Effect of Current Pulse Frequency on Microstructure and Corrosion Behavior of the MAO/Ni-P Composite Coating on the AZ31B Mg Alloy

WEI Jiali; LIU Fei; YUAN Deyong; LIU Xiaohe; DONG Shuai

doi:10.16490/j.cnki.issn.1001-3660.2025.15.011

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Surface Technology ›› 2025, Vol. 54 ›› Issue (15) : 120-133. DOI: 10.16490/j.cnki.issn.1001-3660.2025.15.011

Effect of Current Pulse Frequency on Microstructure and Corrosion Behavior of the MAO/Ni-P Composite Coating on the AZ31B Mg Alloy

WEI Jiali¹, LIU Fei², YUAN Deyong², LIU Xiaohe³, DONG Shuai^{3, *}

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Abstract

Due to the low density, high specific strength and excellent electromagnetic shielding properties, magnesium alloys have shown broad application prospects in aerospace, automotive industry and biomedical fields. However, the poor corrosion resistance and surface hardness of magnesium alloys seriously restrict their long-term service performance in harsh environments. Microarc oxidation (MAO) technology, as a highly efficient surface modification method, can generate a dense ceramic oxide layer on the surface of magnesium alloys in situ, which significantly improves the corrosion and wear resistance of the substrate. However, the inherent porous structure of MAO coating may become a channel for corrosive media penetration, limiting its application in extreme environments. In recent years, composite surface technology has become a research focus to break through the performance bottleneck of the single MAO coating. Electroless nickel plating is widely used to fill the porous defects of the MAO coating because it can form a uniform, dense metal coating with excellent corrosion resistance. By combining MAO with nickel plating, it is expected to form a MAO/Ni-P composite coating, and the synergistic effect of the two can further improve the comprehensive performance of magnesium alloys. In order to investigate the effect of MAO coating with different current pulse frequencies on the electroless nickel plating, the experiment was carried out on 0.5 kHz, 10 kHz and 20 kHz MAO coatings respectively. Meanwhile, the microstructure, phase composition and corrosion resistance of the MAO coating and the MAO/Ni-P composite coating were investigated by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), electrochemical tests, etc. The experiment results demonstrated that the MAO coating effectively enhanced the corrosion resistance of the alloy. Notably, higher frequencies were found to reduce pore dimensions within the coating microstructure, which consequently led to improved anti-corrosion performance. The MAO coating at 20 kHz has the best corrosion resistance, with a corrosion current density of 3.51×10^-7 A/cm². After electroless nickel plating on the MAO coating, the corrosion resistance of the MAO/Ni-P composite coating is better than that of the single MAO coating. Particularly, the 0.5 kHz MAO/Ni-P composite coating exhibits optimized surface morphology characterized by larger and more uniformly distributed Ni-P particulates and increased coating thickness. This structural refinement enhances the electrochemical performance, achieving a minimal corrosion current density of 6.53×10^-8 A/cm² in 3.5wt.% NaCl solution, with corresponding elevation of impedance modulus (|Z|) observed through Nyquist plot analysis. Research has shown that without chemical pretreatment, the micropores in the MAO coating can provide good active sites for the deposition of Ni-P particles in direct electroless nickel plating. The electroless Ni-P coating deposited on the surface of the MAO coating is uniform and tightly aligned, and the intermediate MAO coating avoids the direct contact between the Ni-P plating solution and the anodic Mg substrate, which reduces the risk of corrosion of the alloy. In addition, the larger size of the micropores on the MAO coating and the faster deposition rate of the nickel-plating solution on the MAO coating lead to a better combination of the two and form a better coating morphology. Therefore, the results show that the MAO coating at 0.5 kHz has poor corrosion resistance, while the MAO/Ni-P composite coating has better morphology and the best corrosion resistance.

Key words

magnesium alloys / microarc oxidation / current pulse frequency / electroless nickel plating / MAO/Ni-P composite coating / corrosion resistance

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WEI Jiali, LIU Fei, YUAN Deyong, LIU Xiaohe, DONG Shuai. Effect of Current Pulse Frequency on Microstructure and Corrosion Behavior of the MAO/Ni-P Composite Coating on the AZ31B Mg Alloy[J]. Surface Technology. 2025, 54(15): 120-133 https://doi.org/10.16490/j.cnki.issn.1001-3660.2025.15.011

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