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

为了研制高耐蚀的Ag-Pd-Cu-Au合金，研究了含45%Pd的Ag-Pd-Cu-Au合金在0.1%Na2.S和0.1%NaCI溶液中的腐蚀和变色行为。用动电位极化和分光光度法评价了Ag-45Pd-18Cu-12Au合金经不同热处理后的耐蚀性和变色性与组织的关系。用X射线光电子能谱(XPS)对合金在0.1%Na2S溶液中浸泡后的表面进行了分析，发现该合金为富Pd单相组织，铸态合金中的偏析对腐蚀行为没有明显影响。动电位极化和比色分析表明，该合金在0.1%Na2S溶液中具有很高的耐蚀性，几乎不腐蚀。Ag-45Pd-18Cu-12Au合金在0.9%NaCI溶液中的金属离子释放量远低于工业用的Ag-2OPd-18Cu-12Au合金。少量Ga的加入有效地降低了合金的熔点，改善了合金的铸造性能。少量的Ga(质量分数为1~5%)不会降低合金的耐腐蚀性，实验用的Ag-45Pd-18Cu-12Au合金具有耐腐蚀性高、抗变色性能好、铸造性能好等特点，适合口腔环境下牙科铸造修复。

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.