Pulse Frequency-Induced Structural Evolution and Corrosion Resistance Enhancement of MAO Coatings on AZ31B Magnesium Alloy

This study focuses on how modulating the electrical pulse frequency alters both the topographic features and corrosion resistance of ceramic coatings produced via micro-arc oxidation (MAO) on an AZ31B magnesium substrate. In a silicate-based electrolyte, MAO treatments were executed utilizing four distinct frequencies: 300, 500, 800, and 1000 Hz. Phase configurations, microstructural features, and surface patterns were thoroughly evaluated using X-ray diffraction (XRD), scanning electron microscopy (SEM) integrated with energy dispersive spectroscopy (EDS), and an ultra-depth-of-field microscope. Furthermore, the fabricated coatings underwent rigorous assessments for their porosity, contact angle, Vickers hardness, and electrochemical attributes. The outcomes demonstrate that the surface appearance, phase structure, defect state, and density of the MAO coatings are highly sensitive to variations in pulse frequency. Notably, although the coating prepared at 800 Hz exhibited elevated surface roughness (Ra), it achieved a highly consolidated microstructure and the lowest porosity level, underscoring that roughness alone is not a definitive quality indicator. This structural refinement was driven by the presence of well-crystallized Mg2SiO4 and MgO phases. This group recorded a peak inner barrier layer resistance (Rb) of 1.62 × 104 Ω·cm2 alongside a minimum corrosion current density (Icorr) of 3.38 × 10−7 A·cm−2, confirming its exceptional protective capacity. Consequently, the structural quality and corrosion resistance of MAO coatings can be strategically enhanced by tuning the electrical frequency, offering valuable engineering guidelines for utilizing AZ31B alloy parts under aggressive environmental conditions.