Comparative Study of the Oxidation of NiAl(100) by Molecular Oxygen and Water Vapor Using Ambient-Pressure X-ray Photoelectron Spectroscopy

The oxidation behavior of NiAl(100) by molecular oxygen and water vapor under a near-ambient pressure of 0.2 Torr is monitored using ambient-pressure X-ray photoelectron spectroscopy. O-2 exposure leads to the selective oxidation of Al at temperatures ranging from 40 to 500 degrees C. By contrast, H2O exposure results in the selective oxidation of Al at 40 and 200 C, and increasing the oxidation temperature above 300 degrees C leads to simultaneous formation of both Al and Ni oxides. These results demonstrate that the O-2 oxidation forms a nearly stoichiometric Al2O3 structure that provides improved protection to the metallic substrate by barring the outward diffusion of metals. By contrast, the H2O oxidation results in the formation of a defective oxide layer that allows outward diffusion of Ni at elevated temperatures for simultaneous NiO formation.