# Progress in Electrodeposition of Zinc and Zinc Nickel Alloys Using Ionic Liquids
## 3 Zinc-Nickel Alloy Deposition in Ionic Liquids
[43] studied the electrodeposition of Zn-Ni alloy on copper substrates from DESs/Reline (ChCl/urea = 1:2) and Ethaline (ChCl/EG = 1:2). Though smooth Zn-Ni electrodeposits were achieved, the reactions at the counter electrode (anode) were undetermined and require further investigation. In addition, they also encountered issues associated with the solubility of metal salts (ZnCl\({}_{2}\)), metal ions (Zn(II)), and composition control. The conductivity and purity of the deposit were reported to be lower, thereby affecting the efficiency of the process, and had a significant impact/influence on the morphology of the deposit and deposition rate. Moreover, Zn-Ni coatings are prone to sacrificial dissolution of the zinc in the zinc-nickel coating, which hinders its commercial potential in the electroplating sector. Furthermore, electrodeposition of binary Zn-Ni alloys produced from ILs resulted in the formation of Ni rich alloys, known as anomalous deposit, and displayed more positive corrosion potential than that of ferrous substrates, making the Zn-Ni alloy deposit cathodic to steel, such that they cannot be used for sacrificial protection. Moreover, the deposit obtained using the additive-free ChCl/EG-based ionic liquid did not cover the entire surface of the electrode and resulted in the formation of small crystallites without a defined shape. The hygroscopic nature of ChCl-based ILs calls for a long drying procedure, thereby affecting the overall economy of the process. Addition of water to DESs (such as ChCl/urea/EG) for the deposition of Zn-Ni is considered to be one of the effective ways to control the Ni content while favoring the Zn deposition. The addition of water to these systems, in general, does not change the potential window, owing to the fact that the systems reported in literature (i) are less susceptible to moisture, (ii) shift the reduction potential of zinc(II), and (iii) favor the Zn deposition and suppress the growth of nickel. [PERSON] et al. [44] studied the deposition of Zn-Ni coatings from the reline-xH\({}_{2}\)O system (ChCl/urea = 1:2; x = 0, 1, 3, 5, and 7 wt %). The CV results showed the system was stable in the electrochemical window between -1.12 V and 1.25 V (vs. Ag/AgCl) for x \(\leq\) 5 wt %. The stable electrochemical window of the reline-xH\({}_{2}\)O systems was attributed to the hydrogen bonding between the water molecules and urea, which can suppress the hydrogen evolution reaction to some extent, while increasing the conductivity up to 5 wt % water content in the system. When the water content is increased to 7 wt %, the electrochemical window of the reline-7H\({}_{2}\)O system decreases and becomes unstable, indicating water electrolysis. SEM results displayed a change in morphology from irregular clusters with holes and cracks with no water content to a uniform and compact surface with fine spherical like clusters when the reline (ChCl/urea = 1:2) contained 5 wt % water. Furthermore, X-ray diffraction (XRD) results showed that Zn-Ni alloys deposited from reline (ChCl/urea = 1:2) exhibited a change in phase with the increase in water content. At 0, 3, and 5 wt % water, \(\alpha\)-phase Zn-Ni (an Ni rich phase), a mixture of \(\alpha\) and \(\gamma\)-phase Zn-Ni, and a single \"\(\gamma\)\" (Ni: 12 to 16%) phase Zn-Ni were observed, respectively. The best corrosion protection was observed when Zn-Ni was deposited from reline-5H\({}_{2}\)O system (ChCl/urea = 1:2 containing 5 wt % water). Another suitable way to solve the problem of overcoming the anomalous co-deposition of Zn-Ni alloys is to design ILs with metal complexes as part of their structures [14]. Development of ILs with metal containing cations can facilitate the charge balancing anions owing to their easy accessibility to the electrode surface, reducing the polarization and improving the energy efficiency of the process [45]. Besides, grain size and predominant grain orientation of the metal can be controlled by varying the deposition temperature. [PERSON] et al.