Comparison of isothermal and cyclic oxidation behavior of twenty-five commercial sheet alloys at 1150°C

Abstract

Twenty-five commercial nickel-, iron-, and cobalt-base sheet alloys incorporating chromium or chromium and aluminum additions for oxidation resistance were tested at 1150°C in air for 100 hr in both isothermal and 1-hr cyclic furnace exposures. The alloys were evaluated by sample specific weight change, by type of scale formed, by amount and type of spall, and by sample thickness change and microstructure. In isothermal steady-state oxidation, four types of controlling oxides were observed depending on alloy composition: NiO, Cr₂O₃-chromite spinel, ThO₂-blocked Cr₂O₃, and αAl₂O₃-aluminate spinel. The latter three types are considered protective. In the Cr₂O₃-forming alloys, however, scale vaporization is a critical factor in determining the parabolic scaling rate based on paralinear oxidation. In cyclic oxidation the alloys which form Cr₂O₃-chromite spinel scales were degraded severely when sufficient chromite spinel developed to trigger spalling. The cyclic behavior of the other three types of alloys does not differ greatly from their isothermal behavior. If chromite spinel formation is minimal, the thinner the oxide formed, the less the tendency to spall. Factors contributing to a thin scale are low isothermal scaling rates; reactive element additions, such as thorium, lanthanum, and silicon; and scale vaporization. Scale vaporization may, however, lead to catastrophic oxidation at high gas velocities or low pressures or both. A tentative mass-balance approach to scale buildup, scale vaporization, and scale spalling was used to calculate the critical oxidation parameter—the effective metal thickness change. In general, this calculated thickness change agrees with the measured change to within a factor of 3 if a correction is made for grain boundary oxidation. The calculated thickness change parameter was used to rate the oxidation resistance of the various alloys under isothermal or cyclic conditions. The best alloys in cyclic furnace oxidation tests were either αAl₂O₃-aluminate spinel formers or Cr₂O₃ formers with ThO₂ blockage.