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

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.