Influence of Thermal Fatigue on Hot Corrosion of an Intermetallic Ni-Aluminide Coating

Abstract

The influence of thermal fatigue strain history on the hot corrosion attack of a Ni-aluminide coating was examined. Coatings were applied by pack aluminization to stepped-disk fatigue specimens machined from a monocrystalline Ni-base superalloy (Rene N4). Induction heating of the stepped-disk specimens was used to simulate the severe thermal and strain transients experienced by gas turbine airfoils. Hot corrosion was studied by applying Na2S04 to the specimen, and controlling the partial pressures of 02, SO2 and SO3 in the test atmosphere. Hot corrosion of the Ni-aluminide coatings was found to be strongly influenced by strain history. After 6000 fatigue cycles, between peak strains of -0.26% at 925°C and 0.03% at 650X, extensive hot corrosion attack occurred, with Al and Ni sulfides found throughout the coating. By contrast, only minor surface oxidation of the coating was observed after 6000 cycles between lower peak strains of -0.16% at 925°C and 0.01% at 650°C. The pronounced dependence of hot corrosion attack on strain history is attributed to cracking of protective surface oxide scales during thermal fatigue cycling, allowing direct interaction between molten Na2S04 and the Ni-aluminide coating. This interaction results in an increase in the oxygen ion (02-) activity in the vicinity of the coating/oxide interface, preventing reformation of a protective oxide scale.