Hydrogen and yttria in chromium:: Influence on the high-temperature oxidation kinetics in O2, oxide-growth mechanisms and scale adherence

The effects of hydrogen and Y2O3 on high-temperature oxidation of Cr in 20 mbar O-2 have been studied at 900 degreesC. Oxidation- and O-2-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 less than or equal to1 ppm hydrogen and Cr-1%Y2O3 with less than or equal to1 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-less than or equal to1 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, O-16,16(2) and O-18,18(2), was found that at 900 degreesC the dissociation rate of O-2 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 O-2 and that there is a synergetic effect in combining Y2O3 and hydrogen.