Mechanism of the simultaneous formation of oxide and sulfide at the scale surface during the oxidation of a pure metal in mixed atmospheres

Abstract

During the reaction of a pure metal with complex atmospheres containing both oxygen and sulfur, the formation of sulfide mixed with oxide is often observed at high temperature, contradicting thermodynamic predictions. The mechanism proposed so far to explain the formation of a duplex scale at the scale surface assumes a change in composition of the gas phase in a boundary layer next to the scale-gas interface and a reaction of the metal with the molecules of the two elements. This model is shown to be unable to explain the observed amounts of the less stable phase and the reaction rates when sulfur dioxide is the prevailing reacting species and is substituted by the assumption of a direct reaction with the SO₂ molecules. The thermodynamic equivalence of the two approaches is also pointed out.