Metal-modified micro-arc oxidation (MAO) coatings on magnesium alloys demonstrate exceptional stability in space environments and have been extensively utilized in aerospace electronic components. The properties of MAO-derived thermal control coatings are critically dependent on element doping levels, phase composition, and microstructural morphology of the coatings. Investigating the effects of varying MAO duration on the growth behavior of thermal control coatings on magnesium alloy surfaces offers theoretical foundations for optimizing process parameters and facilitating engineering applications.
In this work, the micro-arc oxidation (MAO) coatings on ZK61M magnesium alloy are fabricated in a sodium phosphate solution system containing ferric citrate under varying oxidation duration (5, 15, 25, and 30 min). This study systematically investigates the coating status, thermal control properties, and corrosion resistance of these MAO coatings, providing crucial insights for optimizing process parameters in practical applications. Coating thickness and roughness are analyzed with a roughness tester and an eddy current thickness gauge. Microstructure, elemental distribution, and phase composition are characterized via scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD). Solar absorptance (αs) and infrared hemispherical emissivity (εH) are evaluated by spectrophotometry and radiometry, while corrosion resistance is assessed through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS).
The results show that prolonged oxidation progressively darkens the coating color from light gray to deep black. Coating thickness increases from 10 μm (initial stage) to 45 μm (30 min), with surface roughness rising from 1.0 μm to 2.7 μm. The oxidation voltage rapidly ascends to 500 V during the initial phase, promoting molten oxide generation and deposition. Subsequent voltage stabilization combined with continuous electrolyte temperature elevation reduce molten material cooling rates, leading to coarse crystallized particles and increase surface roughness. SEM reveals high porosity with uniform pore distribution during early oxidation, while prolonged oxidation induces molten oxide stacking and Fe3O4 enrichment, elevating internal stress and crack width. The coating contains a significant number of closed pores within its structure, with pore sizes ranging from less than 1 μm to over 2.5 μm, and locally elongated pores exceeding 10 μm in length are observed. These features are primarily attributed to voltage variations between the initial and final stages of oxidation, as well as the coverage of molten oxide. XRD confirms MgO, Mg3(PO4)2, and Fe3O4 as primary phases, with Fe exhibiting gradient enrichment from the substrate to the surface. Although surface Fe content remains constant across different duration, Fe3O4 concentration varies as evidenced by XRD peak intensities.
Thermal control performance improves significantly: αs increases from 80% (5 min) to 93% (30 min), and εH rises from 0.78 to 0.85. These enhancements are attributed to microprotrusions, open pores, increased Fe3O4 content (enhancing blackness), and sufficient thickness (>30 μm) to suppress substrate interference. Electrochemical tests demonstrate optimal corrosion resistance at 5 min (Jcorr=0.18 μA/cm², |Z|=431 kΩ·cm2), two orders of magnitude lower than bare substrate (17.7 μA/cm2). However, prolonged oxidation degrades corrosion resistance (Jcorr=0.23 μA/cm2, |Z|=281 kΩ·cm2 at 30 min) due to enlarged pores and microcrack propagation.
In conclusion, the synergistic regulation of oxidation voltage, electrolyte temperature, and Fe doping governs coating thickness, roughness, and coloration. Thermal control properties positively correlate with Fe3O4 concentration, surface roughness, and thickness. However, accumulated internal stress and interfacial stress concentration at Fe3O4/MgO heterophases exacerbate microcracking, compromising corrosion resistance during extended oxidation. These findings provide critical insights for optimizing MAO parameters and advancing engineering applications of magnesium alloy thermal control coatings.
Key words
magnesium alloy /
micro-arc oxidation /
oxidation duration /
microstructure /
thermal control /
corrosion resistance
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