## 4 Discussions
### Evolution of Corrosion Products  
In an urban industrial atmosphere, complex electrochemical reactions occur on steel surfaces. The phase composition of the rust layers depends mainly on the anions in the electrolyte film. The content of dissolved oxygen at the metal/rust layer interface is relatively high, and the cathode is mainly oxygen reduction, an oxygen-absorbing corrosion (Equations (2) and (3)). The initially generated Fe (OH)2 is unstable. Some of the Fe(OH)2 rapidly oxidizes to Fe(OH)3 (Equation (4)), which is partially dehydrated to generate red-brown Fe2O3.nH2O. Some of the Fe (OH)2 may generate FeOOH in the presence of O2 (Equation (5)) [70; 71]. \[\mathrm{Anode:Fe\to Fe^{2+}+2e^{-}} \tag{2}\]  
\[\mathrm{Cathode:O_{2}+4e+2H_{2}O\to 4 OH^{-}} \tag{3}\]  
The main pollutant in Taiyuan is SO2, which is easily soluble in water. The thin liquid film on the steel surface absorbs SO2 from the atmosphere to generate H2 SO3. A small amount of H2 SO3 is gradually oxidized to H2 SO4, which reduces the electrolyte pH, accelerates the dissolution of the anode, and speeds up the corrosion rate of the steel (Equations (6) and (7)) [37; 72]. \[2\mathrm{Fe(OH)}_{2}+1/2\mathrm{O_{2}+H_{2}O\to 2 Fe(OH)}_{3} \tag{4}\]  
\[2\mathrm{Fe(OH)}^{+}+1/2\mathrm{O_{2}+2 OH^{-}\to 2 FeOOH+H_{2}O} \tag{5}\]\[4\text{FeSO}_{4}+6\text{H}_{2}\text{O}+\text{O}_{2}\to 4\text{FeOOH}+4\text{H}_{2} \text{SO}_{4} \tag{6}\]  
\[2\text{H}_{2}\text{SO}_{4}+2\text{Fe}+\text{O}_{2}\to 2\text{FeSO}_{4}+2\text{H}_{2} \text{O} \tag{7}\]  
After two months of exposure to the atmospheric environment, numerous areas with bird's nest structures formed in the corrosion products of Q235 steel (Figure 4). The bird's nest structure is a typical morphology of \(\gamma\)-FeOOH [4; 12; 31; 73; 74]. Therefore, the main corrosion products after two months of exposure are \(\gamma\)-FeOOH. After 12 months of corrosion, the worm's nest structure (labeled \"A\" in Figure 5c,e), the hair-like structure (labeled \"B\" in Figure 5f), and the globular structure with an amorphous appearance (labeled \"E\" in Figure 5c,d), are all the morphologies of \(\gamma\)-FeOOH [73; 74]; the cloud-like structures (labeled \"C\" in Figure 5f) were probably morphologies of \(\alpha\)-FeOOH [31]; the globular structure formed by the agglomeration of cloud-like and hair-like structures (labeled \"D\" in Figure 5f), and the globular formation with a crystalline substructure in the form of a needle(labeled \"F\" in Figure 5e), are all mixtures of \(\gamma\)-FeOOH and \(\alpha\)-FeOOH [73; 74]. Figure 6c shows the typical morphologies of \(\gamma\)-FeOOH and \(\alpha\)-FeOOH formed on steel after 41 months of corrosion. \(\alpha\)-FeOOH appears as laminar petal formations (labeled \"A\") [73], and \(\gamma\)-FeOOH is globular with crystalline substructures (labeled \"B\") [73; 74]. Some petal structures also appear in the globular formations (labeled \"C\") and are a mixture of \(\gamma\)-FeOOH and \(\alpha\)-FeOOH. These features may indicate growing \(\alpha\)-FeOOH that was replacing lepidocrocite. The main corrosion product in the initial stages of corrosion is \(\gamma\)-FeOOH, but with increasing exposure time, \(\gamma\)-FeOOH is converted into amorphous FeOOH through dissolution and precipitation, and then gradually transforms into \(\alpha\)-FeOOH [75] (Equation (8)). [PERSON] et al. [45] also believed that the amorphous inner layer of steel may transform into densely packed goethite nanoparticles during long-term exposure. The XRD results in Figure 10 and Table 5 further support that within one year of atmospheric corrosion, the corrosion products were mainly \(\gamma\)-FeOOH, followed by \(\alpha\)-FeOOH. SO\({}_{2}\) is generally considered to be the most corrosive gas in the atmosphere [76] and contributes to the formation of \(\alpha\)-FeOOH [77; 78].