Investigation of Cyclic Oxidation and Hot Corrosion Behaviour of Plasma-Sprayed NiCrAlY/Cr₃C₂/h-BN/Mo Coatings on T22 Boiler Steel Alloy

At elevated temperatures, coating develops protective oxide scales on the surfaces, ensuring prolonged stability. In this investigation, ASTM-SA213-T-22 (T22) boiler tube steels underwent coating with NiCrAlY/Cr₃C₂/h-BN/Mo matrix using plasma spray to resist oxidation and hot corrosion. The study involved 50 cycles at 700 °C, evaluating hot corrosion in a molten salt atmosphere (Na₂SO₄–60%V₂O₅) and oxidation in static air. Oxidation kinetics was determined via thermogravimetric analysis, while XRD and SEM/EDAX methods characterized the produced oxide scales. The NiCrAlY/Cr₃C₂/h-BN/Mo coated T22 substrate demonstrated reduced oxidation rates compared to uncoated steels, with gradual scale growth kinetics and surface oxides of NiO and Cr₂O₃ in air. In molten salt, accelerated oxidation resulted in metastable Cr₂O₃ formation. Cr, Al, B, and Mo preferentially oxidized along the nickel-rich splat boundary, hindering oxygen penetration through pores and voids, thereby ensuring stable oxidation. The uncoated steel experienced rapid corrosion, characterized by the detachment and flaking off of the unprotected Fe₂O₃ oxide layer.