Hot corrosion behaviour of a Ni + CrAlYSiN composite coating in Na2SO4–25 wt.% NaCl melt

Ni + CrAlYSiN nanocrystalline composite coatings and NiCrAlYSi reference coatings were prepared by vacuum arc evaporation on the Ni-based K438 superalloy. The aim of this work is to identify the hot corrosion resistance of the Ni + CrAlYSiN nanocrystalline composite coatings and the NiCrAlYSi coatings. The hot corrosion tests were carried out in a molten salt (75 wt.% Na2SO4 + 25 wt.% NaCl) environment at 900 °C. X-Ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDX) and electron probe microanalysis (EPMA) technique were employed to characterize the coatings and the corrosion scales. The results indicated that both coatings improved the hot corrosion resistance of K438 in the molten salt environment. For the composite coating, the oxide scales were composed of α-Al2O3, Cr2O3 and minor NiCr2O4. No spallation or crack of the oxide scales was observed. Internal oxidation was detected in the outer part of the coating after 100 h corrosion. Internal TiN and AlN formation in the substrate was detected. In contrast, obvious crack/spallation of α-Al2O3 oxide scales on the NiCrAlYSi coating was observed; internal oxidation and sulphidation in the coating and even in the substrate were detected.