The high-temperature corrosion behavior of Nb-Al alloys in a H2/H2S/H2O gas mixture

Abstract

The corrosion behavior of pure Nb and three Nb Al alloys containing 12.5, 25, and 75 at.% Al was studied over the temperature range of 800–1000°C in a H2/H2S/H2O gas mixture. Except for the Nb-12.5Al alloy consisting of a two phase structure of α-Nb and Nb3Al, other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but fluctuated with increasing Al content. The Nb-75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 1.6–2.2 orders of magnitude lower than those of pure-Nb (depending on temperature). An exclusive NbO2 layer was formed on pure Nb, while heterophasic scales were observed on Nb-Al alloys whose compositions and amounts strongly depended on Al content and temperature. The scales formed on Nb-12.5Al consisted of mostly NbO2 and minor amounts of Nb2O5, NbS2, and γ-Al2O3, while the scales formed on Nb-25Al consisted of mostly Nb2O5 and some γ-Al2O3. The scales formed on Nb-75Al consisted of mostly α-Al2O3 and Nb3S4 atT ≤900°C, and mostly α-Al2O3 , Nb3S4 and some AlNbO4 at 1000°C. The formation of α-Al2O3 and Nb3S4 resulted in a significant reduction of the corrosion rates.