## 3 Results and Discussions
### Potentiodynamic Polarization Tests  
Potentiodynamic polarization measurements were used to estimate the corrosion behavior of the AZ63 Mg alloy uncoated and PEO-coated samples in 3.5% NaCl solution. A three-electrode conventional electrochemical cell connected to a Parstat 4000 potentiostat system was employed for potentiodynamic polarization tests at a constant 30 \({}^{\circ}\)C temperature. After 30 min stabilization, the potentiodynamic polarization curves were measured from la \(-\)0.25 V vs. open circuit potential to +1.5 V vs. open circuit potential at a scanning speed of 1 mV/s. The obtained polarization curves (corrosion current density as a function of measured potential vs. saturated calomel electrode--SCE) are shown in Figure 11.  
The calculated corrosion parameters--constants anodic Tafel slope \(\beta\)a and cathodic Tafel slope \(\beta\)c--were obtained by Tafel extrapolation from the polarization curves; corrosion potential (Ecorr), corrosion current density (i\({}_{\text{corr}}\)) and corrosion rate (V\({}_{\text{corr}}\)), are presented in Table 8. \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **HV/0.3 (GPA)** \\ \hline AZ63 & 0.88 \(\pm\) 0.09 \\ \hline AZ63\_5m & 2.04 \(\pm\) 0.09 \\ \hline AZ63\_10m & 3.42 \(\pm\) 0.17 \\ \hline AZ63\_20m & 4.44 \(\pm\) 0.12 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Coating Vickers micro-hardness. The cathodic reaction (2H\({}^{+}\) + 2e = H\({}_{2}\)) presents bigger values in all cases in comparison with metal corrosion, typically for Mg alloys [4]. Total cathodic current corresponds to the cumulating of the currents for both hydrogen (H) and oxygen (O) reduction reactions and has to be balanced by the single anodic reaction current. Depending on the level of electrolyte agitation, the magnitude of the limiting current for the oxygen reduction will vary. The Ecorr of the PEO coating was lower than that of the Mg alloy, especially for the AZ63_10m sample. For the thinner layer obtained in the first conditions (AZ63_5m), the Ecorr value was near the base material and this fact can be associated with the micro-pores' presence on the PEO coating surface. The electrolyte could penetrate into the inner PEO layer through the micro-pores and cause the corrosion starting and occurring [4,44]. Increasing the layer thickness improves the corrosion resistance behavior of the material by sealing part of the pores, and the contact between the base material and saline solution is reduced much more. Values of icorr generally are connected to a possible corrosion rate of the material. All coated samples presented an improvement in corrosion current on the surface with a decrease of 2 order of magnitude. The results confirmed an improvement of corrosion resistance of the material through PEO coatings. Corrosion resistance increased with an increasing cathodic current density, which may be attributed to low porosity and more compactness of the coatings produced [17,45]. In general, the overall protective effectiveness of PEO coatings is determined by [14]:  
* Coating thickness, crystalline phase composition, roughness, level of porosity and defects for mechanical properties;
* Coating thickness, structure and chemical composition, level of porosity and the integrity of the coating/substrate barrier layer for anti-corrosion properties. Figure 11: Polarization curves for AZ63 magnesium alloy in 3.5% NaCl: AZ63 alloy, 5 min PEO processed (AZ63_5m), 10 min PEO processed (AZ63_10m) and 20 min PEO processed (AZ63_20m).