*3.5. TEM Analysis*

TEM image of the SA106B carbon steel sample was depicted in Figure 10a. Figure 10a, b show TEM bright-field micrograph and selected-area electron diffraction (SAED) patterns, respectively, in the top surface layer of the sample RASP-processed for 5 min. One can note that the crystal grains have been refined into nanocrystal grains of relatively equiaxed and uniformly distributed compared with the irregular crystal grains of the matrix. The mean grain size is 25 nm (Figure 10c). Note that the SAED pattern of the sample consists of rings. It is known that the more continuous the rings, the smaller the grain sizes within the selected field of the view and the more uniform the distribution of grains [29]. The result demonstrates that the grain size of the steel can be markedly reduced when they are RASP-processed for 5 min. It is noteworthy that some spots spreading along the rings (Figure 10b) were found, indicating that there are also some coarse grains in the samples.

**Figure 10.** (**a**) TEM image of the top surface layer of the sample RASP-processed for 5 min; (**b**) the corresponding selected-area electron diffraction (SAED) pattern in (**a**); (**c**) grain size distribution in the top surface layer.

Figure 11 shows typical TEM images of the SA106B low-carbon steel, (a) RASP-processed for 0 min, (b) RASP-processed for 5 min and (c) shows the corresponding SAED pattern. In the carbon steel specimens untreated by the peening, one can see that the cementite is parallel, evenly distributed and the width of lamellar spacing is about 200 nm, as shown in Figure 11a. As shown in Figure 11b, the main result concerns the cementite phase which underwent a dissolution, at least partial, during SPD. The parallel distributions were changed severely. The diffraction spot, as shown in Figure 11c, confirmed the formation of supersaturated carbon atom ferrite. It is plausible that because the dislocation activity and cementite dissolution occurred simultaneously during plastic deformation, the carbon atoms can be dragged out of the cementite by mobile dislocations [30]. One study [31] has shown that cementite dissolution takes place through a global mechanism involving the whole volume of each individual lamellae, resulting in a carbon concentration gradient from cementite to ferrite. Indeed, as cementite partly dissolves, a large amount of carbon atoms is released, and must therefore be partitioned in the ferrite. Owing to the importance of the dissolution, the average carbon content in the ferrite may reach 1–2 at.%. After the cementite dissolution, a solid solution formed, which increased the electrode potential of the matrix [32]. Thus, the number of micro batteries reduced and the corrosion film was more stable and the surface layer of the sample was flattened.

**Figure 11.** TEM images of the SA106B low-carbon steel. (**a**) RASP-processed for 0 min; (**b**) RASP-processed for 5 min; (**c**) The corresponding selected-area electron diffraction (SAED) pattern in (b).

#### **4. Conclusions**

In the present work, the influence of corrosion resistance of a SA106B low-carbon steel with gradient nanostructure produced by RASP was investigated. The main conclusions can be drawn as follows:


(3) Prominent micro-cracks and holes were produced in the steel when the RASP was processed more than 5 min, resulting in the decrease of corrosion resistance.

**Author Contributions:** Writing-original draft preparation, figures and data curation, C.L.; data analysis, C.L., X.C., Y.C. and B.Y.; literature search, X.C.; study design, Y.L. and B.Y.; Writing—review and editing, B.Y.

**Funding:** This research was founded by the Beijing Municipal Natural Science Foundation under No 2162026 and the Beijing Municipal Science and Technology Commission under No Z181100005218005.

**Conflicts of Interest:** The authors declare no conflict of interest.
