Effect of water vapor on high-temperature oxidation of NiAl alloy

•The oxidation with H2O results in a thicker Al2O3 oxide scale than that with O2.•The α-Al2O3 layer of the oxide scale formed in H2O is more defective with a higher porosity than that in O2.•H protons derived from H2O molecules boosting the formation and clustering of lattice vacancies in α-Al2O3. The high-temperature oxidation of NiAl is studied with dry oxygen and water vapor. The oxidation in H2O results in a thicker Al2O3 oxide scale than that in O2. The oxide scale formed initially is a single layer of γ-Al2O3 that subsequently transforms into a α-Al2O3/γ-Al2O3 bilayer structure, in which the inner α-Al2O3 layer formed in H2O has a higher porosity than that in O2. Further density functional theory calculations show that H protons derived from H2O molecules penetrate into the oxide lattice and boost the formation of lattice vacancies in both α-Al2O3 and γ-Al2O3, thus enhancing the oxide scale growth.