## 3 Results
### Morphologies of the Rust Layers  
A digital camera was used to observe the macrosurface morphology of the rust layer exposed to an urban industrial atmosphere. As shown in Figure 3, after 12 months of atmospheric exposure, the rust layers were evenly distributed with red-brown colored loose structures. After 48 months of atmospheric exposure, the rust layers were red-brown with evenly distributed etch pits. After 84 months of atmospheric exposure, the rust layers were yellow-brown and dense, and the etch pits expanded. To summarize, as the corrosion time increased, the color of the rust layers deepened from red-brown to yellow-brown,  
Figure 2: The mass loss of steel samples exposed to an urban industrial atmosphere versus the time in Taiyuan. revealing changes that occur gradually in the composition of the rust layers or the relative content of each component [61]. Furthermore, the rust layers thicken over time. The SEM micro-morphology of the Q235B steel rust surface layer at 2, 12, 41, and 84 months of exposure to the urban industrial atmosphere is shown in Figures 4-7. After 2 months of exposure, the corrosion products of the Q235 steel were relatively loose and flat, with areas of numerous bird's nest formations (Figure 4a,b). After 12 months of corrosion, the corrosion products were mainly small particles, and the local corrosion products were loose and flat (Figure 5a,b). The loose and flat regions present mainly as worm's nest formations (labeled \"A\" in Figure 5c,e). The particle regions (Figure 5c-f) were mainly hair-like formations (labeled \"B\") in the bottom layer, cloud-like formations (labeled \"C\") above, and the agglomeration (labeled \"D\") of the two, globular formations with an amorphous appearance (labeled \"E\"), and globular formations with crystalline substructure in the form of needles (labeled \"F\"). After 41 months of corrosion, the corrosion products exhibited mainly particle shape with increased particle size and a tendency to compact into sheets (Figure 6a). The typical morphologies of corrosion products were laminar petal formations (labeled \"A\"), globular formations with a crystalline substructure (labeled \"B\"), and other globular formations composed of petal formations (labeled \"C\") (Figure 6b,c). The hair-like and cloud-like structures shown in Figure 5f can still be observed after 41 months of corrosion. After 84 months of corrosion, some of the partial corrosion products were still loose, and some had compacted into dense, smooth corrosion layers with cracks (Figure 7a,b). The loose areas were dominated by granular corrosion products with different morphologies, including globular formations with crystalline substructures in the form of lamina (labeled \"A\") and thin and flat sheet structures with whiskers (labeled \"B\") (Figure 7c,e). The main corrosion products in the dense and smooth areas were cotton ball structures (labeled \"C\") and compacted whisker structures (labeled \"D\") (Figure 7d,f). Figure 3: Surface morphology of the rust layers at different atmospheric exposure times: (**a**) 12 months of urban industrial atmospheric corrosion, (**b**) 48 months of urban industrial atmospheric corrosion, and (**c**) 84 months of urban industrial atmospheric corrosion. Figure 4: Surface microscopic morphologies of the rust layer and EDS analyses after 2 months of atmospheric exposure. Figure 5: Surface microscopic morphologies of the rust layer and EDS analyses after 12 months of atmospheric exposure. The corrosion products of Q235 structural steel exposed to the urban industrial atmospheric environment transformed from loose and flat to granular, and then to compact and smooth. Corrosion products of various forms interconnect and agglomerate loosely. They all have cavities and fractures, making the surface rust layers incapable of providing sufficient protection.