The Mechanical and Corrosion Properties of Ceria-Reinforced PEO Coatings on Mg–RE Alloy

Abstract

Limitations in the possibilities of using such interesting materials in terms of the ratio of density and volumetric mechanical characteristics as magnesium alloys with the addition of rare earth elements (Mg–RE), associated with the low corrosion properties of their surface, impose the need to search for and improve methods for its improvement. Of particular importance is the elimination of the phenomenon of microgalvanic corrosion that occurs at the interfaces between the phases of the alloy matrix (a-Mg) and secondary strengthening phases. The mechanical properties of the Mg–RE surface are also insufficient for work under contact conditions. In this research, the method of electrochemical formation of conversion protective ceramic layers—plasma electrolytic oxidation (PEO)—was modified by the addition of cerium dioxide particles CeO₂ into the electrolyte at two modes (medium (250 Hz) and high (1000 Hz)) of technological pulse frequencies. It has been established that the mechanism of incorporation of dispersed particles of cerium dioxide consists of both electrophoretic incorporation of particles into the forming coating and thermokinetic inert incorporation as a result of impact interaction of particles and the oxide layer, and in both cases, preservation of CeO₂ particles in its original state occurs. Modification of coatings with CeO₂ particles allowed increasing resistance to chipping by up to 50 times and reduce the chip area during indentation from ~0.5 up to ~0.01 mm², and the use of high-frequency PEO mode, in general, improves resistance to chipping by ~2–3 times, regardless of the concentration of CeO₂ particles in the electrolyte compared to the mid-frequency. The CeO₂ particles had greatest influence on the wear resistance of the oxide coating—high-frequency PEO with the addition of CeO₂ allowed reducing the values of specific wear to ~0.05–0.1 µm/(N m) at high frequency PEO versus with ~0.3 µm/(N m) for the base coating obtained in mid-frequency PEO. Without the addition of particles, high-frequency PEO allowed reducing the wear of the base coating in the experiment by ~1.5 times compared to the medium-frequency one, which indicates the synergy of the interaction of the CeO₂ additive and high-frequency PEO. Studies of the corrosion properties of coatings have shown that CeO₂ particles slightly reduced the original characteristics, however, with prolonged exposure to a corrosive medium, some restoration of the corrosion properties of the coatings and the formation of a new corrosion-resistant phase in them—Mg_0.667Al_0.333(OH)₂(CO₃)_0.167(H₂O)_0.5 (hydrotalcite)—is observed.