Hydrogen in Chromium: Influence on the High-Temperature Oxidation Kinetics in H2O, 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.