## 3 Results and Discussion
### Fabrication and Characterization of AAM
0.4 M & & & & & & & & & & & \\ H\({}_{3}\)PO\({}_{4}\) & 51.68 & 2.63 & 6.53 & 55.17 & 2.32 & 6.53 & 65.1 & 2.04 & 6.53 & 103.66 & 1.67 & 6.53 \\ Al(OH)\({}_{3}\) & & & & & & & & & & & \\ \hline
0.4 M & & & & & & & & & & & & \\ H\({}_{3}\)PO\({}_{4}\) & 4.04 M & & & & & & & & & & & \\ Al(OH)\({}_{3}\) & & & & & & & & & & & & & \\ \hline
0.4 M & & & & & & & & & & & & \\ H\({}_{3}\)PO\({}_{4}\) & 4.01 M & & & & & & & & & & & \\ Al(OH)\({}_{3}\) & & & & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Morphological features of PAA dependent on the electrolyte composition and passed electrical charge for aluminum anodization at 160 V and 0 \({}^{\circ}\)C.  
Figure 4: FESEM images of PAA formed at 160 V and 0 \({}^{\circ}\)C in 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.095 M Al(OH)\({}_{3}\) after passing 20 C cm\({}^{-2}\): (**a**) inner and (**b**) outer surface. A strict coherence with the curves of Figure 5 can be found in Figure 6, which shows the FESEM images of sample anodized at 0.13 M Al(OH)\({}_{3}\), corresponding to the most external point of the curves of Figure 5. In detail, Figure 6 shows the outer surface of PAA formed at 160 V and 0 \({}^{\circ}\)C in 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.13 M Al(OH)\({}_{3}\) after passing 20 (a), 40 (b), 80 (c), and 160 C cm\({}^{-2}\) (d). It can be observed that more compact layers were formed with passed charge as the concentration of Al(OH)\({}_{3}\) was increased. Besides, at 20 C cm\({}^{-2}\) (Figure 6a), the surface shows irregular shape pore distributed on the surface, while, at 160 C cm\({}^{-2}\) (Figure 6d), the surface morphology appears more structured with an enlarged pore entrance with a more regular shape. Figure 6d shows also the presence of micro-cavity likely due to local oxide dissolution promoting the onset of new pores. Since the increase in charge densities are obtained through longer anodizing times, i.e., longer times in solution, the obtained PAA morphologies can be explained in light of this parameter by considering anodic oxide dissolution during anodizing. Without any attention to argue about the various mechanisms proposed for PAA formation, and above cited, the findings appear consistent with the occurrence of oxide dissolution during anodizing. Otherwise, it should be impossible to explain the curves of Figure 2a whose difference gives just the oxide dissolution extent. The major obstacle in invoking oxide dissolution under anodizing is the flow model of formation of PAA that excludes dissolution [7, 81]. However, the successive in-depth investigation showed the possible coexistence of oxide dissolution under a high electrical field with the flow model, because oxide dissolution has been attributed to field-induced instability, while mechanical instability has been invoked for the oxide flow [8]. On this basis, it is possible to elucidate tentatively the influence on PAA morphology of Al(OH)\({}_{3}\) at constant charge density, and of charge density at constant Al(OH)\({}_{3}\) concentration. In the first case, the addition of Al(OH)\({}_{3}\) to phosphoric acid determines the growth of a more compact oxide because the oxide dissolution during anodizing is progressively inhibited, owing to the ion common effect. Therefore, pore size decreases because the growth of a more compact oxide determines the shrinkage of the channels due to the wall thickening. Such a decrease is more significant at 160 C cm\({}^{-2}\) owing to the longer anodizing time, as shown in Figure 5a. In addition, the practical pore mean size independence of charge passed at 0.13 M Al(OH)\({}_{3}\) is likely due to the close of this concentration value to the saturation one (0.138), where the oxide dissolution is hardly inhibited, independently of anodizing time. According to oxide dissolution over anodizing, it is possible to explain also the variations of pore size and population with the passed charge at constant Al(OH)\({}_{3}\).