Surface chemistry of oxide scale on IN-738LC superalloy: Effect of long-term exposure in air at 1173 K

Abstract

The oxidation kinetics of IN-738LC at 1173 K in dry air up to 1500 hr followed parabolic law. Surface morphology and the oxide phases present in the scale were characterized by SEM, XRD, EDS, FIB, and XPS. FIB investigation exhibited a compact and adherent oxide layer. XRD analysis revealed the presence of NiO, NiAl2O4, NiCr2O4 spinel, and Al2O3 on the top scale surface formed at 1173 K in dry air. Extensive XPS analyses revealed the presence of Cr2O3, CrO2, and CrO3 on the top scale surface formed on IN-738LC after 10 hr of exposure. The presence of TiO2, Al2O3, Cr2O3, NiO, and NiAl2O4 and NiCr2O4 spinels along with the oxides of Ta at the top surface of the scale was observed after 100 hr of oxidation. The TiO2 content was high on the surface and the entire scale cross section was composed mostly of Cr2O3, NiO, TiO2, and Al2O3 after 100 hr of exposure to dry air at 1173 K. The concentration of Al2O3 on the surface of the oxide scale was found to increase after 100 hr of exposure and remained constant thereafter. After 300 and 1500 hr of exposure, the surface oxide was mainly Al2O3 along with oxides of Ni, Ti, and Cr. The oxide scale cross section consisted mostly of Al2O3 along with other oxides such as Cr2O3, NiO, and TiO2. The oxide-scale composition was found to vary significantly with the duration of exposure to dry air at 1173 K.