Comparison investigation of hot corrosion exposed to Na₂SO₄ salt and oxidation of MoSi₂-based coating on Nb alloy at 1000 °C

Hot corrosion behavior of MoSi₂-based coating on Nb alloy exposed to Na₂SO₄ salt was investigated by contrast with the oxidation performance at 1000 °C for 0.5 h, 1 h and 2 h. The phases and microstructures evolutions of the corroded coatings were analyzed by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). With the oxidation time prolonging, the profile of splats disappeared gradually. After oxidation for 2 h, SiO₂ particles were generated on the surface of the coating and the mass loss was 1.59 mg/cm², implying that oxidation is a slow degradation process. MoSi₂-based coating exhibited a serious corrosion with Na₂SO₄ salt, forming corroded products such as cristobalite scale and sodium molybdates. With the elongation of corrosion time, the surface morphology of cristobalite layer underwent the transformation from lotus leaves into granules due to basic fluxing of the cristobalite preferentially on the grain boundary. The formed cristobalite scale would spall off due to its loose structure and coefficient of thermal expansion (CTE) mismatch with MoSi₂-based coating, leading to a serious mass loss with 2.5 mg/cm² after hot corrosion for 2 h.