Use of NaAlO2 additions to extend the corrosion resistance of PEO layer on EV31 magnesium alloy

This study focuses on the evaluation of the influence of NaAlO2 content in the phosphate electrolyte on the properties of the plasma electrolytic oxidation coating formed on magnesium alloy EV31. The additions of NaAlO2 used were 4, 8, 12 and 16 g. The PEO coating was applied using a pulse regime with a frequency of 100 Hz, a duty cycle of 10 % and a current density of 0.05 A cm−2 for 10 min. The time to reach the initial dielectric breakdown was shortened by the addition of NaAlO2. The incorporation of NaAlO2 resulted in diminished porosity, reaching a minimum at an addition of 8 g/l NaAlO2. At the same time, the presence and extent of microcracks in the coating were enhanced. Based on elemental mapping and phase composition analysis by XRD, it can be concluded that the coatings without the addition of NaAlO2 are composed mainly of MgO and Mg3(PO4)2. After the addition of NaAlO2, the spinel phase MgAl2O4 was present in the coating, the ratio of which increased with increasing aluminate content. The highest polarisation resistance after 168 h was achieved by the samples with the addition of 4 g/l NaAlO2 after 10 min of PEO process time.