In order to increase the corrosion resistance of magnesium alloy AZ91 in corrosion environments containing chlorides, the alloy surface has been modified by plasma electrolytic oxidation (PEO). The chemical composition of electrolyte in the PEO process consisted of 12 g/L Na3PO4·12 H2O and 1 g/L KOH, and a direct current was applied to the sample. The corrosion resistance of PEO coating and as-cast AZ91 (sample without PEO coating) was assessed using two different electrochemical methods: electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation (PDP) in 0.1 M NaCl at laboratory temperature. In addition to the electrochemical methods, the morphology of the oxidic coating was observed in the cross-sectional and top surface view by using the SEM technique. For better determination of the microstructure and PEO coating, chemical composition EDX analysis was used. The results of the experiments show that the formation of the PEO coating on AZ91 alloy has a more positive effect on the corrosion resistance in 0.1 M NaCl based on electrochemical methods than in the case of the formed coating on AZ31 alloy from the previous study. Based on electrochemical measurements in the selected environment, the formation of PEO coating on AZ91 was accompanied by a significant increase in polarisation resistance after short-term exposure compared to the as-cast surface. The EIS results showed a 73 times higher Rp value for PEO coated AZ91 when compared to the as-cast AZ91. Correspondingly, a 27 times lower icorr value was observed for PEO coated AZ91 than in the case of substrate AZ91 in 0.1 M NaCl. At the same time, the typically porous and inhomogeneous structure of the formed PEO coating on the magnesium alloy AZ91 was demonstrated.

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This paper is focused on evaluating the surface properties of protective coating prepared by plasma electrolytic oxidation on magnesium alloy EV31. Plasma electrolytic oxidation was performed in DC mode with a constant current density of 0.05 A cm−2 for 14 min. Evolution of voltage was measured and depicted graphically in order to describe the growth of the plasma electrolytic oxidation coating. Morphology and thickness of the plasma electrolytic oxidation coating were examined together with corrosion resistance of the coating by measurements of electrochemical characteristics using electrochemical impedance spectroscopy and potentiodynamic polarization after 1 h in 0.1 M NaCl. Results from potentiodynamic polarization showed decrease in corrosion current density i corr from 10.50 μA cm−2 (substrate) to 1.02 μA cm−2 (plasma electrolytic oxidation coated) and results from electrochemical impedance spectroscopy showed increase in R p by one order of magnitude for plasma electrolytic oxidation coated sample after both 1 h and 168 h exposure times.