The high-temperature corrosion of Fe-28Al and Fe-18Al-10Nb in a H-2/H2S/H2O gas mixture

The corrosion behavior of Fe-28Al and Fe-18Al-10Nb (at.%) was investigated over the temperature range of 700-900 degrees C in a H-2/H2S/H2O gas mixtur e with varying sulfur partial pressures of 10(-2)-10(3) Pa and oxygen parti al pressures of 10(-19)-10(-15) Pa. Ln general, the corrosion kinetics foll owed the parabolic rate law although two-stage kinetics were noted in some cases. The steady-state parabolic rate constants increased with increasing temperature. Based on the equivalent addition (28 at.%) of alloying element s, it was found that Fe-18Al-10Nb exhibited a better corrosion resistance, being 2.1-3.9 orders of magnitude lower than Fe-28Al (depending on temperat ure). The scales formed on Fe-28Al consisted of mostly FeS, alpha-Al2O3, an d minor FeAl2S4, while the scales formed on Fe-18Al-10Nb consisted of mostl y alpha-Al2O3, Nb3S4, Nb3O5, and minor FexNbS2 (FeNb2S4/FeNb3S6) at T less than or equal to 800 degrees C and of alpha-Al2O3, Nb3S4, and FexNbS2 at 90 0 degrees C. The formation of Al2O3 and Nb3S4 resulted in the significant r eduction of corrosion rates. (C) 1998 Elsevier Science S.A. All rights rese rved.