Study On Process Of Hexavalent Chromium Plating With High Efficiency

Aiming at the deficiencies of existing additives, this paper mainly studied the additives and process of hexavalent chromium plating with high current efficiency based on the standard bath.Compared with the standard chromium plating bath, the effects of the HCR, WR-26, BH additive to the bath and coating performance were researched. The results showed that the HCR additive had the best behavior and the current efficiency of its bath could reach 26.3%. But it also had disadvantages, for example, its coating had wider crackle and lower hardness than that of WR-26 or BH additive.According to these shortages metioned above, the HCR additive was optimized by single factor experiment and orthogonal test. Five single additives,including potassium iodide, boric acid,formic acid, malonic acid and sulfanilic acid, were investigated by analyzing their influences to the currert efficiency, hardness and surface morphology. It was found that potassium iodide could improve current efficiency, malonic acid could ameliorate the hardness and surface morphology. Therefore, these two additives were compounded with HCR additive. By contrasting the behaviors of HCR additive before and after optimization, it could be known that the optimized one had higher current efficiency and hardness whose values were 27.8% and HV1001132. So the process of the optimized additive was confirmed as: current density 65 A/dm2, temperature 56℃, malonic acid 1.0g/L,potassium iodide 0.3g/L,HCR 2.2g/L,basic components of the bath unchanged.Additionally, the mechanism of chrome plating with high efficiency was also discussed by using electrochemical workstation, SEM, XRD and so on. The polarization curves indicated that the additives could control the hydrogen evolution reactions and play down the evolution overpotential of chromium to rise the current efficiency. Moreover, the microstructures of coatings showed that the additives could not only refine the crystals but also make them grow in a preferred orientation. For these two reasons , the levelings and corrosion resistances of the coatings got better.At last, the bath of Shenyang electroplating factory with high concentration of impurity ions was treated by diaphragm electrolysis with biscuit firing tube. The experimental results showed that the traditional electrolyte in the tube might lead to the increasing of sulfate in the bath. To solve this problem, the standard bath was diluted 3 times and used as the electrolyte in the tube. The pilotscale experiment pointed out that this method can be used in production successfully.