Kinetic conditions for the simultaneous formation of oxide and sulphide in reactions of iron with gases containing sulphur and oxygen or their compounds

The scaling of pure iron has been investigated in N₂O₂SO₂ and COCO₂COS mixtures between 700 and 900°C. Simultaneous formation of FeO and FeS at the scale/gas phase boundary is observed when the diffusion in the aerodynamic boundary layer or the reaction at the scale/gas phase boundary is the rate-controlling step of the oxidation in O₂N₂ mixtures or of the sulphidation in COCOS mixtures. In those cases the addition of the second oxidant (SO₂ to O₂N₂ mixtures, and an increased CO₂ to COCO₂COS mixtures) increases the rate of the oxidation or sulphidation reactions. When, however, the diffusion of iron ions and electrons through the oxide or sulphide layer respectively, or the reaction at the metal/scale phase boundary are rate-determining, the thermodynamically stable phase (oxide or sulphide) is formed exclusively and the addition of the second oxidant has no influence on the scaling rate. These results may be understood from an evaluation of the equilibria prevailing at the scale/gas phase boundary.