Microstructural evolution and molten salt corrosion failure mechanisms of annealing modified Al2O3 coatings

Corrosion protection of structural materials in high temperature molten salt environments remains a critical challenge for the development of spent fuel reprocessing technologies. In this study, Al 2 O 3 ceramic coatings were deposited on 316L stainless steel by atmospheric plasma spraying (APS), and vacuum heat treatment (VHT) was applied to tailor the phase composition and microstructural characteristics. The high-temperature corrosion behaviors of the VHT-treated and as-sprayed coatings were systematically investigated in LiCl-KCl molten salt at 600 °C. The results showed that annealing the as-sprayed coating at 1000 °C for 4 h resulted in the near disappearance of metastable γ-Al 2 O 3 diffraction peaks and the formation of a δ-Al 2 O 3 dominated phase composition. Meanwhile, the porosity decreased from 5.33% to 2.86%, and an elemental interdiffusion layer of approximately 21 μm formed across the NiCrAlY bond coat and 316L substrate interface. The adhesion/cohesion strength increased from 15.80 ± 0.17 MPa for the as-sprayed coating to 41.07 ± 0.31 MPa for the VHT-treated specimen. During molten salt exposure, the as-sprayed specimen exhibited pronounced spallation after 80 h, whereas the VHT-treated specimen maintained structural integrity even after 320 h. The improved corrosion durability is mainly attributed to VHT-induced phase stabilization, coating densification, enhanced interfacial continuity, and improved bonding performance, which effectively impede molten salt ingress along defect-assisted pathways and suppress spallation associated with the accumulation of interfacial corrosion products.