The acidic and basic solubilities of the potential thermal barrier oxide components CeO2, HfO2, and Y2O3 were measured as a function of solvent basicity for fused Na2SO4-30 mol% NaVO3 at 900 C and 1 atm O2. Y2O3 solubility was higher than that of CeO2 or HfO2. For comparison, the solubility of CeO2 was measured in pure Na2SO4. In the mixed sulfate-metavanadate melts, the oxides formed orthovanadate acidic solutes with a solubility much higher than the corresponding acidic sulfates for the pure Na2SO4 melt.
The results point to a revised mechanism for hot corrosion of alloys by acidic fluxing. Solutions of Na2SO4 with the salts of strong acids (e.g., V2O5, MoO3, and WO3) provide higher acidic solubilities for protective oxides by forming solutes involving the complex anions. These mixed solvent solutions are generally buffered to higher basic values.
