Hydrogen and Yttria in Chromium: Influence on the High-Temperature Oxidation Kinetics in O2, Oxide-Growth Mechanisms and Scale Adherence

Abstract

The effects of hydrogen and Y_2O_3 on high-temperature oxidation of Cr in 20 mbar O₂ have been studied at 900°C. Oxidation- and O₂-dissociation rates were determined from gas-phase measurements. Hydrogen in Cr leads to breakdown of the oxide scale. The oxide scale on Cr–1%Y₂O₃ charged with hydrogen for 4 hr (resulting in Cr–1%Y₂O₃ with approximately 10 ppm hydrogen) is adherent to the metal substrate. The oxidation rate is similar for Cr with ≤1 ppm hydrogen and Cr–1%Y₂O₃ with ≤1 ppm hydrogen, but significantly lower for 4-hr H-charged Cr–1%Y₂O₃. The oxidation rate of Cr–5%Fe–1%Y₂O₃–25 ppm H is also lower than the oxidation rate of Cr–5%Fe–1%Y₂O₃–≤1 ppm H. This indicates that unless hydrogen is present, there are virtually no effects of the addition of 1% Y₂O₃ to Cr. Using labeled oxygen, ^16,16O₂ and ^18,18O₂, was found that at 900°C the dissociation rate of O₂ is higher on Y₂O₃ than on Cr₂O₃. It is suggested that the well-known improvement of the oxidation resistance of Cr as an effect of additions of Y or Y₂O₃ is related to an increase in the dissociation rate of O₂ and that there is a synergetic effect in combining Y₂O₃ and hydrogen.