# Insight into the Role of Cerium (III) Addition to a MgAl-LDH Coating on AA6082
## 3 Results
The wide broad signal between 15 and 35\({}^{\circ}\) can be assigned to the aluminum hydroxide Al(OH)\({}_{3}\) formed as corrosion product after the chloride attack or to aluminum oxide Al\({}_{2}\)O\({}_{3}\), as suggested by the search-match results made in the COD databases. The signals belonging to the crystalline cerium compounds at 15 and 17\({}^{\circ}\) disappeared, suggesting that they solubilize, releasing the cerium ions during the prolonged immersion. According to the XRD results, the top-view of SEM images in Figure 6 highlights a well distinct platelet morphology, especially for C10-4.5 which shows the higher crystallinity (Figure 6b). For a shorter and longer treatment time, Figure 6a,c, the LDH nano-sheets seem to merge in a more refined and compact structure. The thickness of the different LDH layers was measured by SEM and was found to be 11.6 \(\pm\) 1.3 \(\upmu\)m, 5.1 \(\pm\)0.3 \(\upmu\)m, and 9.8 \(\pm\) 0.8 \(\upmu\)m for the C10-1, 4.5, and 9 samples, respectively. In accordance with the XRD pattern, cerium-rich agglomerates, probably Ce(OH)\({}_{3}\), are found on the surface of the LDH coatings, (Figure 7), regardless of the synthesis time. The EDS map collected over the agglomerate in Figure 7a highlight the presence of Ce. The appearance of the LDH coatings after 400 h of exposure to 0.1 M NaCl solution is reported in Figure 8a-c. Among the investigated LDH coatings, only the C10-4.5 structure still shows a platelet-like structure (Figure 8b), even if cracks are present over the surface. On the contrary, the LDH cannot be observed anymore in the samples C10-1 and C10-9 in Figure 8a,c seems that the LDH coating is completely dissolved. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**Lattice Parameter**} & \multicolumn{3}{c}{**Interlayer Distance**} & \multicolumn{2}{c}{
\begin{tabular}{c} **Crystallite** \\ **Size** \\ \end{tabular} } \\ \cline{2-7}  & \(a\) & \(c\) & \(\mathbf{d}_{003}\) & \(\mathbf{d}_{006}\) & \(\mathbf{d}_{009}\) & \(\mathbf{d}_{110}\) & \(\mathbf{D}\) \\ \hline C10-1 & 0.2662 & 2.0591 & 0.6831 & 0.3294 & 0.2217 & 0.1311 & 2.9835 \\ C10-4.5 & 0.2623 & 2.0261 & 0.6755 & 0.3271 & 0.2216 & 0.1312 & 5.5919 \\ C10-9 & 0.2629 & 2.0464 & 0.6830 & 0.3307 & 0.2226 & 0.1315 & 2.8406 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: XRD analysis results: cell parameters a and c and crystallite size D. All values are expressed in nm. Figure 6: SEM images of the top-view of the as synthesized samples: (a) C10-1, (b) C10-4.5, and (c) C10-9. By investigating the surface of the sample C10-4.5 (Figure 9) it is possible to observe different agglomerates: (i) platelet-like clusters whose main constituents are Al, Mg, and traces of zinc (Figure 9a); (ii) corrosion products consisting of Al hydroxides and alloying elements such as Fe, Mn, and Si (Figure 9b); and (iii) cerium-rich agglomerates (Figure 9c). The cerium-rich agglomerates are believed to be amorphous structures since no peaks corresponding to the crystalline form of Ce compounds were found in the XRD pattern after 400 h of immersion (Figure 5). We hypothesize that the crystalline structure such as \(\mathrm{Ce(OH)}_{3}\) or \(\mathrm{CeO}_{2}\) formed during the synthesis in the LDH structure dissolves during exposure to the NaCl solution, thus releasing cerium ions. Gradually, the LDH layer cracks, and the corrosion of the substrate occurs. At the cathodic site, oxygen reduction leads to the formation of an alkaline environment, which promotes the precipitation of amorphous Ce oxides/hydroxides. These compounds are recognized to reduce the cathodic reaction rate, providing mitigation of electrochemical activity of the metal and, thus, reducing the extent of corrosion. Figure 8: SEM images of the top-view of the investigated samples after 400 h of immersion in 0.1 M NaCl: (**a**) C10-1, (**b**) C10-4.5, and (**c**) C10-9. Figure 7: Ce-rich agglomerate on LDH surface (**a**) and the corresponding EDS analysis (**b**).