## 3 Results and Discussions
### Potentiodynamic Polarization Tests
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & \begin{tabular}{c} \(\beta\)**a** \\ **(mV)** \\ \end{tabular} & \begin{tabular}{c} \(\beta\)**c** \\ **(mV)** \\ \end{tabular} & \begin{tabular}{c} \(\textbf{E}_{\text{corr}}\) \\ **(V vs. SCE)** \\ \end{tabular} & \
\begin{tabular}{c} \(\textbf{i}_{\text{corr}}\) **(A/cm\({}^{2}\))** \\ \end{tabular} &
\begin{tabular}{c} \(\textbf{V}_{\text{corr}}\) \\ **(mmpy)** \\ \end{tabular} \\ \hline AZ63 & 20.6 & 222.7 & \(-1.49\) & \(51.0\times 10^{-5}\) & \(11.1\) \\ \hline AZ63\_5m & 77.85 & 404.8 & \(-1.48\) & \(15.6\times 10^{-5}\) & \(4.0\) \\ \hline AZ63\_10m & 30.01 & 647.5 & \(-1.35\) & \(2.91\times 10^{-5}\) & \(2.4\) \\ \hline AZ63\_20m & 37.56 & 419.04 & \(-1.39\) & \(0.83\times 10^{-5}\) & \(0.8\) \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Calculated corrosion parameters. In addition, an important key factor is the structure and chemical composition of the substrate material, in our case the AZ63 Mg alloy, which has a huge influence on the nature of the PEO coating. [PERSON] et al. [26] reported a strong relationship between the protective properties of the PEO coatings (obtained on binary and ternary magnesium alloys in a mixture of sodium silicate, sodium aluminate and potassium hydroxide electrolyte) and the content of aluminum and zinc in the substrate material. They showed that a high content of aluminum promotes the formation of the MgAl\({}_{2}\)O\({}_{4}\) phase, while the role of the Zn alloying element is to increase the coating formation rate. A high ratio of aluminum-related crystalline phase, which corresponds to the formation of the coating from the substrate's \(\beta\)-phase, led to more defective coatings, hence the lower protective properties. As processing time increases, the \(\alpha\)-Mg phase plays a predominant role in coating formation, promoting the increase in MgAl\({}_{2}\)O\({}_{4}\) content. The reduction of the macro-scale defects and level of porosity, along with the increase in overall and barrier layer thicknesses of the PEO samples with longer processing time, are responsible for better corrosion protection, since it will be harder for corrosive species to reach the substrate. Better corrosion protective properties with increasing processing time could be attributed also to the increase of the MgAl\({}_{2}\)O\({}_{4}\) crystalline phase ratio in the coating with a longer processing time, as shown by XRD analysis. It has been shown [14] that an increase in spinel volume fraction is beneficial for corrosion protective properties, being more stable. The higher content of the spinel phase with a longer PEO duration, along with the increasing thickness of the coating and reduced porosity, could also be responsible for the increase in micro-hardness. The MgAl\({}_{2}\)O\({}_{4}\) crystalline phase is much harder than MgO [40]. Further research must be carried out to determine the influence of the aluminum-related crystalline phase on the protective properties of obtained PEO coatings. More detailed mechanical properties investigations, such as wear and adherence, are also future targeted in order to test the quality of the coatings.