demonstrated that this alloy was highly corrosion resistant and practically free of corrosion in 0.1% Na_2S solution. This extremely high corrosion and tarnish resistance was due to passivity attained by the formation of Pd-rich sulphide film that was identified by XPS.The amounts of metallic ions released into 0.9% NaCI solution from the Ag-45Pd-l8Cu-l2Au alloy were much lower than those from a commercial Ag-2OPd-l8Cu-l2Au alloy.The addition of small amount of Ga effectively lowered the melting point and improved the castability of the experimental alloy. Small amount of Ga (1-5 mass %) did not decrease the corrosion resistance of the alloy.The characteristics of the experimental Ag-45Pd-l8Cu-l2Au alloy such as high corrosion resistance, excellent tarnish resistance, and good castability are suitable for dental casting restorations used in the oral environment.