| With the continuous development of decorative gold-plated and industrial gold-plated, pure gold coating is increasingly difficult to meet the requirements because of its low hardness and poor wear resistance. Copper element can increase the hardness and palladium element can improve the wear resistance and contact resistance stability of the gold coating. Therefore, gold-copper-palladium alloy coating has a very broad application prospects. Currently, the most widely used gold alloy plating solution contains cyanide or virulent additives, which is harmful to both environment and human health. Therefore, a non-cyanide innoxious plating solution for gold-copper-palladium alloy is proposed.According to project requirements, a corresponding research plan is determined, which contain process flow, pre-treatment formula, and test methods for performance and electro-chemical testing. On this basis, the influence of auxiliary complexing agents, such as sodium thiosulfate, urotropine and potassium citrate, on coating performance is studied. The results show that the coating brightness and electric contact performance decline after adding sodium thiosulfate or urotropine, but improve after adding potassium citrate which also improves coating binding force.By using single factor method and taking solution stability and coating appearance as major evaluation indicators, micro hardness as minor evaluation indicator, the solution composition and process conditions were optimized. The optimal formulation is composed of lOg/L Au,6g/L Pd(NH3)2Cl2,120g/L (NH4)2SO3,100g/L K3C6H5O7·H2O,40g/L Na2EDTA, and at the pH value of8, temperature of50℃, mixing speed of1000r/min, current density of0.2A/dm2. The coating obtained at the optimal formulation has properties of shiny appearance, good adhesion, low contact resistance, high microhardness, and excellent corrosion resistance. The coating contains84.79%Au,12.68%Cu,2.53%Pd. The alloying elements distribute evenly in the coating, and formed a continuous solid solution which has a crystal structure. The performances of coating meet the requirement of decorative coating and electrical contact materials. The optimized solution presents a good dispersing capacity, covering capacity and stability. During optimizing the solution composition and process conditions, the influence of them on coating composition is also studied, and the influence rule is obtained. According to the influence rule, coating composition can be effectively controlled, and Au-Cu-Pd alloy coating with different performance can be obtained.The electrodeposition behaviors of Au-Cu-Pd alloy are studied by electrochemical testing. The cyclic voltammetric curve with scanning speed at50V/s shows that the electrodeposition of Au-Cu-Pd alloy has experienced three-dimensional nucleation procedure. The cyclic voltammetric curves with different scanning speed shows that the electrocrystallization of Au-Cu-Pd alloy is controlled by diffusion and the procedure is irreversible. The I-t transient curve of plating solutions added different auxiliary complexing agent shows that potassium citrate can greatly shorten the charging time of the electric double layer on the cathode surface and consequently improve coating performance, but the effects of hyposulphite and urotropine are not obvious. The analysis of corresponding nucleation curves shows that the nucleation curve of solution added potassium citrate coincides with theoretical nucleation curve most and its nucleation mode includes continuous nucleation and instantaneous nucleation at the same time. |
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