关键词: 电镀, 1-丁基-3-甲基咪唑硫酸氢盐, 醋酸钠, 三价铬, 铬镀层

Abstract: Using trivalent chromium ions (Cr(III)) as the chromium source for chromium electrodeposition has attracted much attention since it can reduce the toxicity of the whole process. Even though the chromium deposition in Cr(III)-based ionic liquid bath can avoid the most hydrogen evolution problem, CrCl₃·6H₂O is widely used as the Cr(III) precursor, which still contains water and has the stable octahedral structure. As a result, it is difficult to deposit Cr and there is still hydrogen evolution reaction (HER). Moreover, the hydroxyl ions (OH^-) produced during HER react with Cr³⁺ to form Cr(OH)₃, which will affect the performance and property of the Cr layer. To avoid the formation of Cr(OH)₃, 1-butyl-3-methylimidazolium hydro sulfate ([BMIM]HSO₄) aqueous solution was used as the electrolyte in this work. To enhance the depositing ability and to lower the reduction onset potential of Cr(III), NaOAc was used as the additive. Electrochemical measurements such as cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were made to test the electrochemical performance in different electrolytes. Chromium layers were electrodeposited on copper plates at a constant potential of -3.0 V (vs. Pt). The Cr thickness and current efficiency were calculated based on the gravimetric method. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) techniques were used to study the surface morphology, crystalline structure and elemental composition of the deposited Cr layer, respectively. The cyclic voltammetric results showed that the reduction of Cr(III) to Cr(0) is a two-step process. First, Cr(III) reduced to Cr(II) at -1.50 V (vs. Pt). Then, Cr(II) further reduced to Cr(0) at -2.10 V (vs. Pt). Both of the peak current and peak potential followed the Rendle-Sevcik equation, by which the diffusion coefficient of Cr³⁺ at 40 ℃ was calculated to be 1.6 ×10^-8 cm²·s^-1. The XRD and SEM characterizations indicated that the Cr coating layers were composed of Cr nanoparticles with an average particle size of 0.87μm. The NaOAc effect on the electrodeposition of Cr was also studied. After adding NaOAc, the reduction peak potential of Cr(III)shifted to positive direction, indicating less energy required to reduce Cr³⁺. Additionally, the molar ratio of Cr:O in the coating layer increased from 4.48 to 6.28, indicating that OAc- was helpful for the electrodeposition of Cr metal. This was because the addition of OAc- could break the stable octahedral structure of CrCl₃·6H₂O. Overall, the best coating thickness (63 μm) and highest current efficiency (33.5%) were obtained when the molar ratio of NaOAc-[BMIM]HSO₄-CrCl₃-H₂O electrolyte was 0.075:1:0.5:6. Based on this study, it can be concluded that [BMIM]HSO₄-NaOAc aqueous electrolyte might be benefited to electrodeposit pure Cr, instead of Cr(OH)₃, with relatively high current efficiency and low reduction onset potential.

Key words: electrodeposition, 1-butyl-3-methylimidazolium hydro sulfate, sodium acetate, trivalent chromium, chromium coating