High-temperature oxidation and hot corrosion behaviors of the NiCr-CrAl coating on a nickel-based superalloy

The hot-section components in commercial aerogas turbines were typically made of nickel-based superalloys and were protected by coatings. Two kinds of diffusion aluminide coatings, including simple aluminide coating and NiCr-CrAl coating, were prepared on a nickel-based cast superalloy by LP-VCD and pack cementation techniques, respectively. The isothermal oxidation behavior of the cast alloy and the coatings at 1000 and 1100 °C and their hot corrosion behaviors in the presence of 75 wt.% Na2SO4+K2SO4 film at 900 °C were studied using TGA, scanning electron microscopy (SEM)/energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and electron probe microanalysis (EPMA), respectively. The results showed that NiCr-CrAl coating, not only held a very excellent oxidation resistance due to the α-Al2O3 formation on the coating surface during the isothermal process at 1000 and 1100 °C, but also possessed excellent hot corrosion resistance in sulphate melt. Internal oxidation and nitridation were found in the cast alloy during the oxidation process. Moreover, the cast alloy suffered from catastrophic hot corrosion, resulting in the serious internal oxidation and sulfidation. The oxidation resistance of the simple aluminide coating was superior to the cast alloy; the simple aluminide coating was damaged completely and lost its protective effect after the hot corrosion test. In addition, the mechanisms for oxidation and corrosion of the cast alloy and the coatings are discussed simultaneously.