Research On Cyanide-Free Electrodeposited Gold And Gold Alloy Nanocrystalline Coatings With DMH As Coordinator

Gold has good physical and chemical properties and is widely used.Gold plating mostly uses cyanide plating solution,but cyanide is highly toxic,so there is an urgent need to develop a cyanide-free gold plating process with performance close to the cyanide gold plating process.The non-cyanide gold plating process using 5,5-dimethylhydantoin(DMH)as a complexing agent has shown the possibility of industrial application,but the process still has the following problems: coarse grains and loose accumulation of the coating,As a result,the color and gloss of the coating are poor.In order to solve the above problems,this article proposes to optimize the process by looking for appropriate additives.The study found that when the grains are refined to the nanometer level,the coating layer appears golden and bright.In addition,due to the poor hardness and wear resistance of the pure gold coating,it is difficult to meet the requirements in some applications.Therefore,this article develops the plating process of the gold alloy coating on this basis.The composition and process conditions of the optimized electroplated gold nanocrystalline coating are: HAu Cl4 0.015 mol·L-1,DMH 0.25 mol·L-1,K2CO3 0.36 mol·L-1,o-phenanthroline(PHEN)30 mg·L-1,polydiallyl dimethyl ammonium chloride(PDDA)10 mg·L-1.The current density during plating is 0.5-1.2 A·dm-2,the control temperature is 303～323 K,the p H is 9.7,and the stirring speed is 600～1000 rpm.Through the comparison of the macroscopic morphology and SEM photos,it can be seen that the main brightener PHEN functions to refine the crystal grains,and the auxiliary brightener PDDA mainly plays a role in leveling.After TEM observation and Xie Le formula estimation,the refined grain size is about 20 nm.The purity of the plating layer was tested by XPS,and the purity of the gold plating layer was found to be 97.88%.Despite the higher purity,a small amount of C,N,and O elements were still found,presumably due to the adsorption of additives.In order to explore the mechanism of action of the additives,first,through molecular dynamics simulation,the adsorption behavior of PHEN and PDDA molecules on the substrate surface was simulated,both of which could be adsorbed in parallel on the surface,and the adsorption energy was calculated.A series of electrochemical tests were subsequently carried out to further verify the adsorption of the additives: the cathodic polarization curve test showed that the compound additive of PDDA and PHEN can significantly increase the cathodic polarization and can play a synergistic role,so that it can be used for a long time.Bright plating can still be obtained during time electroplating;cyclic voltammetry test shows that the electrodeposition process of gold is divided into two steps,followed by Au(DMH)4-→Au(DMH)2-→Au,the additives mainly affect the second The step deposition process negatively shifts the deposition potential of the second step deposition;the nucleation curve shows that the presence of compound additives does not have a significant impact on the nucleation method.The nucleation process is still continuous nucleation,but the additives make the crystal epitaxial The growth rate has decreased,indicating that the growth of crystals has been inhibited to some extent.The above electrochemical test results can indirectly prove the adsorption behavior of compound additives on the substrate surface.By adding antimony potassium tartrate to the basic gold plating solution and optimizing the concentration of antimony potassium tartrate and various process parameters,a golden and bright Au-Sb alloy coating can be obtained.Both the macro and micro morphology of the coating can be plated with cyanide gold The layers are comparable.In order to further enhance the practicality of the process,potassium sodium tartrate is added to enhance the dispersion ability of the plating solution.By introducing Ni Cl2 and its complexing agent potassium citrate into the basic gold plating solution,as well as the grain refiner polyethyleneimine(PEI),a bright Au-Ni alloy coating can be obtained.The coating color is white golden yellow.It is speculated that the plating layer contains sufficient metal Ni to improve wear resistance.