## 3 Results and Discussions  
### SEM-EDS Sample Surface Analysis  
#### 3.1.1 Morphology and Topography  
The surface morphology for the raw Mg alloy and PEO treated samples was observed by SEM in backscattered electrons mode at different magnifications (Figure 1a-d). Figure 1a presents a typical [18] Mg AZ series alloy surface SEM micrograph, with a majority darker \(\alpha\)-Mg matrix and \(\beta\)-Mg17 Al12 brighter areas. EDS data (not shown here) indicated that in the beta phase area, a rich Al compound, the mean ratio of Al and Mg (approx. 0.7) is 10 times bigger than average ratio for the entire surface of the alloy (approx. 0.06). Figure 1: SEM images obtained at different magnifications (100\(\times\) and 700\(\times\) in zoom box) for raw Mg alloy (**a**) and PEO treated samples: AZ63_5m (**b**), AZ63_10m (**c**) and AZ63_20m (**d**). For all the PEO-coated samples (Figure 1b-d), data showed typical PEO \"pancake-like\" surfaces with different porosity, radial surface cracks and areas with chemical elemental contrast. In addition, in zoom boxes from Figure 1b-d, some sintered particles (or nodules) at the border of the re-solidified melt pool [30] can be seen. Similar structures were reported by other groups dealing with PEO on titanium alloys [31;32]. A representative SEM micrograph in both BSE and SE mode is presented in Figure 2.  
The formation of \"pancake-like\" structures, as in Figure 2, indicates strong penetrating plasma discharges [33] and the dielectric properties of the ceramic PEO coating [5]. In Figure 2a, the main surface morphology characteristics are indicated as follows: 1--\"deep-pores\"; 2--\"shallow pores\"; 3--radial micro-cracks; 4--sintered particles on the coating surface; 5--central \"sink-hole\" of a completely closed pore; 6--the re-solidified melt pool [30]. This characteristic morphology, with deep and shallow pores, was also reported by [PERSON] et al. [34], who studied the surface morphology of PEO AA7075 aluminum alloy. Using ImageJ software for image processing and analysis on SEM images obtained at \(\times\)100 magnification for PEO treated samples, the average apparent surfaces porosity were obtained. A representative example of a used SEM image and its correspondent ImageJ pore identification is presented in Figure 3.  
Considering the SEM image resolution capabilities and PEO micro-discharge channels, pore size constrain was introduced to account only for pores with an area above 78.5 um\({}^{2}\), which is equivalent to a round pore with a size (diameter) above 10 um. No pore shape constrain was added. The data obtained for pore sizes from surface image analysis were quantitatively analyzed, the porosity for each of the three PEO treated samples being calculated as the ratio between total pores area versus total investigated area. The determination of the average porosity was carried out by using SEM (\(\times\)100 magnification) micrographs from five randomly chosen areas of each sample, and the obtained mean values are presented in Table 1.  
The mean porosity values presented in Table 1 show a decrease of the apparent surface porosity with increasing processing time, reaching a level of approx. 11% for the 20 min PEO surface treatment. Higher values (approx. 27%) were reported by [PERSON] et al. [35] for the PEO on AZ91D Mg alloy in NaAlO\({}_{2}\) without additives. A decrease in porosity with increasing processing time could be explained by the PEO coating formation mechanisms [5]: numerous softer plasma discharges in an early stage, followed by lesser, localized and more powerful events when the PEO coatings develop. Figure 2: SEM micrographs in BSE (**a**) and SE (**b**) mode of a representative area of the PEO coating surface for the sample AZ63_20m. Based on the Hitachi SU5000's capabilities and Hitachi Map 3D software, the mean height parameters of the PEO coatings were determined in order to perform a comparative analysis of the coatings' roughness evolution with processing time.