## 3 Results and Discussion
### SEM and EDS  
The morphological evolution during the carbonatation treatment was investigated by SEM. Figure 6 shows micrographs from the samples immersed for 5 min (Figure 6a), 15 min (Figure 6b) and 30 min (Figure 6c,d). The sample immersed for 5 m exhibits the formation of a layer of nanoparticles covering the disk surface. These nanoparticles have typical sizes in the 20-30 nm, shown in the higher resolution image inside Figure 6a. The nanoparticle size may be correlated with an apparent equilibrium size for disordered nuclei grown in solution that are deposited on Mg in the first stage of carbonatation coating. The sample immersed for 15 min (Figure 6b) exhibits the formation of a second and more porous layer of nanoparticle aggregates. The aggregates tend to grow significantly into slightly elongated structures, sized in the hundreds of nms (inset in Figure 6b), leaving small areas of the inner nanoparticle layer still exposed, such as the darker gray area marked with an arrow in Figure 6b. This stage of carbonatation can be correlated with the observation by vibrational spectroscopy of the presence of ACC in the samples immersed for 15 min. The observed nanoparticles correspond with the formation of disordered phases. Figure 6c,d shows micrographs at different magnifications from the samples immersed for 30 min, taken in the top view from the center of the disks. Mg samples immersed for 30 min present rounded secondary aggregates, sized around 500 nm, which cover the whole disk surface and tend to coalesce into a dense layer. The aggregates are composed of much smaller elements, and the constituent primary nanoparticles, with sizes below 50 nm, can be identified at higher magnification images (inset in Figure 6d). At this stage, no faceted crystals can be observed, only short-range order phases. According to Raman and FT-IR, the observed nanoparticle aggregates correspond to amorphous carbonates. Figure 5: Raman spectra showing the evolution for immersion times in carbonated water of 15 min, 30 min and 1h and details of the spectral regions of the Eg modes of calcite at 1300–1800 cm\({}^{-1}\) and of OH stretching at 3500–3800 cm\({}^{-1}\). Figure 7 shows the surface morphology after carbonate coating for 1 h. Faceted calcite crystals, sized up to a few microns, can be observed forming aggregates of intertwined crystals. The surface of the crystals exhibits a patched'mosaic-like' surface texture instead of being completely flat. The observed texture can be attributed to the growth of calcite crystals by the incorporation of amorphous carbonate nanoparticles from the solution on the preexisting crystal. A growth mechanism described previously as 'crystallization by oriented nanoparticle attachment' [39; 40]. Table 1 displays the results of the elemental compositional analysis for bare Mg disks after immersion in carbonated water. Figure 6: SEM micrographs showing the evolution with carbonatation time of Mg disks after (**a**) 5 min, (**b**) 15 min, and (**c,d**) 30 min immersion times. Figure 7: SEM micrographs at different magnifications of Mg disks after (**a,b**) 1 h and (**c,d**) 1 -week immersion, showing rhombohedral calcite crystals (**a,b**) with a nanocrystalline mosaic pattern visible on the facets of growing calcite crystals (inset in (**b**)) and colonies of aragonite needle-like crystallites. Lateral growth is induced by the preferential nucleation and precipitation-deposition of amorphous carbonates at the areas of the Mg surface that are still exposed to oxidation. Crystallization and crystal growth thus permit the lateral growth and the coalescence of the calcite crystals layer after about 1 h immersion. It is worth mentioning that the formation of brucite -magnesium hydroxide- crystals are not observed by any microstructural analysis during carbonatation. However, hydroxide is the natural corrosion product of Mg in an aqueous solution.