Study On Electrodeposition Of Cu-Zn Alloy Transition Layer From Ionic Liquid On Aluminum Alloy

Aluminum?Al?alloy is an indispensable structural engineering material in modern society.Surface plating is an important surface modification technique for aluminum alloys.In the Al alloy electroplating industry,the toxic cyanide zincating pretreatment process is still widely used.This topic has developed a new pretreatment method to replace the process to eliminate the use of cyanide.The new method is green and effective,in which a dense and flat Cu-Zn alloy layer is directly deposited on the surface of the Al alloy by galvanostatic electrodeposition from a mixed system of ionic liquid?[EMIm]TfO?and ethanol?EtOH?.In this paper,the optimum process parameters for the preparation of Cu-Zn alloy layer in the mixed system of[EMIm]TfO-EtOH were determined,and the properties of the alloy layer and cyanide zincating layer were compared.In addition,the electrochemical behavior of Cu-Zn was also studied under the mixed system,and the mechanism of action of the additive2,2-bipyridine was discussed.The addition of ethanol can increase the electrical conductivity of the[EMIm]TfO-EtOH mixed system and reduce the activation energy of the system.The analysis of infrared spectroscopy showed that the increase in conductivity of the system is attributed to the hydrogen bond between the ethanol molecule and[EMIm]⁺.The electrochemical window of[EMIm]TfO-EtOH-2 mixed system is the widest,and the morphology of the coating prepared from this system is relatively optimal.Based on this mixed system,the dense Cu-Zn alloy required for preparation is further determined by single factor experiment.Electroplating process parameters of the coating:ethanol volume fraction 20%,c?CuCl₂?=60 mmol·L^-1,c?ZnCl₂?=300mmol·L^-1,deposition time 30 min,stirring speed 300 r·min^-1,bath temperature 50°C,current density 3 mA·cm^-2,additive 2,2-bipyridine 1.0 g·L^-1.The Cu-Zn alloy coating was used to replace the cyanide zincating Zn layer,and both deposited layers have good adhesion.And the corrosion resistance of Cu-Zn alloy coating is better than that of zincating Zn layer.In addition,the effect of two kinds of deposited layers as the transition layer of aluminum alloy plating was further studied.The Ni layer plated on the surface of two intermediate layers showed good adhesion and corrosion resistance.The Ni layer deposited on the Cu-Zn layer is smoother.The Cu-Zn alloy coating prepared by the electroplating process determined by this subject completely can be used to replace the cyanide immersion Zn layer,thereby eliminating the use of cyanide.The CV test showed that the reduction of Cu in the[EMIm]TfO-EtOH-2 system was a two-step reduction:Cu²⁺?Cu⁺and Cu⁺?Cu,while Zn was a one-step reduction:Zn²⁺?Zn.The presence of Cu²⁺can inhibit the reduction of Zn,and the increase of ethanol content is beneficial to the co-deposition of Cu-Zn.The increase in the concentration of the main salt caused the reduction peak of the Cu-Zn co-deposition to shift negatively.When the concentration of ZnCl₂ in the solution is too low or too high,the co-deposition reaction of Cu-Zn is inhibited.The solution temperature has little effect on the peak potential of the Cu-Zn reduction peak.The deposition of Cu belongs to the multi-electron multi-step heterogeneous reaction under the[EMIm]TfO-EtOH-2 system,and the reaction process belongs to the quasi-reversible process.The co-deposition of Cu-Zn and the reduction of Zn are irreversible processes,and the reaction rate of the process is diffusion controlled.The effects of 2,2-bipyridine on the electrochemical behavior of Cu-Zn were investigated by electrochemical test methods such as CV,LSV and CA.In the binary Cu-Zn system,the addition of additives makes ion reduction difficult,but incre ases the reaction rate of Zn reduction,and also reduces the reaction rate of Cu reduction,but has no effect on the current and potential of Cu-Zn co-deposition.The addition of the additive 2,2-bipyridine can change the nucleation mode of Cu,Cu-Zn and Zn.XRD showed that pure Cu,Cu₅Zn₈ alloy and Zn were obtained under the set potential of-0.85 V,-1.75 V and-2.1 V,respectively.The SEM image showed that the additive2,2-bipyridine could have a great influence on the microstructure of the deposited layer.