Fluxing of protective oxide scales on superalloys via a thermal surface tension gradient

Abstract

An oxide fluxing model involving continuous dissolution and reprecipitation of oxide scales via a thermal surface tension effect is postulated to explain hot corrosion of high teperature alloys and coatings in gas turbines. It is demonstrated that due to the existence of a thermal surface tension gradient, a molten alkali salt film condensed on a turbine vane or blade can undergo a circulating flow motion. A cycle of oxide dissolution at the hot end of the molten salt film and reprecipitation at its cold end can thus be established continuously to remove the protective oxide scales from the surfaces of the turbine components. Under gas turbine operating conditions, the liquid salt film can flow at a velocity as high as 1.5 cm sec⁻¹, and the oxide scale can be continuously fluxed away at a rate over 1.5 mgcm⁻²h⁻¹ (∼5 microns per hour for Al₂O₃). This can, therefore, be one of the mechanisms to sustain hot corrosion, especially under conditions where large surface temperature gradients are encountered.