Microstructure and Corrosion Characteristics of Composite Coating Developed on T22 Steel Using Plasma Thermal Spray Method

High-temperature corrosion is a crucial issue in power plant components such as boilers and superheaters when they operate in high-temperature aggressive environments leading to early component failure. As a result, surface modification is critical to protect parts against various types of degradations and increase operational performance at the lowest possible cost. The present study deals with Metco 42C and cenosphere-based composite coating on ASME A387 Grade 22 boiler steel material. The coating compositions were prepared by weight fraction of 5% and 10% cenosphere with Metco 42C and coated on T22 boiler tube material by plasma spray technique. To check the performance a high-temperature cyclic oxidation study at 600°C for 20 cycles in a molten salt environment of sodium sulfate (40%Na₂SO₄) and vanadium pentoxide (60%V₂O₅) was carried out and simultaneously corrosion kinetics was evaluated after each cycle. Neutral salt spray (NSS) and cyclic corrosion tests were carried out at defined temperature and humidity cycles in a salt mist environment to understand the relative corrosion resistance of coated and uncoated substrates. The characterization techniques such as scanning electron microscope (SEM) and X-ray diffraction (XRD) were utilized to check the morphology of tested samples. The bond strength, porosity, and hardness tests were done independently to characterize the coating. The results of the high-temperature oxidation test showed protective oxide layers formation on coated samples, improving high-temperature oxidation resistance. Additionally, the neutral salt spray and cyclic corrosion test results of coated substrate showed higher relative corrosion resistance as compared to uncoated substrate.