Study On Plating 22K Au-Cu Alloy In Cyanide Free Bath

In order to save gold, increase the microhardness, corrosion resistance of gold coating and decrease the toxicity of bath, the process of plating 22K gold is studied.The bath of 22K gold is completely cyanide free, and use sulfite and citrate as complexant. Orthogonal test was carried out to establish the optimal cyanide-free plating 22K gold process. The gold component, color and gloss, morphology and thickness, micro hardness of the coating were studied systematically by means of X-ray fluorescence, spectro-guide gloss, microscope, micro hardness tester. The hall cell method and inner-pole method was carried out to test the throwing power and covering power of the bath.In order to decrease the toxicity largely, the cyanide free process is studied, which uses Na₃Au(SO₃)₂ as main salt, Na₂SO₃ and citrate Potassium as complexant of Au, DTPA as compexant of Cu,and Cu as alloying metal in the form of DTPA complex. Effects of concentration of CuCl₂, cathode current density and operating temperature on the gold composition and coating appearance are also investigated. The results show that the optimal technological formulation of plating 22K gold is determined to be composed of 50g/L Na₂SO₃, 60g/L citrate Potassium, 30g/L DTPA, 0.8g/L AuCl₂, 8g/L Na₃Au(SO₃)₂ as metallic gold and 40～50g/L K₂HPO₄, while the optimal operational conditions 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 optimal conditions has properties of golden color, uniformity, nicety, between grade II and III brightness, high microhardness, good adhesion and corrosion resistance, according with 22K standard. The optimal cyanide free bath has better throwing power and covering power than that of low cyanide bath. The bath has no toxicity and it is easy to operate and treat waste residue.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 extent when stabilizing agent and brightener are added.22K gold coatings obtained from cyanide free bath have brightly golden color and other excellent properties and can be applied to decorative coating.