Vanadate-Sulfate Melt Thermochemistry Relating to Hot Corrosion of ThermalBarrier Coatings

摘要

The gas turbine industry is moving strongly toward the use of ZrO2-based thermalbarrier coatings (TBCs) on hot section vanes/blades to increase engine efficiency and durability. In some applications (e.g., ship propulsion or electricity generation), such TBCs may be corroded by molten vanadate-sulfate deposits from fuel impurities. This Report provides a synopsis of vanadate-sulfate thermochemistry relating to TBC hot corrosion, and summarizes research conducted on this topic at the Naval Research Laboratory. The interactions of Na2O, V2O5 and SO3, the melt components which determine the composition of the vanadate-sulfate deposits, were examined and clarified. Vanadate-sulfate melts were shown to be nonideal rather than ideal a point of some contention in the past literature and, for melts of Na/V = 1, to have a V205 activity coefficient (gamma) that ranges from 5 x 10(exp -4) up to essentially 1, depending on the S03 overpressure. Different NaVO3/Na2SO4 mixtures gave a detectable but small change in gamma(V2O5), suggesting that the Na/V ratio is relatively unimportant, for melts equilibrated with SO3, in determining gamma(V2O5). The reactions of several candidate ZrO2 stabilizers (MgO, CeO2, Sc2O3, In2O3, SnO2) with vanadate-sulfate melts are categorized and discussed.