Life assessment of thermal sprayed alloy 825 in waste heat incineration environment. (April 2023) - Muthu, S.M.
Abstract: In this study, the cyclic hot corrosion behaviour of Ni-based superalloy 825 before and after thermal spray coating of Cr3 C2 –25%NiCr and NiCrAlY powders was investigated in a superheater section of a simulate waste-heat incinerator. The thermogravimetric (TG) technique is used to analyze the corrosion kinetics of the samples that have undergone hot corrosion tests in Na2 SO4 –40%K2 SO4 –10%KCl–10%NaCl environment at 750 °C for 60 cycles. The X-ray diffraction (XRD) technique was also used to analyze the phases of coating and corrosion products. Scanning electron microscopy (SEM) was used to characterize the surface and cross-sectional morphology of the coatings, while the compositions were analyzed using energy dispersive spectroscopy (EDS). After hot corrosion tests, the cross-sections of the coated samples were investigated using EDS point and line analysis techniques to characterize the oxide scales formed and assess the depth of the corrosion attack. It is confirmed that the bare specimen showed maximum weight gain compared to the HVOF coated specimens. Minor spallation has been observed in both the uncoated and specimens coated with the Cr3 C2 –25%NiCr powder, however, not in the NiCrAlY coated specimen. Based on the weight gain results, the Cr3 C2 –25%NiCr coating exhibits less weight gain than the NiCrAlY coating. However, some failures were noticed in the leading edges of Cr3 C2 –25%NiCr coated samples with minor spallation and sputtering. The NiCrAlY powder-coated specimen displayed better corrosion resistance than the specimens coated with Cr3 C2 –25%NiCr powder due to the development of NiO, Cr2 O3, Al2 O3, and NiCr2 O4 scales on the surface of hot corroded samples. The NiCrAlY coating was found to have 20% reduced weight gain compared to the uncoated alloy. Highlights: HVOF coating of NiCrAlY and Cr3 C2 –25%NiCr coatings were deposited on the Ni-based alloy 825. Hot corrosion study was performed on the bare and coated alloy in simulated waste heat incineration environment at 750 °C. The coatings significantly reduced the corrosion rate of the alloy in salt condition. The coating surfaces and oxide scale was analyzed using SEM and XRD. The corrosion mechanism was clearly explained the scale formation and failure due to molten salt.