# Corrosion Behavior of Corrosion-Resistant Spring Steel Used in High Speed Railway
## 3 Results and Discussion
### The Cross-Section Morphology and Chemical Element Distribution of Rust Layer  
The cross-section morphology and thickness of rust layer of samples corroded by 60 Si2 Mn-A and 60 Si2 Mn-B for 24 h, 72 h, and 360 h were observed by JXA 8530F electron probe. The results are shown in Figure 4 and Table 2. The light-colored part is the metal matrix, the gray part is the rust layer, and the dark part is the Mosaic. It can be seen from Figure 4 that the thickness of the rust layer of the experimental steel increases with the corrosion time, and the structure of the rust layer also changes from loose to dense, especially close to the rust layer on one side of the matrix. At the corrosion time of 360 h, the 60 Si2 Mn-B cross-section rust layer can be seen to have a relatively obvious boundary in the rust layer, forming the inner rust layer and the outer rust layer. As can be seen from Table 2, with the same corrosion period, the thickness of the 60 Si2 Mn-A rust layer is greater than that of the 60 Si2 Mn-B, and with the increase of corrosion time, the thickness difference increases, indicating that the corrosion rate of the 60 Si2 Mn-B is lower. Figure 2: X-ray analysis of corrosion products of experimental steel after different corrosion time: (**a**) 24 h; (**b**) 72 h; and (**c**) 360 h.  
Figure 4: Section micromorphology of rust layer of experimental steel after corrosion for different time: (**a–c**) 60 Si2 Mn-B after corrosion for 24 h, 72 h, and 360 h; (**d–f**) 60 Si2 Mn-A after corrosion for 24 h, 72 h, and 360 h.  
Figure 3: Micromorphology of rust layer of experimental steel after corrosion for different time: (**a–c**) 60 Si2 Mn-A after corrosion for 24 h, 72 h, and 360 h; (**d–f**) corrosion resistant 60 Si2 Mn-B after corrosion for 24 h, 72 h, and 360 h.  
The distribution of Cu, Ni, and Cr elements in the cross-sections of the rust layer of 60 Si2 Mn-A and 60 Si2 Mn-B for 72 h and 360 h were analyzed. In order to distinguish the rust layer from the matrix, Fe elements were scanned for reference. At the time of 72 h corrosion, due to the small content of Cu and Ni in the 60 Si2 Mn-A, there was no significant difference between the matrix and the rust layer. The Cu and Ni elements in the 60 Si2 Mn-B are dispersed in the rust layer. In the experimental steel, Cr element was partially enriched on the side close to the inner rust layer for 72 h after corrosion. As the weight loss rate decreased in the early corrosion stage, the contribution of Cr element was the most obvious. This was mainly because Cr element formed composite oxides with other metals on the steel surface to protect the matrix [26]. As its content increases, it forms \(\alpha\)-(Fe\({}_{1-x}\), Cr\({}_{x}\))OOH (Cr-Goethite) in the rust layer. The finer crystals make the rust layer denser, causing the rust layer to open up and the potential to move forward, weakening the corrosion tendency of the material. Moreover, the OXIDE containing Cr has cationic selectivity, which can effectively prevent corrosive media, such as Cl\({}^{-}\) and SO\({}_{4}\)[2]\({}^{-}\), from invading from the outer rust layer to the inner rust layer [27]. The element distribution of rust layer after 360 h corrosion is shown in Figures 5 and 6. There is no obvious difference between the matrix and the outer layer in 60 Si2 Mn-A. In 60 Si2 Mn-B, element Cr has obvious bias and is uniformly distributed in the rust layer close to the matrix, and elements Cu and Ni also produce enrichment in the rust layer close to the matrix. In addition to the corrosion resistance mechanism mentioned in chemical element design, Cu can also promote the transformation from \(\gamma\)-FeOOH to the most stable \(\alpha\)-FeOOH. According to the surface enrichment theory, Cu enriched in the matrix precipitates from the matrix and forms oxides together with Fe, which increases the impedance of the rust layer, causes anodic passivation, and slows down electrochemical corrosion. The oxide formed can repair the holes and other defects of the rust layer and improve the densification of the rust layer.