Competing effects of water vapor on Al₂O₃ scale growth and crack propagation in nickel-aluminide coatings at 950°C

• Dual role of water vapor revealed : H₂O simultaneously accelerates Al₂O₃ scale growth and triggers longitudinal cracks in nickel-aluminide coatings at 950°C. • H⁺-mediated defect chemistry : First-principles calculations demonstrate that H⁺ reduces Al vacancy formation energy, enabling rapid outward Al³⁺ diffusion for dense Al₂O₃ formation. • Competitive cracking mechanism : H₂ gas generated from H₂O dissociation penetrates the oxide-coating interface, initiating cracks, while Al₂O₃ growth suppresses crack inward propagation via pore filling. • Environment-dependent microstructure : Water vapor promotes a uniform, sheet-like Al₂O₃ layer, contrasting with needle-like oxides in pure O₂, enhancing short-term oxidation resistance. • Biomass corrosion implications : The trade-off between H₂O-enhanced Al₂O₃ protection and H-induced embrittlement provides guidelines for crack-resistant coating design. The dual role of water vapor in high-temperature oxidation of nickel-aluminide coatings remains a critical but poorly understood issue for biomass superheater applications. In this study, nickel-aluminate coatings were obtained by powder aluminizing method, and oxidation experiments of coatings were conducted at 950°C in oxygen and water vapor environments. The mechanism of the effect of water vapor on the short-term oxidation of coatings was systematically investigated. Results showed that the Al 2 O 3 protective layer was formed by selective oxidation in both atmospheres, and the alumina formed under oxygen conditions had a needle-like structure, while in water vapor, the alumina existed in a sheet-like structure. Combined with first principles calculations, it was found that H + in water vapor intensified the formation of Al and O vacancies, which accelerated the outward diffusion of Al to form Al 2 O 3 , resulting in a thicker and denser Al 2 O 3 layer. Meanwhile, water vapor induced a competition effect: on one hand, H 2 and water vapor generated by the reaction diffused along cracks to the inside of the coating, leading to the expansion of the cracks; on the other hand, the protective Al 2 O 3 generated by the reaction filled the pores of the coating and inhibited the expansion of cracks to the inside of the coating.