• The CUSR process induces a gradient nanocrystalline surface layer with uniform Volta-potential and compositional homogeneity in sensitized stainless steel. • Twinning-assisted nanocrystallization facilitates the rapid formation of a compact passive film and reduces potential differences associated with sensitization-induced segregation. • Remarkable enhancement in corrosion resistance is achieved by CUSR, surpassing both non-sensitized and RTUSR-treated counterparts through synergistic microstructural and electrochemical modifications. Corrosion is a leading cause of catastrophic safety and environmental accidents in harsh service environments, particularly for stainless steels prone to sensitization. This study demonstrates that cryogenic ultrasonic surface rolling (CUSR) can markedly enhance the corrosion resistance of sensitized stainless steel. At cryogenic temperatures, the reduced stacking fault energy promotes the formation of deformation twins at the early stages of cryogenic ultrasonic rolling. With continued deformation, dislocations accumulate within the twins, forming dislocation walls that subdivide lenticular twins into elongated nanograins bounded by twin and low-angle grain boundaries. Further strain induces grain rotation and boundary bulging, transforming these structures into equiaxed nanograins surrounded by high-angle grain boundaries. This multi-stage refinement constitutes a twinning-assisted gradient nanocrystallization process. The resulting gradient nanostructure improves compositional uniformity and mitigates potential differences arising from sensitization-induced secondary phases along grain boundaries. Moreover, the refined substructures diminish potential variations between grain interiors and boundaries, facilitating the simultaneous nucleation and growth of passive films across multiple grain boundaries. This accelerates rapid, efficient passivation and yields a dense, protective film. Overall, the CUSR strategy provides an effective post-sensitization treatment that enhances corrosion resistance by promoting compositional homogeneity and rapid passivation in sensitized stainless steel.
Liu et al. (Sun,) studied this question.