Influence of Preparation Parameters on Thermal Control and Corrosion Behavior of ZnO Nanoparticle-Incorporated MAO Coatings on Magnesium Alloy

Abstract

In this study, ZnO nanoparticle-incorporated micro-arc oxidation (MAO) coatings were prepared on AZ31 magnesium alloy in a sodium silicate-based electrolyte. The effects of applied current density (3–6 A·dm−2), oxidation time (3–15 min), ZnO concentration (4–10 g·L−1), pulse frequency (400–700 Hz), and duty cycle (10–40%) on the morphology, phase composition, thickness, roughness, thermal control properties, and corrosion resistance were systematically investigated. The results show that ZnO particles are successfully incorporated into the MAO coatings through electrophoretic migration and discharge-assisted sintering, as confirmed by EDS and XRD. Increasing current density and oxidation time increase thickness and roughness and enhance both solar absorptivity and infrared emissivity, but excessive values introduce microcracks and degrade corrosion resistance. Appropriate ZnO concentration (8 g·L−1) improves thermal control performance, while excessive concentration leads to agglomeration and defects. Pulse parameters show relatively minor effects on coating properties. Electrochemical tests reveal that coatings prepared at moderate conditions (4 A·dm−2, 10 min, 8 g·L−1 ZnO) exhibit the lowest corrosion current density. After 28 days of immersion in 3.5 wt% NaCl solution, corrosion products deposit on the surface, slightly increasing thickness, decreasing roughness, maintaining absorptance, and significantly enhancing emissivity.