## 3 Results and Discussions
### SEM-EDS Sample Surface Analysis
#### 3.1.2 Surface Elemental Composition  
First, EDS area scans were performed on a raw Mg alloy in order to extract chemical elemental concentrations. The measured average values (Table 3) from five determinations correspond to an AZ63 Mg alloy [1].  
\begin{table}
\begin{tabular}{c c c} \hline
**Sample Code** & **Sq**\(\pm\) SD (\(\mu\)m) & **Sa**\(\pm\) **SD (\(\mu\)m)** \\ \hline AZ63\_5m & 5.11 \(\pm\) 2.08 & 4.28 \(\pm\) 2.04 \\ \hline AZ63\_10m & 5.24 \(\pm\) 0.36 & 4.17 \(\pm\) 0.29 \\ \hline AZ63\_20m & 6.67 \(\pm\) 1.29 & 5.23 \(\pm\) 1.03 \\ \hline \end{tabular}
\end{table}
Table 2: Surface roughness analysis.  
Figure 4: Example of a 3D SEM-BSE topographical analysis performed on sample AZ63_10m: the four BSE images acquired on the same surface by the four channels of the BSE detector (**a**); generated 2D surface height map (**b**); 3D surface rendering (**c**); topographic cross-section profile (**d**).  
Figure 5 shows the point scans used for identifying the chemical elemental composition corresponding to different surface morphological structures.  
Three EDS measurements for each selected structure from representative areas were used to analyze the chemical composition variation of the main elements present in the coatings. The mean values are presented in Table 4.  
The qualitative analysis of chemical composition in the measured point scans indicates the formation of magnesium and/or aluminum oxide compounds (a mixture of magnesium oxide, aluminum oxide and magnesium aluminate phases).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{3}{c}{**Element \(\pm\) SD (wt\%)**} \\ \hline
**Mg** & **Al** & **Zn** & **Mn** & **Si** \\ \hline
91.55 \(\pm\) 0.05 & 5.36 \(\pm\) 0.04 & 2.83 \(\pm\) 0.03 & 0.17 \(\pm\) 0.01 & 0.09 \(\pm\) 0.01 \\ \hline \hline \end{tabular}
\end{table}
Table 3: EDS area scan quantitative chemical elemental analysis of raw Mg alloy.  
Figure 5: Surface point-scans EDS analysis on “pancake-like” areas: P1—deep pore; P2—shallow pore; P3—re-solidified melt pool; P4—sintered particles for AZ63_5m (**a**), AZ63_10m (**b**) and AZ63_20m (**c**) samples.  
The cross-section SEM-EDS analysis show a structure formed by a barrier layer, local Al/Mg rich areas and sandwich structures of alternating Mg and Al increased concentration layers.  
Figure 6 indicates a complex, non-uniform structure of the PEO coatings, which evolves with the processing time. [PERSON] et al. [38] showed in their study that the concave regions, rich in aluminum (Figure 6a), appearing in the PEO coatings were the result of the coating formation above the constituent \(\beta\)-phase of the magnesium alloy. They assumed that the coating layers formed above the Mg\({}_{17}\)Al\({}_{12}\) phase were formed by lateral growth of the layers formed above the alpha phase. In Figure 5b,c, it can be seen that areas rich in aluminum are covered by areas rich in magnesium, which is why we believe that a similar growth mechanism is responsible for the formation of PEO coatings on the AZ63 magnesium alloy. The presence of closed cavities in the structure of PEO coatings could be attributed to a mechanism of plastic deformation of the constituent material under the influence of the gas evolution within the coating [32].  
Figure 7: Cross-section EDS line-scan analysis for sample AZ63_5m (**a**), AZ63_10m (**b**) and AZ63_20m (**c**).  
Both EDS line-scans and mapping data showed that, for all three investigated samples, there was a non-homogenous ceramic PEO coating with variable cross-section Mg/Al ratios.