The rule of hydrogen and rare earth metals in the formation of protective metal-oxides

Sammanfattning: The effects of hydrogen and rare earth metal (REM) additionson the oxidation behavior of metals and alloys that formchromia, alumina and zirconia in 20 mbar O2and/or H2O have been investigated at elevated temperatures.The metal-gas reactions were studied by the use of isotopicallylabeled gas in combination with mass spectrometry and bymeasuring changes in the total pressure in a closed reactionchamber. The position of oxide growth was examined bysequential oxidation in16,16O2and18,18O2followed by secondary ion mass spectrometry (SIMS)analysis.The oxide formed in O2at 900°C, on Cr containing ? 1 wt.ppmhydrogen, is flat and adherent to the metal substrate while theoxide is wrinkled and the adherence is poor when Cr contains? 5 wt.ppm hydrogen. The hydrogen increases the outwardtransport of chromium and thereby reduces the portion of oxidegrowth at the metal-oxide interface. On Cr containing Y2O3, the oxide is adherent to the metal substrate evenwhen the substrate contains up to 10 wt.ppm hydrogen.The oxide formed in H2O on Cr with ? 1 and 5 wt.ppm hydrogen isflat and adherent to the metal substrate. In accordance withthis there is a relatively large fraction of oxide growth atthe metal-oxide interface. However, as is the case when theoxide is formed in O2, hydrogen in the metal increases the outwardchromium transport in the scale.It was found that the dissociation rate of O2is higher on Y2O3than on Cr2O3at 900°C. It is suggested that an effect of Yand/or Y2O3additions on the oxidation mechanism of Cr isrelated to a Y- or Y2O3-induced increase in the dissociation rate of O2. The oxidation rate is similar for Cr with ?1 wt.ppm hydrogen and Cr-1wt.%Y2O3with ? 1 wt.ppm hydrogen but lower forCr-1wt.%Y2O3with 10 wt.ppm hydrogen. This indicates that,unless hydrogen is present, there are only small effects of 1%Y2O3and, when both Y2O3and hydrogen are present, a well-balancedtransport of cations and anions is obtained in the oxide scale.A combined effect of hydrogen and REM that can be used as a"tool" in the formation of more protective metal-oxides hasalso been observed in the oxidation of Cr-5wt.%Fe-1wt.%Y2O3, Fe-20wt.%Cr-5wt.%Al-1wt.%Y,Fe-25wt.%Cr-35wt.%Ni-1wt.%Si-0.05wt.%Ce, Zircaloy-2 and Zr. Inaddition, platinum-coating on zirconia has been observed togive results comparable with ceria-coating.Several advantages in using gas phase analysis instead ofthermogravimetry in studies of high-temperature oxidation ofmetals are presented.Key words:chromium; high-temperature corrosion; oxidegrowth; scale adherence; hydrogen; yttria; REM; reactiveelement effect; gas phase analysis; two-stage oxidation; oxygendissociation; secondary ion mass spectrometry.