Abstract: The Al₂O₃ scale serves as a critical inert component in aluminide coatings, improving the high-temperature oxidation resistance of the alloy substrate. However, its stability is often compromised in corrosive conditions. In this work, the stability of the Al₂O₃ scale in the presence of environmental species was systematically investigated through first-principles calculations. These calculations assessed the influence of S, Cl, and H on defect formation and elemental diffusion, the adsorption and dissociation behavior of O₂ and H₂O on the α-Al₂O₃ surface, as well as the dynamic stability of the solid-liquid interface between α-Al₂O₃ and molten NaCl or Na₂SO₄. Experimentally, a β-NiAl coating with a pre-grown oxide scale was prepared, and its corrosion behavior was evaluated in dry and humid atmospheres under various salt deposits. Variations in the concentration of oxygen vacancies were observed under different corrosion conditions. Through a combination of short- and long-term corrosion tests, the evolution of corrosion products was tracked to assess the protectiveness and phase transformation of the Al₂O₃ scale. By integrating theoretical and experimental results, the corrosion reactions and degradation mechanisms of the coating exposed to Na₂SO₄ in NaCl fog were comprehensively discussed.

Key words: Aluminide coating, Al₂O₃ scale, First-principles calculations, High-temperature corrosion, Degradation mechanism