The ceramic coatings on the 22 vol.% Al 18 B 4 O 33 w/AZ91 Mg metal matrix composite (MMC) were fabricated by plasma electrolytic oxidation (PEO) technique in silicate electrolyte. The optical emission spectroscopy (OES) in the plasma discharge process was collected to evaluate the evolution of plasma parameters with oxidation time. The microstructure, compositions and phase constituents of the PEO coatings were characterized. The electrochemical corrosion behavior of the PEO coatings in 3.5 wt.% NaCl was evaluated. In addition, the fabrication and properties of PEO coatings on AZ91 Mg alloy were also compared. It is found that the electron temperature for both AZ91 alloy and Al 18 B 4 O 33 w/AZ91 composite is about 3500 K in the plasma discharge channels. The Al 18 B 4 O 33 whisker reinforcement is firstly molten and then react with MgO to form MgAl 2 O 4 spinel phase under the local high temperature, meanwhile, some Al 18 B 4 O 33 whiskers as well as Mg matrix are also dissolved into the electrolyte solution. The B spectral line in OES, high concentration of B in solution and the formation of MgAl 2 O 4 spinel phase provide direct evidences that the reinforcement takes part in the chemical reactions in PEO process. PEO treatment significantly improves the corrosion resistance of AZ91 alloy and Al 18 B 4 O 33 w/AZ91 MMC, and their pitting corrosion sensitivity is also greatly suppressed. The corrosion current density of PEO-coated AZ91 alloy and PEO-coated Al 18 B 4 O 33 w/AZ91 MMC are -4.48×10 -9 A·cm -2 and -3.73×10 -7 A·cm -2 respectively, which is about two orders of magnitude lower than that of uncoated substrate. Meanwhile, the low-frequency impedance (|Z| 0.01Hz ) of AZ91 alloy and Al 18 B 4 O 33 w/AZ91 MMC also increases two orders of magnitude after PEO treatment.
Liu et al. (2026) studied this question.