离子液体中18-冠醚-6添加剂对三价铬电沉积的影响

在1-丁基-3-甲基咪唑硫酸氢盐（BMIM]HSO₄）电镀液中,探究了18-冠醚-6添加剂对电沉积铬的影响. 紫外-可见光谱结果表明,18-冠醚-6与Cr³⁺形成配合物,使最大吸收波长发生红移. 循环伏安研究表明,Cr³⁺的还原经历了两步. 18-冠醚-6的添加使Cr³⁺的峰电位和起始还原电位均正移了220 mV. 能谱仪（EDS）结果显示,在18-冠醚-6的作用下镀层中铬含量有所提高. 铬镀层的扫描电子显微镜（SEM）表征结果表明,加入18-冠醚-6后,所得镀层的颗粒变大. 18-Crown-6/CrCl₃/BMIM]HSO₄/H₂O电镀液中工艺优化的结果为：在温度为50 ^oC、pH值为3.5、电流密度为1200 A·m^-2、电镀时间为1.5 h的最佳工艺条件下,铬镀层的厚度达到72.5 μm,电流效率为42.3%.

Effect of 18-Crown-6 Additive on Chromium Electrodeposition in Ionic Liquid

Trivalent chromium ion (Cr³⁺) is used for electrodeposition due to its low toxicity. Electrodeposition in ionic liquids can greatly solve for hydrogen evolution problem. However,as a widely used Cr(III) precursor, chromium chloride hydrate (CrCl₃·6H₂O), still contains water. In the presence of water, Cr³⁺ will form a complex coordination structure with water molecules (Cr(H₂O)₆]³⁺), which is a stable octahedral structure and is difficult to be directly reduced to chromium metal. Therefore, coordination agents should be added into the bath. In this work, the effect of 18-Crown-6 additive on chromium electrodeposition was investigated in CrCl₃/BMIM]HSO₄/H₂O plating solution. The UV-Vis spectra showed that 18-Crown-6 formed a complex with Cr³⁺, destroyed the stable coordination structure formed by Cr³⁺ and water molecules, making a red shift in the maximum absorption wavelength. The cyclic voltammograms indicated that the electroreduction of Cr³⁺ occurred in a two-step process, namely, Cr³⁺ + e → Cr²⁺ and Cr²⁺ + 2e → Cr⁰. Both of the peak potential and initial reduction potential of Cr³⁺ had positive shifts by 220 mV after adding 18-Crown-6. The reason for this phenomenon was that when 18-Crown-6 was added to the plating solution, the stable structure of Cr(H₂O)₆]³⁺ was destroyed, Cr³⁺ became more readily to be reduced, thereby, lowered the reduction potential of Cr³⁺. The EDS data showed that the chromium content in the coating was increased under the action of 18-Crown-6. The improvement indicated that 18-Crown-6 was beneficial to the chromium electrodeposition. The SEM characterizations indicated that the coating obtained in 18-Crown-6/CrCl₃/BMIM]HSO₄/H₂O plating solution had a larger particle size. Tafel curves suggested that the corrosion resistance of chromium coating was better than that of brass substrate. The optimized process in 18-Crown-6/CrCl₃/BMIM]HSO₄/H₂O plating solution could be proceeded at the temperature of 50 ^oC, pH of 3.5, current density of 1200 A·m^-2, and plating time of 1.5 h. The thickness of the chrome plating reached 72.5 μm and the current efficiency was 42.3%.