Comparison of Isothermal with Cyclic Oxidation Behavior of "Cr-Aluminide" Coating on Inconel 738LC at 900 A°C

A two-step pack cementation technique was used to cover nickel-based IN-738LC superalloy with a Cr-aluminide coating layer. The IN-738LC/Cr-aluminide samples were oxidized at 900 A degrees C for 350 and 700 h, isothermally, and 175 and 350 h, cyclically. Each cycle lasted for 25 h. Effects of the isothermal and the cyclic oxidation on (a) morphology, (b) new-phase formation, (c) weight gain, and (d) oxidation reactions were investigated by scanning electron microscopy, electron dispersive spectroscopy, X-ray diffraction, and weight gain measurements. From results, it was concluded that the oxygen attack was more severe during the cyclic oxidation than the isothermal procedure. Kinetics of the reactions indicated a parabolic rate law for the isothermal process, while the cyclic oxidation firstly followed a similar parabolic rate law which was trailed later on by a severe surface spallation.