Sulfur and water have a fundamental impact on the corrosion rate and potential failure of materials. It is therefore necessary to understand the mechanisms, rates, and potential means of transport, as well as the reactions of these elements with an alloy. This paper investigates the effect of water vapor in the initial stages of SO 2 corrosion of an Fe-9Cr-0.5Mn model alloy at 650°C in situ under laboratory conditions using energy-dispersive x-ray diffraction analysis. Two separate experiments were run, one with a 99.5% Ar + 0.5% SO 2 atmosphere and one with a 69.5% Ar + 0.5% SO 2 + 30% H 2 O atmosphere. With a wet atmosphere, the alloy formed a scale with decreasing oxygen content towards the scale-alloy interface. Sulfides were identified above and below a (Fe, Cr) 3 O 4 layer in the inner corrosion zone. In contrast to this, the overall scale growth was slower in a dry SO 2 atmosphere.

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