Revealing What Enhance the Corrosion Resistance beside Grain Size in Ultrafine Grained Materials by Severe Plastic Deformation: Stainless Steels Case

Studies have shown that the corrosion resistance of stainless steels in passive environments is enhanced by grain refinement into the order of submicron or nanoscale via various methods, including severe plastic deformation (SPD). This beneficial effect has been attributed to the enhanced protective nature of the passive film due to a greater Cr enrichment in the film. Two independent mechanisms for the greater Cr enrichment in passive films have been proposed: enhanced selective dissolution of Fe and faster Cr diffusion. Both mechanisms originate from high density grain boundaries. However, recent studies have used high-resolution scanning transmission or in-situ atomic force microscopy to visualize the near atomic-scale passivation process and suggest that the increased protectiveness of passive films caused by the Cr enrichment is limited to a zone in the vicinity of grain boundaries. This finding suggests that both these mechanisms, facilitated by grain refinement, might be capable of the homogeneous passive film formation over the entire surface if the grain size is extremely small (＜100 nm), which most classical SPD methods, represented here by equal channel angular pressing, cannot achieve. Therefore, for the formation of a uniform and homogeneous passive film inside all the grains, the role of factors other than that of grain size might be involved. A fresh review of the literature on the corrosion behavior of ultrafine grained (UFG) stainless steels with grain size smaller than 1 μm and nanocrystalline ones smaller than 100 nm, generated by classical SPD, surface SPD, and other physical methods, was undertaken in light of the uniformity of the passive film. The possible role of high internal stress and residual dislocations, which are common constituents of UFG materials obtained by SPD, on the formation of the protective passive film was discussed.