Hot corrosion of materials

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Abstract
Alloys and ceramics may experience accelerated corrosion at high temperatures when their surfaces are coated by a thin film of fused salt in an oxidizing environment. Because of their high thermodynamic stabilities, fused alkali sulfates are frequently deposited or condensed onto hot substrates from the combustion product gases or else by the oxidation of heavy metal contaminants in the fuel, e.g. vanadium. Solid-state electrochemical probes have been used in extensive determinations of the solubilities, and their dependencies on oxygen and Na 0 activities, for many important oxides in fused 3a2SO4 at 1200K. The expectation for accelerated synergistic dissolution kinetics for two oxides of differing basicity has been confirmed experimentally. Solid-state electrochemical probes have been attached at the surface of coupons of preoxidized nickel which were subjected to hot corrosion by a fused Na2S0 film in an S02/02 gas environments. The probes were a%le to identify the onset of hot corrosion, and the local values for oxygen activity and the basicity in the salt film during the attack. The resulting data supported the Goebel-Pettit model for basic fluxing of nickel oxide and the negative solubility gradient criterion of Rapp-Goto. The acidic solubilities of thermal barrier candidate oxides Y20 , Ce02 and Hf02 in a fused 0.7Na2S04- 0. 3NaV03 sojution greatly exceed their solubilities in pure Na2S04, a problem which should exist for all oxides. A modeling of the constituent vanadates in the Na-V-S-0 system permits a prediction of the dependence of oxide solubility on vanadium content and melt basicity.

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