| In order to save gold, increase the microhardness of gold coating and decrease the toxicityof bath, the process of plating 22K gold is studied. First, the bath of 22K gold has low freecyanide. Then the bath of 22K gold is completely cyanide free, and use sulfite and citrate ascomplexant.The low cyanide bath use KAu(CN)2 as main salt, KCN and Na2SO3 as complexant, andCu as alloying metal in the form of cyanide complex. The technological formulation isdetermined to be composed of 5～10g/L KAu(CN)2, 8～14g/L CuCN, 1～2g/L KCN, 8～20g/LNa2SO3, 3～5g/L KH2PO4, 2～4mL/L H3PO4, 0.2～0.3 mL/L glycerin, and some brightener.And the operational conditions are suggested as pH value 7～7.5, operating temperature60～70℃, cathode current density 0.1～0.2A/dm2, mechanical agitation. The results show thatthe coating has properties of golden color, uniformity, nicety, gradeⅡbrightness, highmicrohardness, good adhesion and corrosion resistance, according with 22K standard. Thebath has properties of stable solution, well throwing power and covering power. Sulfite isused as assistant complexant which can decrease the toxicity and danger of the bath.In order to decrease the toxicity largely, the cyanide free process is studied, which usesNa3Au(SO3)2 as main salt, Na2SO3 and citrate Potassium as complexant, and Co as alloyingmetal in the form of citrate complex. Effects of concentration of COSO4, cathode currentdensity and operating temperature on the gold composition and coating appearance are alsoinvestigated. The results show that the optimal technological formulation of plating 22K goldis determined to be composed of 160g/L Na2SO3, 120g/L citrate Potassium, 30g/L COSO4,9g/L Na3Au(SO3)2 as metallic gold and 40～50g/L K2HPO4, while the optimal operationalconditions are suggested as operating temperature 40℃, cathode current density 0.2A/dm2,pH value 8.80～9.20 and swept cathode. The 22K gold coating obtained at the optimalconditions has properties of golden color, uniformity, nicety, between gradeⅡandⅢbrightness, high microhardness, good adhesion and corrosion resistance, according with 22Kstandard. The. optimal cyanide free bath has better throwing power and covering power thanthat of low cyanide bath. The bath has no toxicity and it is easy to operate and treat wasteresidue.In order to improve the stability of cyanide free bath and the brightness of the coating,effects of stabilizing agent and brightener on the bath and the coating are investigated. The results show that bath stability and coating brightness have been improved in some extentwhen stabilizing agent and brightener are added.22K gold coatings obtained from low cyanide bath or cyanide free bath have brightlygolden color and other excellent properties and can be applied to decorative coating.
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