Abstract
The oxidation of pure chromium in H2-H2O-H2S gas mixtures was studied as a function of gas composition at 900°C. Oxidation kinetics were measured using a thermogravimetric apparatus, and the oxidation products were characterized using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). Chromia scales formed when the H2O/H2S ratio was about 10 or greater. Scales that comprised a mixture of Cr2O3 and chromium sulfides formed when the H2O/H2S ratio was about 3, even though Cr2O3 was the thermodynamically stable phase under these conditions; i.e., a kinetic boundary exists for pure chromium in H2-H2O-H2S gas mixtures. The transition from chromia scale formation to the formation of scales containing both oxide and sulfide with a change in gas composition (decrease in the H2O/H2S ratio) is associated with an inhibition of the overgrowth of growing, metastable sulfide nuclei by the thermodynamically stable Cr2O2 phase. Presulfidation experiments confirmed that metastable chromium sulfide can continue to grow after H2O is added to the gas phase when the H2O/H2S ratio in the gas phase is less than a critical value at the temperature of interest.
Published Version
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