## 5 Conclusions  
This study investigated the corrosion behavior and mechanisms of Q235 steel exposed to the typical industrial atmosphere (Taiyuan) for seven years. The conclusions are obtained as follows:  
1. The corrosion kinetics of Q235 steel exposed to the urban industrial atmosphere for a long term follow a two-stage corrosion power function law. The atmospheric corrosion evolution law for short-term exposure differs from that for long-term exposure. Therefore, the short-term corrosion test results fail to fully reflect the corrosion performance of Q235 steel in these environments. SO\({}_{2}\) significantly affects the corrosion behavior of steel.
2. The morphology of the corrosion products of Q235 steel in the urban industrial atmosphere changes from loose and flat to granular, and finally to compact and smooth. Although the outer rust layer was easy to fall off, the thickness of the inner and outer rust layers increases with increased exposure time. The various forms of  
Figure 14: The schematic diagram of the corrosion mechanism.  
corrosion products connect and agglomerate with each other and possess cavities, cracks, and spallation, reducing the rust layer's protection.
* The rust layer crystalline phase contains \(\alpha\)-FeOOH, \(\gamma\)-FeOOH, Fe\({}_{3}\)O\({}_{4}\)/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), and \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) in the urban industrial atmosphere. Corrosion products in the initial period are mainly \(\gamma\)-FeOOH, followed by \(\alpha\)-FeOOH, and only a small amount of Fe\({}_{3}\)O\({}_{4}\)/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\). With the increase in exposure time, \(\alpha\)-FeOOH and Fe\({}_{3}\)O\({}_{4}\)/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) in the rust layer increases. The concentration of SO\({}_{2}\) in the atmosphere affects the formation of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\). In long-term atmospheric corrosion, the increase in Fe\({}_{3}\)O\({}_{4}\)/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) content is beneficial to improving the corrosion rate.
* In the urban industrial atmosphere, \(\gamma\)-FeOOH typically has bird's nest structures, worm's nest structures, globular structures with crystalline substructures, and hair-like structures; \(\alpha\)-FeOOH has laminar petal structures, cloud-like structures, laminar structures with whiskers, and cotton ball structures.
* The corrosion rate is impacted by the composition of the rust layer and can be measured with the \(\alpha\)/\(\gamma\) ratio. When the exposure time is no more than 48 months, \(\alpha\)/\(\gamma\) decreases with the increasing exposure time, indicating weak rust layer protection and a high corrosion rate. After 84 months of corrosion, \(\alpha\)/\(\gamma\) increases, indicating increased rust layer protection and a decreased corrosion rate.  
Conceptualization, H.W., Y.L. and G.Z.; methodology, H.W.; validation, Y.L. and G.Z.; formal analysis, H.W.; investigation, H.W.; resources, H.W.; data curation, H.W.; writing--original draft preparation, H.W.; writing--review and editing, Y.L. and G.Z.; visualization, H.W.; supervision, Y.L.; project administration, Y.L.; funding acquisition, H.W. and G.Z. All authors have read and agreed to the published version of the manuscript.  
This work is supported by the Applied Basic Research Program of Shanxi Province (NO. 201901D211177), to which the authors are very grateful.  
Not applicable.  
The authors declare no conflict of interest.