Effects of aluminizing on the oxidation and hot corrosion behaviors of two-phase nickel aluminides

In this work, aluminizing treatment to improve the oxidation resistance of two-phase [NiAl+Ni3Al] Ni aluminides was performed and their surface characteristics were compared with the same alloys without aluminizing treatment in terms of microstructure, oxidation behaviors at 1100 °C and hot corrosion resistance at 900 and 950 °C. Two-step treatment of high Al activity low temperature (HALT) aluminizing was performed to form a NiAl compound layer of approximately 230 μm thickness on the surface. Isothermal oxidation test at 1100 °C showed that at the early stage of oxidation up to 2 h a metastable θ-Al2O3 phase was observed on the surface, which in turn would transform into dense equilibrium α-Al2O3 phase in the later stage of oxidation. Due to these dense phase formed on the surface, much improved oxidation resistance in the aluminized two-phase Ni aluminide, especially in the Ni–32 at.%Al–0.5 at.%B alloy could be obtained. Also improved resistance against hot corrosion up to 950 °C was resulted from aluminized two-phase Ni aluminides.