Sulfates, formed by the interaction of SO3, SO2, and O2 in flue gas with alkalies from the fuel and iron oxides are invariably found in corrosion areas. Alkali sulfates, such as Na2SO4, are formed rapidly. Iron sulfates, both FeSO4 and Fe2(SO4)3, can also occur, although the thermodynamics are unfavorable in a low-SO3 atmosphere. Alkali sulfates and iron sulfates can react to form alkali iron trisulfates, for example Na3Fe(SO4)3. These are highly corrosive but their formation also requires more SO3 than is found in bulk flue gas. Recent work has shown that the high SO3 level needed to produce these complex sulfates can be provided by catalysis at surfaces, even though the SO3 is too low in the bulk gas stream. Studies reported here show quantitatively that the SO3 concentration immediately next to an Fe3O3 surface can be many times higher than in the bulk stream of gas, and can be ample to provide the SO3 necessary to form the trisulfates.

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