The effect of SO3on vanadate-induced hot corrosion

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Abstract
Unrefined fuel oils may contain considerable amounts ofboth sulphur and vanadium. Despite this, exposure to combustion gases seldom yields deposits which consist ofboth vanadates and sulphates. Calculations for the sodium sulphate/vanadate system show that this is due to thermodynamics ofthe system. Sodium sulphate cannot exist in equilibrium with fused vanadate unless the sulphur trioxide pressure in the ambient atmosphere is high or the vanadium to sodium ratio in the deposits is less than one. Thermogravitmetric studies which delineate the conditions for simultaneous sulphate- and vanadate-induced corrosion at 650 to 8OO°C have been performed. NiCrAlY coatings with K-sodium vanadyl vanadate deposits and the deposit alone have been exposed to oxygen containing 4% sulphur dioxide at 650 to 8OO°C. The results generally confirm the calculations from available thermodynamic data, but the solubility ofsulphur oxide in fused sodium vanadate is higher than expected from the literature values. The ' corrosion mechanism changes from initial vanadate-induced to essentially sulphate-induced hot corrosion when the sodium trioxide pressure is high enough that sodium sulphate may be formed.

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