| Nickel plating has a wide range of applications in metallic protective and decorative coatings.The current nickel plating process is mostly carried out in aqueous electrolytes.The problems of this process are that nickel plating can produce hydrogen embrittlement,anode passivation easily,the composition of bath is complex,the coating quality has high dependence on additives and the treatment of electroplating waste liquid is difficult.In view of these problems existing in the current nickel plating process,this paper makes use of low eutectic ionic liquid with several advantages such as green environmental protection,wide electrochemical window,good chemical thermal stability,low vapor pressure,and so on.A new process of electroplating bright nickel using Betaine·HCl-6EG eutectic ionic liquid is proposed.The properties of Betaine·HCl-6EG eutectic ionic liquid are systematically studied.The influence of technological parameters on the quality of electroplating bright nickel in Betaine·HCl-6EG eutectic ionic liquid and the mechanism of electrode reaction are discussed.The research work has important scientific value and broad application prospect for the sustainable development of nickel plating industry.The main findings are as follows:?1?Using Betaine·HCl-6EG-nNiCl2·6H2O and Betaine·HCl-6EG-nNiSO4·6H2O solvents as electrolytes and nickel plates as anode,the concentrations of NiCl2·6H2O and NiSO4·6H2O in electrolyte,the temperature of electroplating solution and the cell voltage were adjusted respectively,and no additives were added in the electroplating process.Bright nickel coating with good corrosion resistance and strong bonding with the substrate was obtained on carbon steel substrate.?2?The cyclic voltammetry curves showed that the electrochemical window width of Betaine·HCl-6EG eutectic ionic liquid could fully meet the requirements of nickel electroplating.The physical properties of Betaine·HCl-6EG eutectic ionic liquid are measured.Betaine·HCl-6EG eutectic ionic liquid could dissolve NiCl2·6H2O and NiSO4·6H2OandformBetaine·HCl-6EG-n NiCl2·6H2Oand Betaine·HCl-6EG-nNiSO4·6H2O solvents with low viscosity and good conductivity at room temperature.TheviscosityofBetaine·HCl-6EG-nNiCl2·6H2Oand Betaine·HCl-6EG-nNiSO4·6H2O solvents were 3.28243.64 mPa·s and 3.9255.19 mPa·s,respectively.And the electrical conductivity of Betaine·HCl-6EG-nNiCl2·6H2O and Betaine·HCl-6EG-nNiSO4·6H2O solvents were 0.3141.92 Sm1 and 0.1191.109 S·m-1,respectively.Whenthetemperatureincreased,theviscosityof Betaine·HCl-6EG-nNiCl2·6H2O and Betaine·HCl-6EG-nNiSO4·6H2O solvents decreased,the conductivity increased.Increasing the concentration of Ni?II?,the viscosity of the electrolyte system increased and the conductivity decreased.?3?The effects of the process parameters such as the concentration of Ni?II?,electroplating temperature and cell voltage on the electroplating nickel in Betaine·HCl-6EG-nNi?II?solvents were studied.It was found that controlling the Ni?II?concentration in Betaine·HCl-6EG-nNi?II?solvents was the key factor for electroplating bright nickel.When Betaine·HCl-6EG-nNiCl2·6H2O and Betaine·HCl-6EG-nNiSO4·6H2O solvents were used as electrolytes and the Ni?II?concentration was controlled at a range from 0.137 to 0.141 mol·L-1,the thickness of nickel coatings along the substrate was uniform and they had good bonding with the carbon steel substrates.The crystal structure of the nickel coatings was face-centered cubic structure.According to the results of SEM and EDS as well as XRD and combining with single-factor conditional experiments,the optimal electroplating parameters for nickel electroplating using Betaine·HCl-6EG-nNi?II?solvents as electrolytes were obtained as follows:cell voltage 0.6 V and electroplating temperature333K for Betaine·HCl-6EG-nNiCl2·6H2O solvents;cell voltage 1.8V and electroplating temperature 323K for Betaine·HCl-6EG-nNiSO4·6H2O solvents.The thickness of nickel coatingsobtainedundertheoptimalconditionswere11.17?m?Betaine·HCl-6EG-nNiCl2·6H2O solvents?and 11?m?Betaine·HCl-6EG-nNiSO4·6H2O solvents?respectively.?4?When the same concentration of NiCl2·6H2O and NiSO4·6H2O were added to Betaine·HCl-6EG eutectic ionic liquids,it was found that the dissolution processes of nickel plates in Betaine·HCl-6EG-nNi?II?solvents could be described by the same anodic dissolution model.However,the passivation potential of nickel plate in Betaine·HCl-6EG-nNiCl2·6H2O electrolyte system was more positive than that in Betaine·HCl-6EG-nNiSO4·6H2O electrolyte system.The results showed that it was easy to passivate in the former electrolyte system.No nucleation and retention ring was observed from the cyclic voltammetry curves of Betaine·HCl-6EG-NiCl2·6H2O and Betaine·HCl-6EG-NiSO4·6H2O that recorded on carbon steel electrode.Meanwhile,in Betaine·HCl-6EG-NiCl2·6H2O electrolyte system,the initial reduction potential was more positive and the cathodic reduction current was higher.The results showed that Ni?II?was easily reduced to metallic nickel in this system.The analysis of the data of cathodic peak current,cathodic peak potential and half-peak potential read from the cyclic voltammetry curves that were obtained at different temperatures and potential scanning rates revealed that the electrochemical reduction process of Ni?II?on carbon steel electrode was determined to be a quasi-reversible process controlled by diffusion.The initial electrocrystallization of Ni?II?on carbon steel electrode was determined by using the dimensionless equation of the theoretical current-time transient curve.The current-time transient curves were fitted and analyzed.It was found that the electrocrystallization mechanism of Ni?II?in Betaine·HCl-6EG-nNi?II?electrolyte did not conform to the typical continuous or instantaneous nucleation growth model.Therefore,the electrode reaction mechanisms of electroplating bright nickel in BetaineHCl-6EG-nNi?II?eutectic ionic liquids were revealed. |
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