Effect of vacuum annealing on defect structure and LiCl–KCl molten salt corrosion behavior of atmospheric plasma-sprayed Y3Al5O12 coatings

Y 3 Al 5 O 12 (YAG) is a promising barrier material for chloride molten salt environments, but the lamellar boundaries, pores, and microcracks inherent to atmospheric plasma-sprayed (APS) coatings can severely compromise its protection. In this work, NiCrAlY/YAG coatings were deposited on 316L stainless steel by APS and subsequently vacuum-annealed at 900–1300 °C for 2–4 h. The effects of annealing on phase constitution, defect structure, and corrosion behavior in eutectic LiCl-KCl molten salt at 600 °C were systematically investigated. Vacuum annealing significantly reconstructed the coating microstructure. Among the tested conditions, HT/YAG-1100 °C/2 h showed the highest YAG phase integrity, the lowest porosity, the lowest surface roughness, and the highest apparent contact angle, indicating the lowest liquid accessibility. In contrast, excessive annealing promoted local secondary-phase crystallization, as reflected by weak YAP and Y 2 O 3 -related reflections, and induced crack re-opening. During 320 h of molten-salt corrosion, both coatings exhibited an initial weight gain followed by weight loss, whereas the annealed coating showed a ∼14% lower final apparent mass loss than AS-YAG. The improvement is attributed to reduced defect connectivity and more tortuous penetration paths, which limit salt access to the YAG/NiCrAlY interface, reduce internal salt retention, and keep most corrosion damage near the coating surface within 320 h.