Diffusion-limited sulfidation of wustite

Abstract

The sulfidation of wustite in H₂S−H₂O−H₂−Ar atmospheres has been studied at temperatures of 700, 800, and 900°C with thermogravimetric techniques. Polycrystalline wustite wafers were equilibrated in a flowing H₂O−H₂−Ar gas stream and then sulfidizedin situ. During an initial transient stage a protective layer of FeS formed on the sample, and an intermediate layer of Fe₃O₄ formed between the FeO and FeS layers. Subsequently, the reaction followed a parabolic rate law. The parabolic rate constant varied from 0.22×10⁻² mg² cm⁻⁴ min⁻¹ at 700°C to 6.5×10⁻² mg² cm⁻⁴ min⁻¹ at 900°C. The reaction rate was limited by the diffusion of iron through the intermediate Fe₃O₄ layer which grew concurrently with the FeS layer and at the expense of the FeO core. After the FeO core was completely converted to Fe₃O₄, the process entered a passive stage during which no further mass changes could be detected.