THE HIGH-TEMPERATURE CORROSION BEHAVIOR OF FE-MO-AL ALLOYS IN H2 H2O/H2S MIXED-GAS ENVIRONMENTS/

The corrosion behavior of 11 Fe-Mo-Al ternary alloys was studied over the temperature range 700-980-degrees-C in H-2/H2O/H2S mixed-gas envir onments. With the exception of Fe-10Mo-7Al, for which breakaway kineti cs were observed at higher temperatures, all alloys followed the parab olic 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-degrees-C The corrosion rates of Fe-20Mo and Fe-30M o were found to be reduced by five orders of magnitude at all temperat ures by the addition of 9.1 or higher wt. % aluminum. The scales forme d on low-Al alloys (less-than-or-equal-to 5 wt.% Al) were duplex, cons isting of an outer layer of iron sulfide (with some dissolved Al) and a complex inner of Al0.55Mo2S4, FeMo2S4, Fe1.25Mo6S7.7, FeS, and uncor roded FeAl and Fe.3Mo2. Platinum markers were always located at the in terface between the inner and outer scales for the low-Al alloys, indi cating 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-con tent alloys (9.1 and 10 wt.%) formed only Al2O3 which was responsible for the reduction of the corrosion rates.