关键词: 三价铬, 1-丁基-3-甲基咪唑硫酸氢盐, 18-冠醚-6, 铬镀层, 电流效率

Abstract:

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%.

Key words: trivalent chromium, 1-butyl-3-methyl imidazole sulfate, 18-Crown-6, chromium electrodeposition, current efficiency