# Progress in Electrodeposition of Zinc and Zinc Nickel Alloys Using Ionic Liquids
## 3 Zinc-Nickel Alloy Deposition in Ionic Liquids  
Alloying of zinc with metals such as nickel improves mechanical properties like hardness and wear resistance, and provides more corrosion protection than pure zinc deposits [33; 34; 35]. Zn-Ni alloys coatings can not only be used for traditional corrosion mitigation, but also for other applications such as electrocatalysis. Zn-Ni alloy electrodeposits exhibit different phases as a function of their alloy composition and are assigned based on their distinct reduction potentials from cyclic voltammetric data. Zn-Ni deposits with less than 10 wt % Ni content are a mixture of \(\gamma\) and \(\eta\)-phases, while alloys of 10-17 wt % Ni content consist of \(\gamma\)-phase Ni in the Zn-Ni alloy composition. Ni contents greater than 17 wt % in the Zn-Ni alloy are categorized as a mixture of \(\gamma\) and \(\alpha\)-phases. Commercially attractive bright Zn-Ni deposits developed are usually obtained with a nickel content of 12-15 wt % and exhibit \(\gamma\)-phase Zn-Ni alloy. These are mostly electrodeposited from alkaline baths as they are formulated very close to the conventional alkaline non-cyanide Zn bath and differ in terms of addition of Ni generally in small amounts. Zn-Ni alloy coatings, when deposited at low current densities, result in the formation of complex structure with a flat and bright surface, which corresponds to the \(\gamma\)-phase and contain Ni in the range 12 to 16 wt %. \(\gamma\)-phase Zn-Ni alloys behave like a monometallic deposit with higher hydrogen overpotential than pure zinc, are anodic to steel, and are considered to be a possible substitute to cadmium. In addition, these alloys offer an excellent hardness in the range 260-400 HV and corrosion resistance with a good degree of micro-cracking [36; 37]. To obtain homogeneous \(\gamma\)-phase Zn-Ni alloy coatings, organic amines are often added to the aqueous electrolyte solution to stabilize nickel. However, cyanide is formed owing to the anodic oxidation of amines during electrolysis of alkaline Zn-Ni baths, indicative of electrolyte breakdown products formation, and poses environmental challenges. As per the reports by [PERSON] et al. [38], the average cyanide formation rate is 7 mg/Ah to 10 mg/Ah. With cyanide generation on one hand and addition of Ni source to maintain the concentration on the other hand, this shall not only make the process difficult, but also increases the operation cost. Unlike the deposition of zinc, the deposition of Zn alloys is more difficult owing to the differences in redox potentials of the alloying elements in aqueous electrolytes. Additionally, Zn-Ni alloy coatings deposited in aqueous electrolytes resulted in anomalous deposition, with preferential deposition of the less noble zinc owing to the pH rise at the surface of the metal substrate (generally ferrous). Considering the limitations associated with aqueous-based electrolytes, employing ILs in Zn-Ni electrodeposition shall make the reduction of either Ni(II) ions or Zn(II) ions difficult, thereby influencing alloy composition and morphology. Before deciding which type of ionic liquid is most suitable for the deposition of a given metal, it is important to consider the reduction potential of the metal and the potential window of the liquid. This is not always trivial as reduction potentials are significantly affected by the coordination of the anion to the metal. Electrodeposition of Zn-Ni alloys were performed in ILs including DESs such as traditional chlorozincate-based ILs [39; 40] and ChCl-based DESs [41; 42], owing to their tailoring properties. One of the key advantages of employing ILs for the metal alloy deposition such as Zn-Ni centers around its ability to tailor the redox potentials through metal speciation. Tailoring of redox potentials through metal speciation in these ILs allows for the co-deposition of metals that generally have a large difference in redox potentials and resolve the problems of bath instability and formation of anamolous deposits. [PERSON] et al.