Oxidation behavior of nickel-base superalloys and High Strength Low Alloy (HSLA) steels at elevated temperatures

Alloy C-22 (UNS N06022) and High Strength Low Alloy (HSLA) steels are candidate materials for use in outer layer of waste storage packages and as rock bolts in the underground roof supports at Yucca Mountain nuclear waste repository respectively. Oxidation kinetics of three Ni-base Superalloys and two HSLA Steels, Split Set Friction Rock Stabilizers (SS-46) and Swellex Mn-24, have been determined by isothermal high temperature continuous measurement thermogravimetry at temperatures ranging between 600°C to 1100°C in pure oxygen atmosphere for predetermined periods of exposures (48 hours for the Superalloys and 100 hours for HSLA steels).;The two other Ni-base Superalloys selected were Alloy-263 (UNS N07263) and Alloy-282. These are similar in their Cr composition to Alloy C-22 and have variations in the contents of other alloying elements namely Co and Mo. The alloys were selected for comparison of their oxidation resistance with C-22 as a baseline material. All three Superalloys are known chromia formers. All the superalloys were evaluated for determining their kinetic parameters and the activation energies for the superalloys were also calculated. The activation energy for the parabolic regime of Alloy-282 is found to be 232 kJ/mol. The slope of the curves on a plot of kp as a function of (1/T) show Alloy-282 to have better oxidation resistance up to 980°C and thereafter the rate constants are similar for all three alloys, but when activation energies over the whole temperature range are calculated, Alloy-263 shows the best average oxidation resistance.;Surface characterization by means of microscopy as well as X-ray photoelectron spectroscopy showed the nature of oxides formed. Based on the kinetics and the characterization, proposed mechanisms for oxidation of these alloys at high temperatures are put forth.;Temperature modulated thermogravimetry was used for studies on HSLA steels. The imposed sinusoidal temperature modulations on the isothermal temperature did not have any noticeable effect on the weight gain characteristics during oxidation. Weight gain data on the steels indicate two distinct regions with different oxidation profiles, where a definite change in rates of oxidation is observed: a first oxidation regime where the steels followed a rate law y = kt0.40-0.63 (changing index of rate law depending upon steel and temperature) and a second stage oxidation regime that follows the parabolic law.;The rate constants determined for the first oxidation region for the steels show that the activation energies of both the HSLA steels is comparable at 82 kJ/mol for Mn24 and ∼102 kJ/mol for SS46. Activation energy was calculated assuming an average time exponent of the rate law of 0.52 for SS-46 and Mn-24. Activation energies of ∼82 kJ/mol for Swellex Mn-24 and ∼83 kJ/mol for SS-46 were determined for the second stage parabolic oxidation regime based on calculated parabolic rate constants.;The results of characterization of the oxide films using SEM/EDAX, X-ray diffraction and Synchrotron white beam x-ray microdiffraction are presented. The oxidation data of the steels presented here is expected to be useful for characterizing those steels for use in underground rock-bolt system and as roof support for the DOE proposed Yucca Mountain Nuclear Waste Repository. To the best of our knowledge this is the first time thermogravimetric studies of this kind have been done on these steels. (Abstract shortened by UMI.)...