# Progress in Electrodeposition of Zinc and Zinc Nickel Alloys Using Ionic Liquids
## 5 Substrate Effect
One type of IL is formed by mixing choline chloride (ChCl) with different hydrogen bond donors: urea, EG, malonic acid (mal), and glycerol (Gl), with appropriate molar ratios. The other type is formed by mixing ChCl with a metal halide, ZnCl\({}_{2}\). They investigated the corrosion behavior and reported the corrosion resistance (in terms of corrosion currents), as shown in Table 3. On the basis of the works reported by [PERSON] et al. [63], from Table 3, the corrosion currents (I\({}_{\text{corr}}\)) for the Zn deposition on ChCl/urea and ChCl/Gl ILs were observed to be 0.48 and 0.49 mA cm\({}^{-2}\), respectively. However, Zn deposited using ChCl/Gl IL suffered from pitting of the magnesium substrate and also displayed few microcracks. These cracks are reported to be eliminated by changing the mode of deposition from constant current to pulse current. The authors concluded that an increase in the nucleation rate of the electrodeposited atoms resulted in the formation of smaller grains, producing a shinier, smoother deposit with a lower number of cracks. In addition to the choice of ionic liquid, the temperature and water content of the ionic liquid plays a major role in influencing the interaction with substrates. However, the presence of water content alone in the ChCl/Gl did not guarantee the pitting of pure Mg substrate. Table 4 shows the works on the Zn and Zn-Ni carried out on different commercial substrates. [PERSON] et al. [64] investigated the electrochemical reduction of Zn(II)/Zn on Mg alloy substrates in N-butyl-N-methyl-pyrrolidinium dicyanamide ([BMP][DCA])-based IL by performing constant voltage deposition in the range -2.3 V to -2.5 V (vs. Fc/Fc\({}^{+}\)). A low melting point water stable IL with good metal (Zn) solubility is formed by mixing dicyanamide ([DCA]) and 1-butyl-1-methylpyrrolidinium [BMP]. The authors reported the formation of complex ions between Zn(II) and DCA anions. The high solubility of ZnCl\({}_{2}\) in the BMP-DCA-based IL was attributed to the formation of complex ions between Zn(II) and DCA anions. In addition, ([BMP][DCA]) IL is reported \begin{table}
\begin{tabular}{c c c c c c} \hline
**System** & **Substrate** & **Temperature, °C** & **OCP (mV vs. P0)** & **E\({}_{\text{corr}}\) (mV vs. P0)** & **I\({}_{\text{corr}}\) (uA cm\({}^{-2}\))** \\ \hline ChCl/ZnCl\({}_{2}\) & Mg & 60 & \(-1253\) & \(-1207\) & \(3.86\) \\ ChCl/ZnCl\({}_{2}\) & Mg & 25 & \(-1863\) & \(-1857\) & \(3711.17\) \\ ChCl\({}_{2}\)ZnCl\({}_{2}\) & Low carbon steel & 25 & \(-664\) & \(-651\) & \(109.42\) \\ ChCl/urea & Mg & 60 & \(-1531\) & \(-1446\) & \(0.48\) \\ ChCl/EG & Mg & 60 & \(-1489\) & \(-1347\) & \(12.72\) \\ ChCl/Gl & Mg & 60 & \(-1594\) & \(-1557\) & \(0.49\) \\ ChCl/Mal & Mg & 60 & \(-1962\) & \(-1940\) & \(31.58\) \\ \hline \end{tabular}
\end{table} 
Table 3: The electrochemical parameters obtained from the polarization of Zn deposition in different ionic liquids on different substrates [63]. to have a wider cathodic potential window than 1-ethyl-3-methylimidazolium ([EMIm][DCA])-based IL. The results suggested that [Zn(DCA)\({}_{3}\)]- complex anion was formed through the reaction of Zn(II) with DCA- anions, which later was reduced to Zn metal through a single-step electron transfer process. Current transient results confirmed that the electrodeposition of Zn involved three-dimensional instantaneous nucleation under diffusion control. SEM and adhesion test data results confirmed that a compact and well adherent Zn coating can be deposited on the Mg alloy in the ionic liquid when deposited in the potentials up to \(-2.4\) V. The corrosion behavior of deposited Zn coatings, produced at \(-2.3\) V and \(-2.4\) V, was shown to be better in comparison with the bare Mg substrate. The lower protective performance of the Zn deposited at \(-2.5\) V was attributed to its high porosity and surface area owing to its spongy structure. A large pore size would allow a rapid mass transport of species, permitting the chloride species to reach the Mg substrate and corrode the surface.