# Concentration Influence of Complexing Agent on Electrodeposited Zn-Ni Alloy
## 3 Results and Discussion
### Electroplating Performance  
Figure 5a shows the variation in thickness as a function of current density with different TPEA concentrations. The thickness was in a wide range from 1 \(\upmu\)m to 11 \(\upmu\)m. The decrease in thickness can be observed according to the decrease in current density. This could be explained by the distance between the anode and cathode because the higher anode-cathode distance in the Hull cell tank induces the lower current density as mentioned above. The longer anode-cathode distance causes the longer and more difficult transportation of cations, leading to the poor ability of cations to stick on the cathodic surface, the uneven cathodic surface, and the thinner coating. At current densities higher than 2 ASD, thickness decreased with the increase in TPEA concentration. For example, the thickness of Zn-Ni coating was 11, 10.5, 9.8, and 9 \(\upmu\)m with 0, 0.035, 0.07, and 0.1 M, respectively. This was caused by the increase in TPEA concentration diminishing the diffusion rate of metal ions, thereby reducing the thickness [28]. For current densities lower than 2 ASD, most thickness values were identical, except for those with 0 M of TPEA concentration, which were lower than the others. Furthermore, Figure 5b shows that Ni content decreased with the decrease in current density. In addition, the increase in TPEA concentration boosted Ni content at any current density. This result was supported by Ni content obtained through EDS mapping (Figure 6). A similar trend was observed in the case of EDS mapping (Figure 7), indicating that the increase in TPEA concentration significantly boosted the Ni content of Zn-Ni alloy coating. Notably, a TPEA concentration of 0.1 M could increase the Ni content by up to 20%, which is a high amount compared to previous studies [34]. The increase in Ni content owing to TPEA can be explained by the fact that TPEA acts as a complex agent that binds with  
Figure 3: Crystallite size obtained from (600) peaks as a function of current density; enlargement of (600) peaks at 1 ASD with TPEA concentrations of (**a**) 0 M, (**b**) 0.035 M, (**c**) 0.07 M, and (**d**) 0.1 M, and at 2 ASD with TPEA concentrations of (**e**) 0 M, (**f**) 0.035 M, (**g**) 0.07 M, and (**h**) 0.1 M.  
Ni ions to prevent the precipitation of Ni hydroxide, thereby enhancing the deposition of metallic Ni on the cathode surface [27; 28; 34]. Figure 4: SEM images of Zn-Ni coating with 0.07 M TEPA at (**a**) 0.5 ASD, (**b**) 1 ASD, and (**c**) 2 ASD, and at 2 ASD with TEPA concentrations of (**d**) 0 M, (**e**) 0.035 M, (**f**) 0.07 M, and (**g**) 0.1 M TEPA. Figure 5: (**a**) The thickness of coating, and (**b**) Ni content obtained by XRF as a function of current density with different TEPA concentrations. Electroplating allows creating a multi-component alloy coating at low temperatures. The anomalous co-deposition densities an electrochemical deposition whereby the less noble metallic element is preferred to deposit from a bath consisting of various metallic ions, resulting in a higher content of the less noble metallic element in the deposit than other metallic elements. One previous study proved that the anomalous co-deposition occurs at a high current density, and the pH at the cathodic surface rises rapidly [35]. The rapid increase in surface pH causes the formation of hydroxides of the less noble metal, which adsorb on the cathodic surface, thereby suppressing the reduction of other metallic ions. This occurrence is recognized in the co-deposition of iron-group metallic elements, or iron-group elements with Zn or Cd [36]. The Zn-Ni alloy electrodeposition is called anomalous co-deposition, in which the less noble metallic element, Zn, is preferentially deposited. To investigate the deposition behavior of each component of Zn-Ni alloy, the partial polarization curves of Zn and Ni deposition could be determined using the following method.