## 3 Results and Discussion
### Characterization of Corrosion Products after 28 Days of Immersion in a 5% NaCl Solution
#### 3.3.2 XRD of the Corrosion Products  
The XRD results of the corrosion products after 28 d of immersion in a 5% NaCl solution are shown in Figure 8. The Cu phase disappeared in the corrosion products of the Cu coating, which was attributed to the delamination of the coating or the formation of thick corrosion products, which hindered the formation of the coating. However, Cu\({}_{2}\)O remained in the coating after immersion in the NaCl solution. This means that Cu\({}_{2}\)O was not dissolved in the solution. Along with Cu\({}_{2}\)O, CuO and CuCl\({}_{2}\)(H\({}_{2}\)O)\({}_{2}\) were also formed as corrosion products. The formation of CuCl\({}_{2}\)(H\({}_{2}\)O)\({}_{2}\) corroborated the EDS analysis, based on which Cl was present in high amounts. NaCl was found in the corrosion products of the Cu coating, suggesting that it penetrated the defects/pores of the coating and filled them, as detected by XRD. No phases other than TiO and Ti\({}_{3}\)O were observed in the corrosion products of the Ti coating. These phases were identical to those found in the Ti coating after deposition. This result indicates that NaCl does not affect the deterioration of Ti coatings deposited via a twin-wire arc thermal spray process. In contrast, the corrosion products of the 85 Zn-15 Al coating were Zn and Al along with simonkolleite (Zn\({}_{5}\)(OH)\({}_{8}\)Cl\({}_{2}\)\(\cdot\)H\({}_{2}\)O) and NaCl. Simonkolleite is typically formed in the corrosion products of Zn when exposed to saline conditions [43]. NaCl filled the defects and pores of the corrosion products, as corroborated by the SEM images (Figure 7c) and observed by XRD.  
The V\({}_{\text{f}}\) (%) values of each phase of the corrosion products, as detected by XRD, are listed in Table 6. The V\({}_{\text{f}}\) value of Cu\({}_{2}\)O was almost identical to that observed after the coating application, suggesting its stability. As observed in the XRD plots, there was no Cu, which means it was delaminated or transformed into other phases. Therefore, CuO, CuCl\({}_{2}\)(H\({}_{2}\)O)\({}_{2}\), and NaCl were observed. CuCl\({}_{2}\)(H\({}_{2}\)O)\({}_{2}\) and NaCl were 48.92% and 11.50%, respectively. This result is well-corroborated with the EDS analysis results, according to which Na and Cl were in the highest amounts. TiO and Ti\({}_{3}\)O were the corrosion products of the Ti coating and were found to be 78.54% and 21.46%, respectively. There was a slight increase in the V\({}_{\text{f}}\) of TiO and a reduction in Ti\({}_{3}\)O, indicating the greater stability of TiO. Therefore, at longer exposure durations, the Ti coating would be highly resistant to corrosion, thus acting as a protective layer in buildings and infrastructure. The V\({}_{\text{f}}\) of Zn in the corrosion products of 85 Zn-15 Al was almost reduced twice that of the coating, suggesting the severe deterioration of the coating. Moreover, the formation of 31.09% simonkolleite can provide some resistance to corrosion; however, owing to the porosity of the corrosion products, it cannot serve as a protective agent for longer durations.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Coatings**} & \multicolumn{6}{c}{**Elements (wt.\%)**} \\ \cline{2-9}  & **Cu** & **Ti** & **Zn** & **Al** & **O** & **Na** & **Cl** & **Ca** \\ \hline Cu & 46.17 & 0 & 0 & 0 & 5.26 & 24.38 & 22.96 & 1.24 \\ \hline Ti & 0 & 77.01 & 0 & 0 & 22.40 & 0.39 & 0.21 & 0 \\ \hline
85 Zn-15 Al & 0 & 0 & 41.48 & 10.97 & 26.70 & 12.99 & 7.86 & 0 \\ \hline \end{tabular}
\end{table}
Table 5: EDS analysis of corrosion products formed in 5 wt.% NaCl solution after 28 days of immersion.