Preparation and characterization of micro-arc oxidation coatings on AZ91D Mg alloy fabricated in a TEOS electrolyte

Amorphous SiO 2 /MgO/MgSiO 3 coatings formed on AZ91D via MAO in TEOS electrolyte. • Due to its dense layer, this coating exhibits high corrosion resistance at 50 Hz. • Amorphous SiO 2 /MgO/MgSiO 3 coatings show optimal thermal control at 500 Hz. • Explains the growth mechanism of the coating. In this paper, AZ91D magnesium alloy is used as the substrate, and thermal control and corrosion resistant composite coatings are prepared by micro-arc oxidation technology in tetraethyl orthosilicate hydrolysis electrolyte at different pulse frequencies (50 Hz, 500 Hz, 1000 Hz, 2000 Hz). The effects of pulse frequency on the microstructure, corrosion resistance, and thermal control performance of the coatings are systematically studied. The results show that the coatings are mainly composed of SiO 2 , MgO, and MgSiO 3 . As the frequency decreases, the content of amorphous SiO 2 in the coating increases, and both the thickness and roughness increase significantly. Electrochemical tests show that the coating prepared at 50 Hz exhibits the optimal corrosion resistance, with a corrosion current density as low as 1.017 × 10⁻⁸ A/cm², and its dense inner layer resistance increases by 2.8 orders of magnitude compared with that of the coating prepared at 2000 Hz. In terms of thermal control performance, the coating at 500 Hz shows the best comprehensive thermal control performance, with an emissivity of 0.89 and an absorptivity as low as 0.30, which is attributed to the moderate thickness (about 143 μm) and roughness, the dense structure, and the intrinsically low absorptivity and high emissivity characteristics of amorphous SiO 2 . In this study, SiO 2 /MgO/MgSiO 3 composite coatings are prepared on the Mg substrate surface, which provides a new idea for the preparation of multifunctional coatings.