| The composite coating of aluminide-yttrium has shown excellent corrosion resistance under cyclic high temperature hot corrosion environment. To understand the effect of the yttrium on the stability of the composite coating, the specimens were prepared with various coating parameters such as thickness of Y layer, sequence of post heat treatment, and surface condition before Y ion plating. Performance of the composite coating was evaluted by the isothermal oxidation and cyclic high temperature hot corrosion. Test results of isothermal oxidation show that the Y in the composite coating helps to form a thick and dense Al₂O₃ scale which is ductile and resistant to thermal stress. The Y in Al₂O₃ may act as a donor which leads to increase in the concentration of interstitial oxygen and thus increase in oxidation rate. The presence of Y₂O₃ and (Y, Al)O type compounds in the grain boundaries of Al₂O₃ itself and the boundaries between the Al₂O₃ and NiAl effectively prohibits the fast diffusion of oxidants, and Al along the grain boundaries. Consequently, it may induce slow diffusion through the matrix, and thus the corrosion resistance of the conposite coating under cyclic hot corrosion increases substantially. |
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