Technology And Mechanism Of Electrodeposition Of Ni And Ni Alloys From [BMIM]HSO₄ Ionic Liquid

At present, the technology of electroplating Ni and its alloy coatings from aqueous solutions have two major problems. Firstly, some organic addictives are needed in the plating solution. However, they may participate in the electrodeposition process, which results in the appearance of impurities in the deposits, thus, the coatings are generally amorphous. Secondly, the Ni and its alloy coatings electrodeposition from aqueous solutions are always accompanied by intensive hydrogen evolution reaction resulting in profound effect on quality of the deposits. In contrast to aqueous solutions, ionic liquids can provide an ideal kind of solvents for the electrodeposition of Ni and its alloy coatings since they usually possess a wide electrochemical window, high electrical conductivity and they can prevent hydrogen evolution reaction, moreover, it is possible to prepare crystal coatings directly from ionic liquids according to their electrochemical stability. In addition, the [BMIM]HSO4 ionic liquid presents stable chemical and physical properties, its cost is low, and the technology and mechanism of the electrodeposition of Ni and Ni alloys from [BMIM]HSO4 ionic liquid system is not reported yet. Therefore, the technology and electrochemical mechanism of Ni and its alloy coatings plating in 1-butyl-3-methylimidazolium hydrosulfate([BMIM]HSO4) are investigated.The effects of electrodeposition potential, electrolyte temperature and electrodeposition time on the coating thickness and metal content in [BMIM]HSO4 ionic liquid have been studied. The optimum conditions for preparing Ni coatings from [BMIM]HSO4 ionic liquid are as follows. Electrodeposition potential is-1.45 V, electrolyte temperature is 65℃, electrodeposition time is 80 min. The Ni coatings with the thickness of 13.0 μm can be prepared under this condition. The optimum conditions for preparing Ni-Fe alloy coatings from [BMIM]HSO4 ionic liquid are as follows. Electrodeposition potential is-0.28 V, electrolyte temperature is 55℃, electrodeposition time is 70 min. The Ni-Fe alloy deposits of 34.09 percent Fe content with the thickness of 13.8 μm can be prepared under this condition. The optimum conditions for preparing Ni-Co alloy coatings from [BMIM]HSO4 ionic liquid are as follows. Electrodeposition potential is-0.55 V, electrolyte temperature is 65℃, electrodeposition time is 70 min. The Ni-Co alloy deposits of 4.68 percent Co content with the thickness of 13.7 μm can be prepared under this condition.The electrodeposits are characterized by SEM, EDS and XRD. SEM analysis shows that the electrodeposits obtained on Cu electrodes are smooth, dense and crack-free. XRD pattern of the electrodeposited layers reveals the crystalline structures of the deposits. The polarization curves reveal that the Ni deposits prepared under higher potential present better anti-corrosion property, the Ni-Fe and Ni-Co deposits present better anti-corrosion property than that of pure Ni.Modern electrochemical methods are employed to investigate the mechanism in the electrodeposition process. The results from the cyclic voltammetry showed that the reduction of Ni and its alloys occurs by a one-step process and they are quasi-irreversible. Chronoamperometry results provid direct evidence that Ni and its alloys electrodeposition take place through an instantaneous nucleation and three-dimensional growth mechanism with diffusion controlled in [BMIM]HSO4...