离子液体中18-冠醚-6添加剂对三价铬电沉积的影响

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

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Cr^(3+)形成配合物,使最大吸收波长发生红移.循环伏安研究表明,Cr^(3+)的还原经历了两步.18-冠醚-6的添加使Cr^(3+)的峰电位和起始还原电位均正移了220 m V.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℃、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72.5μm,电流效率为42.3%.

Trivalent chromium ion(Cr^(3+))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,Cr^(3+)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 Cr^(3+),destroyed the stable coordination structure formed by Cr^(3+)and water molecules,making a red shift in the maximum absorption wavelength.The cyclic voltammograms indicated that the electroreduction of Cr^(3+)occurred in a two-step process,namely,Cr^(3+)+e→Cr2+and Cr2++2e→Cr0.Both of the peak potential and initial reduction potential of Cr^(3+)had positive shifts by 220 m V 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,Cr^(3+)became more readily to be reduced,thereby,lowered the reduction potential of Cr^(3+).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/H2 O plating solution could be proceeded at the temperature of 50℃,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 72.5μm and the current efficiency was 42.3%.