The high-temperature corrosion behavior of Fe-Mo-Al alloys in H2/H2O/H2S mixed-gas environments

Abstract

The corrosion behavior of 11 Fe-Mo-Al ternary alloys was studied over the temperature range 700–980°C in H2/H2O/H2S mixed-gas environments. With the exception of Fe-10Mo-7Al, for which breakaway kinetics were observed at higher temperatures, all alloys followed the parabolic rate law, despite two-stage kinetics which were observed in some cases. A kinetics inversion was observed for alloys containing 7 wt.% Al between 700–800°C. The corrosion rates of Fe-20Mo and Fe-30Mo were found to be reduced by five orders of magnitude at all temperatures by the addition of 9.1 or higher wt.% aluminum. The scales formed on low-Al alloys (≤5 wt.% Al) were duplex, consisting of an outer layer of iron sulfide (with some dissolved Al) and a complex inner of Al0.55Mo2S4, FeMo2S4, Fe1.25Mo6S7.7, FeS, and uncorroded FeAl and Fe3Mo2. Platinum markers were always located at the interface between the inner and outer scales for the low-Al alloys, indicating that outer-scale growth was due mainly to outward diffusion of cations (Fe and Al), while the inner scale was formed primarily by the inward flux of sulfur anions. Alloys having intermediate Al contents (7 wt.%) formed scales that consisted of FeS and Al2O3. The amount of Al2O3 increased with increasing reaction temperature. The high-Al-content alloys (9.1 and 10 wt.%) formed only Al2O3 which was responsible for the reduction of the corrosion rates.