Insights into the enhanced oxidation resistance of Incoloy 800H coating prepared via Electro-spark deposition

An Incoloy 800H coating was deposited on AISI 316L stainless steel (SS) via Electro-spark Deposition (ESD) to improve high-temperature steam oxidation resistance. ESD induced a unique non-equilibrium microstructure featuring refined columnar grains, a high fraction of low-angle grain boundaries (LAGBs) and random high-angle grain boundaries (RHAGBs), and significant compressive residual stress. This contrasts with the equiaxed grains and tensile stress in bulk Incoloy 800H and AISI 316L SS. During 650 °C/500 h steam oxidation, the ESD Incoloy 800H coating demonstrated superior oxidation resistance. It formed an ultra-thin oxide film, approximately 50 times thinner than the thick, Fe-rich oxides (hematite/magnetite) observed on the bulk AISI 316L and Incoloy 800H. A composite protective oxide film formed on the coating: an outer Cr-rich layer (Cr₂O₃ and (Mn, Ni)Cr₂O₄ spinel) and a critical inner, semicontinuous layer of SiO₂ and Al₂O₃ at the interface of oxide/metal. This exceptional performance is due to a synergistic protection mechanism. The high density of LAGBs/RHAGBs with high diffusivity provided rapid diffusion pathways, accelerating Si and Al transport and favoring the formation of the semicontinuous inner barrier layer, which stabilized the outer Cr-rich oxide film. The ESD-induced compressive stress benefited the stability and adhesion of the protective oxide film.