Abstract
Pure iron was exposed at 800°C to flowing, catalyzed-gas mixtures of N2/CO2/CO/SO2 adjusted to control the partial pressures of SO2, S2 and O2. The equilibrium gas compositions were such that iron oxide was thermodynamically stable with respect to sulfide. The reaction product scale was invariably a mixture of oxide plus sulfide, and grew according to parabolic kinetics at high PSO2 values and by linear kinetics in dilute gases. In both cases the reactant gas species was SO2, not molecular oxygen or sulfur. The relative amounts of sulfide and oxide corresponded to stoichiometric reaction of SO2 at high PSO2 values, but not in dilute gases. At low PSO2 values, the relationship between scale-sulfide volume fraction and PSO2 corresponded to two independent scale-SO2 reactions leading to oxide and sulfide growth. The two-phase mixture was lamellar, with platelets oriented approximately parallel to the mass-transfer direction. An inverse relationship between lamellar spacing and linear scaling rate is interpreted as evidence of a cooperative (cellular) growth mechanism.
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Unsworth, J., Young, D.J. Amounts and Distribution of Phases in Sulfide Plus Oxide Scales on Iron. Oxidation of Metals 60, 447–465 (2003). https://doi.org/10.1023/A:1027334819454
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DOI: https://doi.org/10.1023/A:1027334819454