Synergistic Protection: HVOF-Sprayed 304L Coatings Mitigate Cyclic Oxidation in T11 and Superfer800 via Cr-Rich Scale Engineering

High-temperature components such as boiler tubes and heat-exchange surfaces often face premature degradation because conventional alloys offer limited resistance to cyclic oxidation. To mitigate this problem, the behaviour of HVOF-deposited 304L stainless-steel coatings on T11 and Superfer800 substrates was examined under repeated thermal exposure at 700 °C. The coated layers exhibited a compact, uniform morphology and a significant increase in surface hardness, rising to 975 HV0.3 for T11 and 1075 HV0.3 for Superfer800. Cyclic mass-change measurements over 50 heating–cooling cycles highlighted the protective role of the coating. Bare T11 showed the greatest mass gain, whereas the coated T11 specimens observed roughly two-thirds lower oxidation, yielding performance comparable to uncoated Superfer800. EDS analysis indicated the development of a stable chromium-rich oxide scale on the coated surfaces, acting as an effective diffusion barrier. The comparative results reveal that a single 304L HVOF coating can provide a cost-effective means of strengthening oxidation resistance across alloys of differing baseline performance. Overall, the study demonstrates the practicality of employing stainless-steel thermal-spray coatings to extend the service life of materials used in boiler and heat-exchanger environments.