Oxidation kinetics of some nickel-based superalloy foils in humidified hydrogen and electronic resistance of the oxide scale formed part II

Foil specimens of Haynes 230, Hastelloy X, Inconel 718. and Inconel 625 of 4.5 mil (similar to 113 mum) thickness were oxidized in wet hydrogen for several hundred hours at temperatures between 700 and 1100 degreesC. Chromia was determined to be the predominant oxide phase in the scales of these alloys, consistent with the results of a previous study(1) of these alloys in air, as shown using X-ray diffraction and electron probe microanalysis. The oxidation kinetics of ail the alloys, investigated by thermogravimetry, in a wet hydrogen atmosphere exhibited parabolic behavior. Haynes 230 and Hastelloy X exhibited the slowest oxidation kinetics of all the alloys studied. Further, the oxidation kinetics of all the alloys were faster at 800 degreesC in a wet hydrogen atmosphere compared to oxidation in air at 800 degreesC. However, the oxidation kinetics of these alloys in a wet hydrogen atmosphere were slower than those in air at 1100 degreesC. The oxide scale formed in wet hydrogen exhibited a higher resistance than that formed in air. This result Is attributed to the expected p-type conductivity of the oxide scales formed in both air and wet hydrogen. (C) 2001 The Electrochemical Society. All rights reserved.