Theoretical Screening Of Complexing Agents And Additives For Eleroplating Zinc-Nickel Alloy From An DMH-based Bath

Owing to their excellent corrosion resistance and mechanical characteristics,Zn-Ni coatings have been widely used to protect the steel against corrosion.However,the acid baths used in Zn-Ni alloy deposition have poor throwing power and cannot be used for steel parts of complex shape.The alkaline zincate baths often suffer from low current efficiency and cannot be used in the industrial production.Therefore,it is essential to develop a new alkaline bath with high current efficiency for Zn-Ni electrodeposition.In this study,a combined approach using quantum chemical?QC?calculations,molecular dynamic?MD?simulations and electrochemical experiments was employed to investigate potential auxiliary complexing agents and additives in the Zn-Ni bath.The bath composition and plating conditions were optimized and the feasibility of Zn-Ni coatings used as replacement of Cd coatings was investigated.Electrochemical experiments were employed to study the electrochemical behaviors of Zn-Ni alloys.QC calculations and MD simulations were employed to predict the auxiliary complexing agents used in 5,5'-dimethylhydantoin?DMH?-based alkaline bath.Pyrophosphate was selected as the auxiliary complexing agent due to its larger ?E and weaker adsorption capacity on Fe surface.The QC calculations show that the most possible and stable structures for zinc?II?and nickel?II?in the electrolyte are the complexes with one pyrophosphate and two DMH as the complexing agents.The potential additives were predicted in the alkaline with DMH and pyrophosphate as complexing agents.According to QC calculations and MD simulations,vanillin and coumarin were predicted as the most promising additives due to their relatively higher EHOMO,smaller ?E values and stronger adsorption capacity on Fe surface.This has been verified by larger cathodic polarization with decreased grain size obtained from the bath with vanillin or coumarin.The current efficiency is over 85% at the optimized Zn-Ni bath.The phase structure,microhardness,grain size and corrosion resistance of deposits are directly dependent on Ni content in deposits.When Ni content in deposits is in the range of 13% 16%,phase structure of Zn-Ni coatings is single ? phase,the microhardness of deposits is higher and corrosion resistance of deposits is better.Among Zn-Ni alloys,Zn and Cd coatings,the corrosion resistance of Zn-Ni alloys is significantly higher compared to Zn and Cd coatings.Therefore,Zn-Ni alloy coatings can be used as replacement of Cd coatings.Vanillin and coumarin can adsorb on metal surface and inhibit the reduction of Zn²⁺ and Ni²⁺.Thus,the cathodic polarization increases.The inhibition effect of vanillin is more pronounced than that of coumarin in the electrolyte through preferential adsorption on the cathode,indicating that vanillin is the main brightening agent.However,coumarin has a more prominent effect on the structure of Zn-Ni alloys compared to vanillin.Coumarin can thus be considered as a brightening promoter.The bright coatings with smoother appearance is obtained with composite additives.The bright coatings display superior corrosion resistance relative to the dull coatings.It is found that simonkolleite,hydrozincite and zinc oxide are the main corrosion products for both dull and bright Zn-Ni coatings.Better corrosion resistance is observed for bright Zn-Ni alloys compared to that of dull Zn-Ni alloys.This behavior could be due to the smaller grain size and hydrophilic surface of bright Zn-Ni alloys during the initial stage of immersion.After the long immersion time,the better corrosion resistance of bright Zn-Ni samples is related to the increased amount of simonkolleite in the corrosion products.The bright Zn-Ni alloys with better wear resistance are obtained with the addition of additives.The bright Zn-Ni coatings exhibit a much lower and more stable value of the friction coefficient.The wear rate of bright Zn-Ni alloys is about 7.95% of that of dull Zn–Ni alloys,indicating improved wear resistance of Zn-Ni alloys with additives.The wear mechanisms of dull and bright Zn-Ni alloys are adhesive,abrasive and oxidative wears.Electrochemical experiments showed that DMH is the main complexing agent in the Zn-Ni bath.Different behaviors of cyclic voltammetry?CV?curves on Pt and GC electrodes were observed.Only one cathode peak is observed on Pt electrode.However,one or two cathode peaks can be seen on GC electrode.This can be explained according to the difficulty of nucleation on Pt and GC electrode.Furthermore,the effects of Ni²⁺/Zn²⁺ ratio and bath temperature on CVcurves can be used as a guidance to choose the Ni²⁺/Zn²⁺ratio and bath temperature of the electrodepositing parameters in the bath.The transfer coefficient ? and diffusion coefficient D were calculated by CV and chromoamperometry?CA?,respectively.The ? of Zn-Ni alloy is lower than that of Zn only.In contrast,the D of Zn-Ni alloy is larger than that of Zn only.According to CV and linear sweep voltammetry?LSV?,the addition of additives can increase the cathodic polarization.This correlates with the adsorption capacity of additives.The additives can adsorb on metal surface and inhibit the reduction of Zn²⁺ and Ni²⁺,resulting in increased cathodic polarization.The presence of additives has no effect on the rate controlling step and nucleation process of Zn-Ni deposition.The Zn-Ni deposition process is dominated by three-dimensional instantaneous nucleation with mixed controlled step with or without additives.