Effect of machining on corrosion behavior of Ni-based alloy GH3535 in molten LiF-NaF-KF salt

Ni-based alloys are extensively used in harsh service environments, where the condition of their surfaces often dictates component performance. This study investigates the profound effect of machining on the corrosion behavior of Ni-based alloy GH3535 in static molten LiF-NaF-KF (FLiNaK) salt at 650 °C. Unlike generic cold‑working, which introduces uniform deformation, machining creates a distinct gradient layer comprising a nanocrystalline region, a severely deformed region, and a slightly deformed region near the surface. The corrosion response of a machined surface has been directly compared to that of a polished surface, which serves as a proxy for the bulk alloy. While both surfaces exhibited intergranular corrosion, the machined specimen degraded far more severely. This is evidenced by significantly higher weight loss, and notably deeper corrosion penetration. After 1000 h, the average corrosion depth of the machined specimen (6.9 ± 0.4 μm) is approximately seven times greater than that of its polished counterparts (1.0 ± 0.3 μm). Microstructural analysis reveals that the subsurface nano-sized, severely deformed layer in the machined specimen undergoes recrystallization, grain coarsening, and secondary carbide precipitation during exposure. The accelerated corrosion is primarily attributed to the high-density grain boundary network in this layer, which provides rapid diffusion pathways for Cr dissolution. Consequently, intergranular attack leads to the dislodgment of these fine grains, substantially enhancing weight loss. Additionally, the precipitation of carbides also contributes to the accelerated corrosion behavior. These findings underscore that surface finishing is a critical, governing factor in the degradation of Ni-based alloys in molten fluoride salts.