Hot Corrosion Behavior of a New Powder Metallurgy Superalloy in Molten Na2SO4-NaСl Salts

Hot corrosion behavior of a powder metallurgy superalloy (Alloy 1) in molten 95% Na2SO4+5%NaCl salts at 700 °C, 750 °C and 800 °C are investigated with average corrosion rate calculation, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy spectrum analyzer (EDS). Experimental results show that the average corrosion rate of Alloy 1 increases as the temperature elevates. The corrosion layers obtained after 100h of hot corrosion were mainly composed of Cr2O3, TiO2, Al2O3 , NiO and Ni3S2 at each temperature, meanwhile NiCr2O4 and Cr2S3 appeared at 750 °C and 800 °C respectively. The cross-sectional morphologies and corresponding elemental maps indicate that lots of Ni-Ni3S2 eutectic appeared in corrosion layers and a large amount of sulfides and oxides appeared in internal substrate. According to these results, the combined mechanism of oxidation and sulfuration in Na2SO4-NaCl salts for Alloy 1 is confirmed. Compared to Alloy 2, the increased Co and Al content in Alloy 1 with better hot corrosion resistance at 800 °C promoted the rapid formation of continuous Cr2O3 and Al2O3 protective films on the alloy surface in which Co inhibited internal oxidation of Al and reduced internal diffusion of S through the third element effect.