## 3 Results and Discussion
### Afm  
Figure 8 displays the tri-dimensional (Figure 8a) and the bi-dimensional (Figure 8b) AFM images of a bare Mg disk, scanned over an area of (5 \(\upmu\)m \(\times\) 5 \(\upmu\)m), presented in so-called \"enhanced contrast\", together with a typical profile-line (Figure 8c) that is a plot of a line scan collected along the scanning direction, marked to guide the eye by the red line in Figure 8b. The bare disk exhibits a succession of folds and valleys, arranged preferentially along one direction and probably formed during the cutting of the disks. However, the surface is flat, as can be observed from the line scan in Figure 8c, which shows that the surface features are located in a vertical range of \(\sim\)80 nm (see the vertical scale of the surface profile, from \(-\)40 to +40 nm). The root mean square (RMS) roughness of the image presented in Figure 8b is 18.5 nm, while the average roughness value equals 14.1 nm. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **C (at.\%)** & **O (at.\%)** & **Ca (at.\%)** & **Mg (at.\%)** \\ \hline Bare Mg disk & \textless{}0.3 & \textless{}0.3 & - & 99.7 \\ Carbonated disk & 23 & 58 & 18 & \textless{}1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: EDS compositional analysis after immersion treatment for 1 h.  
Figure 8: Tri-dimensional (**a**) and bi-dimensional (**b**) AFM images and line profile (**c**) of bare Mg. Figure 9 shows AFM images of the Mg disk after immersion for 15 min in carbonated solution at (8 \(\upmu\)m \(\times\) 8 \(\upmu\)m) scale. Figure 9a,b display a compact layer of ACC resulting from carbonatation, forming the massive agglomeration of material that covers the Mg surface and a top layer of rounded protruding aggregates of a few hundred nm in diameter, such as the particle marked between the two red arrows in Figure 9c (~190 nm diameter). The layer shown in the 3D AFM image (Figure 9a) has increased values of roughness (Table 2), for example, Rq = 328.7 nm, while the peak-to-valley parameter, on the whole area, is Rpv ~1.93 \(\upmu\)m, estimated based on the 2D AFM image in Figure 9b, therefore about 25 times higher than that of the bare Mg surface (Figure 8). AFM images of the Mg disk after 1h immersion (Figure 10), scanned over an area of (8 \(\upmu\)m \(\times\) 8 \(\upmu\)m), present a completely different morphology, consisting of stacked faceted crystallites of hundreds of nm overlapped in the form of steps and terraces (stairs-like). As suggested by the 3D image from Figure 10a and the linescan from Figure 10c, the spatial arrangement of the faceted crystallites leads to a stepped surface, as proved by the increased values of the roughness parameters: Rq = 509.6 nm, and Rpv = 3.69 \(\upmu\)m.  
Further on, for an immersion time of 1 day, the AFM images in Figure 11, also scanned over an area of (8 \(\upmu\)m \(\times\) 8 \(\upmu\)m), show much larger faceted crystals, with dimensions of a few microns (see Figure 11a,b). The coating preserves the compact morphology, but a finer surface texture is suggested by the aspect of some exposed crystal facets. However, at the (8 \(\upmu\)m \(\times\) 8 \(\upmu\)m) scale, the roughness parameters are similar to the values obtained from the sample immersed for 1 h in Figure 10, as follows: Rq = 478.1 nm, and Rpv = 3.21 \(\upmu\)m (Figure 11c). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **0** & **15 min** & **1 h** & **1 day** \\ \hline Rq & 18.5 nm & 328.7 nm & 509.6 nm & 478.1 nm \\ Ra & 14.1 nm & 245.9 nm & 403.6 nm & 387.8 nm \\ Rpv & 152.9 nm & 1929 nm & 3695 nm & 3219 nm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Roughness parameters (Rq, Ra, Rpv) evolution with immersion time. Figure 9: Tri-dimensional (**a**), bi-dimensional (**b**) AFM images taken on areas of 8 \(\upmu\)m \(\times\) 8 \(\upmu\)m and AFM line profile (**c**) of Mg after immersion for 15 min, along the scanning direction marked to guide the eye by the red line in (**b**). Figure 12 shows the measured contact angles of the Mg disks before and after the carbonate coating treatment.