Study On Electrodeposited Nickel-chromium And Iron-chromium-nickel Coatings From Deep Eutectic Solvents

Ni-Cr and Fe-Cr-Ni alloys have been widely used as protective-decorative coatings in industrial production and social life because of their excellent properties such as wear resistance,corrosion resistance,high resistance,beautiful appearance and so on.However,the traditional electrodeposition process of Ni-Cr and Fe-Cr-Ni alloys is carried out in acidic or alkaline aqueous solution,and a variety of complexing agents and additives need to be added.At the same time,the carbon in the organic additive will participate in the deposition process,so that the alloy coating is doped with carbon elements,the composition of the coating is difficult to control,and accompanied by hydrogen evolution reaction,the operation difficulty increases and the current efficiency decreases.Deep eutectic solvents have good conductivity and wide electrochemical window in electrodeposition of metals.They can not only be electrodeposited at room temperature to obtain most of the metals obtained in aqueous solution,but also can be electrodeposited to obtain metals from high temperature molten salts.Without side reaction,the obtained metal has good quality and high current efficiency.DESs have become a new electrolyte for electrodeposition of Ni-Cr and Fe-Cr-Ni alloys.In this paper,ChCl-EG-NiCl₂·6H₂O-CrCl₃·6H₂O and ChCl-EG-CrCl₃·6H₂O-FeCl₂·4H₂O-NiCl₂·6H₂O were selected for electrodeposition.Bright and smooth Ni-Cr and Fe-Cr-Ni alloy coatings with excellent corrosion resistance were obtained by electrodeposition.The optimum process conditions for electrodeposition are explored and no additives are added to the plating process.The main research results are as follows:（1）The existence of Ni（II）and Cr（III）ions in the solution of ChCl-EG-NiCl₂·6H₂O-CrCl₃·6H₂O was studied.Ni（II）and Cr（III）were found to form[Cr（H₂O）₂Cl₄]^-and[Ni（H₂O）₂Cl₄]^2-with anion in the presence of Ni（II）and Cr（III）ions.The conductivity and viscosity of ChCl-EG-NiCl₂·6H₂O-CrCl₃·6H₂O solution were studied.It was found that the viscosity decreased with increasing temperature.The conductivity of ChCl-EG-NiCl₂·6H₂O-CrCl₃·6H₂O solution increases with the increase of metal salt concentration（NiCl₂·6H₂O+CrCl₃·6H₂O）.The relationship between the conductivity and the temperature is in accordance with the kohlraush empirical formula.The conductivity of ChCl-EG-NiCl₂·6H₂O-CrCl₃·6H₂O decreases with the increase of metal salt concentration（NiCl₂·6H₂O+CrCl₃·6H₂O）,mainly due to the formation of ions and oligomers with low degree of polymerization.（2）The cyclic voltammetry curve indicates that two successive steps of reduction of Cr（III）occur in ChCl-EG-CrCl₃,that is,:Cr（III）→Cr（II）→Cr.The deposition potential difference of Ni and Cr in ChCl-EG-CrCl₃-NiCl₂is reduced,which means that the Ni-Cr-induced underpotential deposition of Ni can lead to a co-deposition of the Ni-Cr alloy.When Ni-Cr alloy is electrodeposited,the deposition process of Ni and Cr is controlled by diffusion.The diffusion coefficient of Ni（II）is D_Ni（II）=1.38×10^-6cm²/s and that of Cr（II）is D_Cr（II）=6.25×10^-7cm²/s.The diffusion activation energy of Cr（II）ion is greater than that of Ni（II）ion.（3）The effects of metal total ion concentration,deposition temperature and deposition potential on electrodeposition of Ni-Cr alloy and Fe-Cr-Ni alloy were studied.The optimum technological conditions for electrodeposition of Ni-Cr alloy are determined as follows:the addition of NiCl₂·6H₂O and CrCl₃·6H₂O is c _Cr（III）+c _Ni（II）=0.4 M,c _Cr（III）:c _Ni（II）=1:6,the temperature is 333 K,and the deposition potential is-0.90V（vs.Ag（I）/Ag）.The lowest content of Cr in Ni-Cr alloy is 2.1 at.%,and the highest content is 28.8 at.%.The Ni-Cr alloy coating prepared in the experiment has good bonding with the substrate,and the coating is very flat and uniform in thickness,about 9 mm.The optimum technological conditions for electrodeposition of Fe-Cr-Ni alloy are as follows:the temperature is 323k and the deposition potential is-0.90v（vs.Ag（I）/Ag）.The analysis of AFM shows that the lower temperature has a positive effect on improving the smoothness of Fe-Cr-Ni coating.The content of fe in the Fe-Cr-Ni alloy obtained by different deposition conditions is about 70 at.%,and the composition of the alloy is Fe_70.4Cr_18.9Ni_10.7,under the condition of 323k,-0.90v（vs.Ag（I）/Ag）.Close to the composition of commercial stainless steel GB/T 14976 304 and GB/T 14976 316.（4）Of all the Fe-Cr-Ni alloy coatings obtained in the experiment,the corrosion resistance of Fe_70.8Cr_16.7Ni_12.5is the best.For all Ni-Cr alloy coatings,the corrosion resistance of Ni-Cr coatings with Cr content of 25.2 at.%is the best,even better than that of commercial chromium sheets and Fe-Cr-Ni coatings with Fe_70.8Cr_16.7Ni_12.5.