## 3 Results
### Morphologies of the Rust Layers
EDS analysis was performed on the surface of the rust layers, as presented in Table 4. The results show that the bird's nest (Point 8 (Figure 4b)), worm's nest (Point 14 (Figure 5c)), hair-like (Point 17 (Figure 5f)), and cloud-like morphological structures (Point 19 (Figure 5f)) and those formed from the agglomeration of cloud-like and hair-like structures (Point 13 (Figure 5c)) all contained sulfur. Sulfur was not detected in the laminar petal morphological or globular formations with crystalline substructures, or other globular formations composed of petal formations (Points 10-12 (Figure 6c)). Meanwhile, thin and flat sheet structures with whiskers and globular formations with crystalline substructures in the form of lamina (Points 41 and 43 (Figure 7e)) and compacted whisker structures also did not contain sulfur (Point 45 (Figure 7d)). Sulfur may come from the pollutants in the industrial atmosphere. The other elements that were detected, namely Si, Mn, Al, K, P, and In, were present on the surface of the rust layers, indicating that dust particles suspended in the atmosphere are deposited on the surface. Figure 6: Surface microscopic morphologies of the rust layer and EDS analyses after 41 months of atmospheric exposure. Figure 7: Surface microscopic morphologies of the rust layer and EDS analyses after 84 months of atmospheric exposure. Figure 8 shows the cross-sectional morphologies of the rust layers at different urban industrial atmospheric exposure times. Distinguisable inner and outer rust layers were observed regardless of the exposure time. The inner layer was attached to the steel substrate with an irregular corrosion profile, i.e., the thickness of the inner rust layer was not uniform, which was mainly due to the non-uniform distribution of physical and chemical factors, such as environmental deposits and the direction of crystallites in the steel [3]. With the increase in corrosion time, the thickness of the inner rust layer increased; the thickness of the inner rust layer was about 0-50 mm after 12 months of atmospheric exposure (Figure 8a), 50-100 mm after 41 months (Figure 8b), and 100-200 mm after 84 months (Figure 8c). The dense inner rust layer, compactly attached to the steel, prevents the intrusion of corrosive media and protects the substrate from further corrosion. The thickness of the outer rust layer was uniform and tended to increase as the corrosion time increased. However, the overall thickness was thin because the stress caused by large temperature fluctuations and alternating wet/dry cycles [62] weakened the adhesion of the rust layers and caused them to detach easily. The outer rust layer was loose, had more cavities, cracks, and gaps, and was not tightly connected to the inner rust layer. Large cracks easily penetrate between the inner and outer rust layers, providing paths for corrosive media, mainly O\({}_{2}\) and SO\({}_{2}\). The outer layer had little inhibition against corrosive media and allowed further corrosion of the steel substrate to occur. The EDS line surface scan image shown in Figure 8d corresponds to the yellow line in Figure 8b. Elemental analysis of the EDS spectrum shows a distinct boundary between the inner rust layer and the uncorroded steel substrate, where O and Fe content differences are most obvious. The corrosion products of the inner rust layer were mainly compounds that contained Fe and O. Corroded steels in urban industrial atmospheric environments have an obvious O and Fe content transition zone (about 5 \(\upmu\)m in length) between the rust layer and steel substrate. The existence of the transition zone indicates that the corrosive media permeates and diffuses into the intact steel substrate adjacent to the rust layer through grain boundaries and adjacent areas, causing local corrosion (Figure 9).