Coupled effect of substrate welding and MAO treatment on surface and corrosion behavior of ZM21 magnesium for orthopedic fixation devices

• A combined FSW–MAO strategy was developed to improve magnesium alloy performance for biodegradable implant applications. • Grain refinement from FSW promoted uniform discharge during MAO, leading to reduced pore size and increased coating thickness (∼35.7 µm). • Electrochemical tests showed significantly enhanced corrosion resistance of CW samples, with polarization resistance increasing over prolonged immersion. • Bioactivity was confirmed by dense apatite deposition with Ca/P ≈ 1.68 on CW surfaces, indicating strong osteoconductive potential. This study investigates the synergistic effects of friction stir welding (FSW) and micro arc oxidation (MAO) on the corrosion resistance and bioactivity of ZM21 magnesium alloy intended for temporary orthopedic implant applications. The alloy was subjected to FSW followed by MAO coating to study the coupled effect on surface and corrosion behavior of the weld joint. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) revealed that the MAO-coated weld sample exhibited the highest charge transfer resistance (22.98 kΩ) and the lowest corrosion current density (0.733 µA/cm²) at 1 h, indicating superior corrosion resistance. Surface characterization showed enhanced roughness (R a = 10.46 µm) and improved hydrophilicity (contact angle = 80°) in coated weld sample, which facilitated apatite nucleation in simulated body fluid. After 14 days, dense, globular Ca-P layers with a Ca/P ratio of 1.68 were observed on coating carried out on weld sample, confirming excellent in vitro bioactivity. The combined effect of FSW-induced grain refinement and MAO-derived bioactive surfaces enabled a multifunctional coating with controlled degradation and enhanced osteogenic potential. These results highlight the potential of the FSW–MAO strategy in advancing the performance of biodegradable magnesium-based orthopedic implants.