Effect of surface preparation on crack initiation of Ni-based alloy GH3535 in molten LiF-NaF-KF salt under creep testing condition

The combined effects of high-temperature creep and molten salt corrosion present critical challenges for structural materials in molten salt reactors (MSRs). In the present study, the influence of surface preparation on the crack initiation behavior of Ni-based alloy GH3535 under simultaneous creep deformation and molten salt corrosion was investigated. Specimens with three different surface conditions were prepared and tested in molten LiF–NaF–KF salt at 700 °C under identical constant stress. The post-test specimens were then examined using multi-scale techniques. The results show that extensive intergranular cracking occurred in the as-machined specimen. Removing the machined layer by grinding and polishing produced specimens with relatively uniform microstructure, which suppressed subsurface crack nucleation but promoted the initiation of shallow intergranular cracks from the surface. In the high temperature annealed specimen, the machined layer was eliminated due to recrystallization. The annealing process suppressed subsurface creep crack initiation but promoted localized intergranular corrosion, leading to the detachment of recrystallized grains from the surface and subsequent surface driven cracking. These findings demonstrate that the near surface microstructure critically governs the synergy between corrosion and crack initiation of the alloy GH3535 in molten salt. While removing the machining induced zone avoids subsurface cracking, it may increase the susceptibility to intergranular corrosion and surface crack initiation, thereby providing essential guidance for surface state optimization strategies for materials in MSR applications.