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

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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.