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

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 degrees C in a H-2/H2S/H2O gas mixture. Except for the Nb-12.5Al alloy consisting of a two-phase structure of alpha-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 gamma-Al2O3 while the scales formed on Nb-25Al consisted of mostly Nb2O5 and some gamma-Al2O3 The scales formed on Nb-75Al consisted of mostly alpha-Al2O3 and Nb3S4 at T less than or equal to 900 degrees C, and mostly alpha-Al2O3, Nb3S4 and some AlNbO4 at 1000 degrees C. The formation of alpha-Al2O3 and Nb3S4 resulted in a significant reduction of the corrosion rates.