Multiscale-optimized hydrothermal composite coatings for magnesium alloys: corrosion resistance mechanisms

The hydrothermal method presents significant advantages in terms of low cost, environmental sustainability, and high efficiency for the corrosion protection of magnesium alloys. However, the experimental stability and protection performance can be restricted by specific details of technologic process such as the direction of sample placement and defects in coating encapsulation. This study synthesized a composite coating comprising Mg(OH) 2 -CaSiO 3 /CaCO 3 and proposed a synergistic optimization strategy based on multi-physics field to address these issues. By constructing a joint simulation model using Fluent-transient structure analysis, the mechanism behind coating defects caused by the eddy current effect in the reactor and the geometric stress concentration of the sample was elucidated. A synergistic approach involving chamfering and horizontal placement was developed to achieve complete coating encapsulation. The results indicate that the dense Mg(OH) 2 bottom layer effectively inhibits charge transfer, while the CaSiO 3 /CaCO 3 surface composite layer extends the diffusion path for corrosive media. The polarization resistance of the optimized sample reaches 4.06 × 10 9 Ω·cm 2 , with corrosion current density reduced by five orders of magnitude compared to that of bare substrate. After 168 h of immersion testing, the coating continued to provide effective protection. The corrosion product Mg(OH) 2 has been shown to continuously fill the micropores. forming a secondary protective layer. The integrated approach, combining simulation, mechanism and process optimization, contributes to significantly expanding the industrial application for protective coatings on magnesium alloys.