Effects of processing parameters on the adhesion and corrosion resistance of oxide coatings grown by plasma electrolytic oxidation on AZ31 magnesium alloys

In this work, the plasma electrolytic oxidation (PEO) process using a pulsed bipolar direct current (DC) power supply and sodium aluminate-based electrolyte was employed to grow the oxide layer on the AZ31 Mg alloy. For achieving a higher adhesion or a better corrosion resistance of oxide layer, the Taguchi L9 orthogonal array was adopted to determine the optimal PEO parameters, including anode on-time, anode current setting, cathode current setting, and cathode on-time. According to the signal-to-noise ratio and analysis of variance, the anode on-time was the most important control factor for improving adhesion and corrosion resistance properties of oxide coatings. The surface morphologies of all oxide coatings were typical crater-like microstructures. The oxide layer thickness can be increased by longer anode on-time and higher anode current. The hardness of the oxide layer was between 12.7 and 17.7 GPa due to the formation of MgAl2O4 phase. Through the confirmation experiments using the optimized PEO processing parameters, the maximum adhesion critical load of 41.8 N can be obtained. On the other hand, the highest polarization resistance of 6.68 × 103 kω˙cm2 was achieved, which was 6485 times better than the bare AZ31 substrate due to its compact microstructure, and a dense and continuous inner barrier layer at the coating/substrate interface.