The influence of reactive-element coatings on the high-temperature oxidation of pure-Cr and high-Cr-content alloys

The influence of various reactive-element (RE) oxide coatings (Y2O3, CeO2, La2O3, CaO, HfO2; and Sc2O3) on the oxidation behavior of pure Cr, Fe-26Cr, Fe-16Cr and Ni-25Cr at 900 degrees C in O-2 at 5 x 10(-3) tory has been investigated using the O-18/SIMS technique. Polished samples were reactively sputter-coated with 4 nm of the RE oxide and oxidized sequentially first in O-16(2) and then in O-18(2). The effectiveness of each RE on the extent of oxidation-rate reduction varied with the element used Y2O3 and CeO2 coatings were found to be the most beneficial, whereas Sc2O3 proved to be ineffective, for example, for the oxidation of Cr. SIMS sputter profiles showed that the maximum in the RE profile moved away from the substrate-oxide interface during the early stages of oxidation. After a certain time the RE maximum remained fixed in position with respect to this interface, its final relative position being dependent on the particular RE. The position of the RE maximum within the oxide layer is also varied with the substrate composition. For all coatings O was found to have diffused through the oxide to the substrate-oxine interface during oxidation, the amount of oxide at this interface increasing with increasing time. The SIMS data confirm that for coated substrates there has been a change in oxide growth mechanism to predominantly anion diffusion. The RE most probably concentrates at the oxide grain boundaries, generally as the binary oxide (RE)CrO3. Cr3+ diffusion is impeded, while oxygen diffusion remains unaffected.