## 3 Results and Discussions  
### Surface Morphology and Roughness  
The effect of the manufacturing process on the sample surface was primarily evaluated by morphological studies. Figure 1 shows SEM images of the aluminum alloy samples prepared by the three roughening processes: Boiling (A_WS), HF/HCl etching (A_FS), and HNO3/HCl etching (A_NS). Clear modifications were observed on all the surfaces after the roughening treatment. However, the silane self-assembled monolayer did not affect the surface morphologies. Combined micro/nano-structures were formed after the boiling treatment and the two types of acidic etching. The different surfaces are characterized by high homogeneity thanks to the efficient and no localized pretreatment. However, different sorts of surface morphologies can be identified. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Code** & **Step 1** & **Step 2** \\ \hline A\_R & – & – \\ A\_RS & – & Silane \\ A\_W & Boiling water & – \\ A\_WS & Boiling water & Silane \\ A\_N & HNO3/HCl & – \\ A\_NS & HNO3/HCl & Silane \\ A\_F & HF/HCl & – \\ A\_FS & HF/HCl & Silane \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Samples details. Several flower-like petals with thicknesses of about 20-30 nm and a great deal of cavities with diameters around 100 nm appear at the boiled aluminum surface. The adjacent flower-like petals overlap and connect with each other, creating, during its growth, a micro/nano-structure. In fact, in the initial stage of the aluminum substrate immersion in boiling water, a reaction between the Al and H\({}_{2}\)O occurs:  
\[2\mathrm{Al}+(3+\chi)\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{Al}_{2} \mathrm{O}_{3}.\chi\mathrm{H}_{2}\mathrm{O}+3\mathrm{H}_{2}\uparrow \tag{1}\]  
Thus, Al at the surface of the AA6082 transforms into Al\({}_{2}\)O\({}_{3}\).\(\chi\)H\({}_{2}\)O. This latter reacts with H\({}_{2}\)O to generate an AlO(OH) crystal called bochmite, resulting in the flower-like rough structure [6; 12; 26; 27]. Moreover, the surface etched with HNO\({}_{3}\)/HCl solution appears as a multiple-scale structure with dimensions of 2 to 4 \(\upmu\)m (microstructure). Nanometric pits, with dimensions lower than 200 nm, are randomly nucleated on the aluminum surface of these scales [25]. This bimodal structure is formed thanks to the associated action of the two acids [28]. The vigorous reaction of the aluminum alloys with the hydrochloric acid probably induced the formation of the microscales following the crystallographic orientation. However, the nitric acid, which is an oxidizing acid that can have a passivation property, has low corrosive action on the aluminum alloy. Thus, it can participate only on the formation of nano-pits [29]. Figure 1: Low-magnification and high-magnification SEM images of aluminum alloy sample prepared by (**a**) boiling (A_WS), (**b**) HNO\({}_{3}\)/HCl etching (A_NS), and (**c**) HF/HCl etching (A_FS). On the other hand, the structure formed by the HF/HCl etching is totally different from the HNO\({}_{3}\)/HCl one. In fact, this etching induced the formation of large deep attack zones in the microscales and a large amount of clean-cut and rugged nano-ladder structures over the AA6082-T6 surface. The main reason for the deep attack zone's appearance is the presence of the several dislocation defects and the crystal boundaries in the aluminum alloy with a relatively greater energy in crystalline metals that are prone to damage when attacked by etchants [30; 31]. In particular, HF has a great effect in dissolving the silicon (Si) impurities distributed on the aluminum alloy surface [32]. In order to quantify the different surface morphologies in the surface structure, the roughness parameters: Ra, Rq, and Sku, were determined using a Mitutoyo profilometer. Ra is the arithmetic average of the absolute values of the profile heights, Rq is the root-mean-square average of the profile roughness, and Sku is a measure of the height distribution and provides information about the number of valleys and hills on the surface [33].