Ultrafine grain stabilization and corrosion studies on supersonic cold-sprayed Al-5Si/Al2O3 coating via thermal annealing

In order to enhance the corrosion protection of magnesium alloys , a novel eutectic Al-5Si/Al 2 O 3 composite coating was applied to an AZ31 substrate by supersonic cold-spray technique, followed by annealing at 400 °C. The as sprayed and thermally treated coatings were subjected to systematic characterization for microstructural, mechanical and corrosion analyses. The results demonstrated that thermal input led to pronounced increase in the fraction of low-angle grain boundaries (LAGB), from 54 % to 70 %, while the average grain size value remained in the ultrafine regime (<1 µm). Although the thermal treatment rendered hardness and porosity slightly changed, a substantial enhancement in corrosion resistance was observed, with the protection efficiency increasing from 40 % to 53 % and corrosion current density ( I corr ) decreasing by one order of magnitude. This significant improvement in corrosion resistance primarily originated from substantial increase in LAGB length fraction, decrease in grain boundary energy, as revealed by electron backscatter diffraction (EBSD), a reduction in porosity due to annealing, and the formation of Al 2 O 3 , Al(OH) 3 , SiO 2 , Al 2 SiO 5 , and albite on the surface, as confirmed by X-ray diffraction (XRD). These findings underscore the influence of annealing in optimizing both the microstructural features and corrosion resistance properties of Al-5Si/Al 2 O 3 composite coating .