## 4 Discussions
### Evolution of Corrosion Products
\[\gamma-\text{FeOOH}\rightarrow\text{FeO}_{x}(\text{OH})_{3-2x}\rightarrow \alpha-\text{FeOOH} \tag{8}\]  
As the corrosion reached 48 months, the content of \(\alpha\)-FeOOH exceeded that of \(\gamma\)-FeOOH, and the Fe\({}_{3}\)O\({}_{4}\)/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) content increased to 22.2-34.3% (Table 5). The formation of Fe\({}_{3}\)O\({}_{4}\) content indicated that the reduction reaction from Fe\({}^{3+}\) to Fe\({}^{2+}\) occurs in the rust layer [64; 79; 80; 81], and the rust layer on the steel surface acts as a strong oxidant during the wetting process. That is, some of the \(\gamma\)-FeOOH becomes a cathode depolarizing agent and is reduced to black Fe\({}_{3}\)O\({}_{4}\) (Equation (9)) [73; 82], while some is dissolved to form \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\). The Fe\({}_{3}\)O\({}_{4}\) content after 48 months of exposure was higher than that after 12 months (Figure 10; Table 5). This is due to the fact that the rust layer becomes thicker after 48 months of exposure, which makes it difficult for the inside of the rust layer to dry out. The longer the rust layer remains in the infiltration state, the more conducive it is to the formation of Fe\({}_{3}\)O\({}_{4}\). In addition, Fe\({}_{3}\)O\({}_{4}\) is typically formed near the steel substrate [83; 84], where the lower oxygen availability favors its development (Equation (10)) [85; 86]. The continuously generated \(\alpha\)-FeOOH increased the stability of the rust layer and strengthened its protectivity. This decreases the corrosion rate. However, magnetite that usually forms close to the steel substrate has high conductivity and acts as a large cathode area, which, from an electrochemical perspective, accelerates subsequent corrosion stages (as described by Equation (10)). Therefore, higher levels of magnetite are associated with lower protection by the formed rust layer, and increased magnetite content is conducive to an increased corrosion rate in long-term atmospheric corrosion [87]. \[8\gamma-\mathrm{FeOOH}+\mathrm{Fe^{2+}}+2\mathrm{e^{-}}\to 3\mathrm{Fe_{3}O_{4}}+4 \mathrm{H_{2}O} \tag{9}\]  
\[4\mathrm{Fe_{3}O_{4}}+\mathrm{O_{2}}+6\mathrm{H_{2}O}\to 12\gamma-\mathrm{FeOOH} \tag{10}\]  
According to studies, \(\mathrm{Fe_{3}O_{4}}/\gamma\)-\(\mathrm{Fe_{2}O_{3}}\) formed by long-term exposure to the atmosphere usually exists in the inner rust layer [30,88], implying that it is difficult to observe the micromorphology of \(\mathrm{Fe_{3}O_{4}}/\gamma\)-\(\mathrm{Fe_{2}O_{3}}\) in the SEM on the outermost surface of the rust layer. After 84 months of corrosion, the content of \(\alpha\)-FeOOH continued to increase, eventually reaching 39.2%, and the \(\mathrm{Fe_{3}O_{4}}/\gamma\)-\(\mathrm{Fe_{2}O_{3}}\) content increased, reaching 35.6% (Figure 10; Table 5). The globular formations with crystalline substructures in the form of lamina (labeled \"A\" in Figure 7e) are \(\gamma\)-FeOOH [73,74]; the thin and flat structures with whiskers (labeled \"B\" in Figure 7e) and cotton ball structures (labeled \"C\" in Figure 7f) are \(\alpha\)-FeOOH [4,31]. From the surface morphologies and the rust layer cross-sections (Figures 3c, 7 and 8), we can see that the thickness of the rust layer continuously increased (Figure 8c) and became denser after 84 months of long-term corrosion, while it is very easy to find very open structures such as large porosity (Figure 7c), spallation (Figure 13a), and cracking (Figures 7d and 13b) in the rust layer, providing access for corrosive media. Irregular cracks (Figures 7d and 13b) are typical manifestations of highly aggressive atmospheric corrosion [4]. The more corrosive the atmosphere is, the easier it is to find open structures [4]. The water-soluble \(\mathrm{FeSO_{4}}\) is expected to be present, and although the EDS results detected sulfur in the morphometric regions (Figures 4 and 5, Table 4), \(\mathrm{FeSO_{4}}\) was not detected by the EDS (Figure 10, Table 5). This may be due to its insufficiency for detection by the instrument.