The Effect of Ceria Particles Incorporation on Protective Properties of Oxide Layers Formed by PEO on Mg Alloy with LPSO-Phase

The progress in the magnesium alloy design technology has enabled outstanding characteristics to be achieved for alloys containing both the strengthening LPSO phase (long-period stacking-ordered phase, LPSO) and rare earth elements (Mg–Gd–X, Mg–Nd–X, Mg–Y–X, Mg–Sc–X, and other systems): increase the allowable continuous service temperatures to ~250–300°C, the tensile strength to ~930 MPa, the relative elongation to ~30%, and the hardness to ~190 HV. However, the phenomenon of microgalvanic corrosion arising from the potential difference between the LPSO phase and the α-Mg matrix at their interface, as well as insufficient surface hardness, are the main problems of these alloys, necessitating surface modification for most applications. In this study, the plasma electrolytic oxidation (PEO) technology for treatment of light alloys was modified by adding cerium dioxide (CeO2) particles to the electrolyte in order to improve the characteristics of oxide layers formed on a Mg–Y–Zn–Yb–Zr alloy with the LPSO phase. The additive concentration (1–5 g/L, with a 1 g/L increment) and the forming pulse frequency of the process current (250 and 1000 Hz) were varied. Consequently, the high-frequency (1000 Hz) PEO with the addition of 4 g/L СeO2 to the electrolyte allowed the average microhardness to be increased from ~350 to ~690 MPa, i.e., by ~2 times, and the adhesion strength to be improved by 40% relative to the basic variant. The incorporation of CeO2 had a negligible effect on the corrosion resistance of the layers; the most pronounced positive effect was observed at a PEO frequency of 1000 Hz and at a CeO2 particle concentration of 1 g/L: the polarization resistance of the oxide layer was ~8.2 MΩ cm2, the corrosion current density was ~3.8 nA/cm2, and the impedance modulus was ~4.2 MΩ cm2 in comparison with the respective characteristics of ~4.3 MΩ cm2, ~9.8 nA/cm2, and ~3.0 MΩ cm2 for the basic oxide layer, which is due to the inert (no chemical reactions involved) incorporation of cerium oxide particles into the layer, its compacted structure, and reduced porosity.