Influence of Ta content on hot corrosion behaviour of a directionally solidified nickel base superalloy

Hot corrosion behaviour of a directionally solidified nickel base superalloy with different tantalum (Ta) addition in fused sodium sulphate (Na2SO4) under an oxidizing atmosphere at 900 °C has been studied. It was shown that the hot corrosion resistance was improved by increasing of Ta content. The hot corrosion kinetics of the alloy with lower Ta content deviated from parabolic law after 60 h corrosion test, whereas the corrosion kinetics of the alloy with high Ta content followed the parabolic law before 60 h and with less mass change afterwards. A detailed microstructure study using scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD) was performed to investigate the corrosion products and mechanisms. The beneficial effect of Ta was found to be resulted from a Ta-enriched (Cr, Ti)TaO4 layer inside the corrosion scale, which led to the retarding of the element diffusion so as to decrease the hot corrosion kinetics.