Improvement of Corrosion resistance of Functionally Graded Ag-Pd-Au-Gu Alloy Casting Treated by Selective

提高合金中贵金属Pd、Au等的比率，防止由于Ag-Pd-Cu-Au合金的硫化而变色。本研究的目的是通过去除引起变色和腐蚀的元素来提高合金的耐腐蚀性，而不增加贵金属。在合金的次氯酸钠溶液之后，浸入a）硝酸溶液或B）硫代硫酸钠或氢氧化铵溶液中诱导Ag和Cu的选择性溶解，增加Pd和Au，该合金具有多相结构，各相之间的腐蚀行为因偏析程度不同而不同。在选择性溶解后，在合金的某些部分上发现了腐蚀坑。选择性溶解处理的铸件经650和700℃固溶处理后变色，富银相变色，在750、800和850℃时变色较小，表面光滑。高温热处理和选择性溶解处理似乎有助于提高该合金的耐腐蚀性，但在处理后的样品的EPMA图像上没有显示出选择性溶解的外观，因此需要更多的研究这一机制。在较高温度下进行固溶处理，除了提高合金的耐腐蚀性外，还可以提高合金的力学性能，这可以作为合金在选择性溶解处理之前的适当处理。

Increasing the rate of noble metal, Pd, Au etc. in the alloys, prevents discoloration due to sulfuration of Ag-Pd-Cu-Au alloys. The aim of this study was to improve corrosion resistance of the alloy by removing the elements that cause tarnishing and corrosion, without increase of noble metal Immersion in a) nitric acid solution or b) sodium thiosulfate or ammonium hydroxide solution following sodium hypochlorite solution of the alloy has induced selective dissolution of Ag and Cu, increasing of Pd and Au, suggested the possibility of improvement of corrosion resistance.This alloy has a multiphase structure, and corrosion behavior is different among phases and due to the degree of segregation. After the selective dissolution, etch-pit was found on some portions of the alloy. The tarnishing test of selective-dissolution-treated castings following solution treatment at 650 and 700℃ has caused discoloration of Ag-rich phase, little discoloration at 750, 800 and 850℃ showing smooth surface condition. Heat treatment at higher temperature and selective dissolution treatment seemed to contribute to an improvement of corrosion resistance of this alloy, however, no appearance of selective dissolution was shown on EPMA image of the treated sample, so that more investigation about this mechanism is required. Solution treatment at higher temperature related to improvement of mechanical properties in addition to corrosion resistance of the alloy, which can be the appropriate treatment of the alloy before selective dissolution treatment.