Formation of compositionally graded grains and molten-salt corrosion behavior in wire-arc additive manufactured NiMoCr alloy-cladded steel

A nickel-based alloy containing Mo and Cr as the primary alloying elements (NiMoCr) was deposited onto 316 L stainless steel via wire-arc additive manufacturing (WAAM), and its microstructural evolution and high-temperature corrosion behavior in molten FLiNaK salt at 750 °C were investigated. The as-deposited cladding exhibited a highly textured dendritic γ-Ni matrix with significant Mo segregation and minor carbide formation in interdendritic regions. At the cladding-substrate interface, compositionally graded grains (CGGs) developed across a transition zone, displaying smooth chemical and crystallographic continuity without a distinct boundary. Corrosion testing for 500 h revealed corrosion rates of 0.11 mm/year for the NiMoCr cladding, 0.29 mm/year for the steel substrate, and 0.18 mm/year for the bistructure, indicating a gradient in corrosion resistance across the system. Post-exposure analysis and thermodynamic modelling showed that the steel substrate underwent intergranular corrosion, driven by rapid Cr diffusion and depletion along grain boundaries, further accelerated by galvanic coupling with the NiMoCr cladding. In contrast, Mo segregation in the NiMoCr alloy suppressed Cr diffusion and promoted the dynamic formation of corrosion-resistant σ-phase precipitates. These precipitates, along with the surrounding Mo-enriched matrix, mitigated galvanic interactions and shifted the dominant corrosion mode from interdendritic to intradendritic. Moreover, the CGGs helped maintain interface integrity by forming a transition zone that did not undergo preferential degradation.