EFFECT OF YTTRIUM ON THE STABILITY OF ALUMINIDE YTTRIUM COMPOSITE COATINGS IN A CYCLIC HIGH-TEMPERATURE HOT-CORROSION ENVIRONMENT

A composite coating of aluminide-yttrium has shown excellent corrosion resistance in a cyclic high-temperature hot-corrosion environment. To understand the effect of yttrium on the stability of the composite coating, the specimens were prepared with various coating parameters of Y thickhess, sequence of post heat treatment and surface condition before Y-ion plating, Performance of the composite coating was evaluated by isothermal oxidation and cyclic high-temperature hot corrosion. Isothermal-oxidation-test results 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 an increase in concentration of interstitial oxygen and, thus, increases in oxidation rate. The presence of Y2O3 and (Y, Al) O-type compounds in grain boundaries of Al2O3 and boundaries between the Al2O3 and NiAl effectively prohibits the fast diffusion of oxidants (such as O and S) and Al along grain boundaries, Consequently, it may induce slow diffusion through the matrix, and thus the corrosion resistance of the composite coating under cyclic hot corrosion increases substantially.