# Corrosion Behavior of Corrosion-Resistant Spring Steel Used in High Speed Railway
## 3 Results and Discussion
### Corrosion Mechanism  
When steel is exposed to the atmosphere, chemical corrosion occurs on the surface of steel under extremely dry air conditions. When the atmospheric relative humidity is high (generally more than 60%), a layer of liquid film is formed on the steel surface due to the adsorption of water, and the corrosion medium is dissolved in the liquid film. The formed electrolyte liquid film creates conditions for the electrochemical corrosion of steel in the atmospheric environment. Corrosion mechanism of steel in atmospheric environments without a rust layer is as follows:  
Cathodic reaction of electrochemical reaction:  
\[\text{O}_{2}+2\text{H}_{2}\text{O}+4\text{e}{\rightarrow}4\text{OH}^{-} \tag{2}\]  
Cathodic reaction of electrochemical reaction: in the heavily polluted industrial atmospheric environment, the following reactions occur:  
\[\text{SO}_{2}+\text{O}_{2}+2\text{e}{\rightarrow}\text{SO}_{4}^{2-} \tag{3}\]  
The anodic reaction is:  
\[\text{Fe}+n\text{H}_{2}\text{O}{\rightarrow}\text{Fe}^{n}+\text{H}_{2}\text{O }+n\text{e} \tag{4}\] \begin{table}
\begin{tabular}{c c c} \hline
**Element** & **Value** & **Error/\%** \\ \hline \(R_{sol}\) & 131 & 8.82 \\ \(C_{all}\) & 3.925 \(\times\) 10\({}^{-9}\) & 6.77 \\ \(R_{rust}\) & 710.3 & 5.18 \\ \(Q_{all}\) & 0.00115 & 11.55 \\ \(R_{ct}\) & 185 & 9.68 \\ \(W_{s}\) & 3.862 \(\times\) 10\({}^{-4}\) & 4.84 \\ \hline \end{tabular}
\end{table} 
Table 4: Values of the equivalent circuit fitting data of 60 Si2 Mn-B, the rusty sample. Figure 10: Fitting curve for the equivalent circuit of 60 Si2 Mn-B of the rusty sample. \begin{table}
\begin{tabular}{c c c c c} \hline
**Corrosion Time/h** & **Steel** & \(R_{sol}\)**/**(\(\Omega\cdot\)cm\({}^{2}\))** & \(R_{rust}\)**/**(\(\Omega\cdot\)cm\({}^{2}\))** & \(R_{ct}\)**/**(\(\Omega\cdot\)cm\({}^{2}\))** \\ \hline
24 h & 60 Si2 Mn-B & 114.2 & 449.1 & 107.1 \\
60 Si2 Mn-A & 117.5 & 399.6 & 92.1 \\ \hline
360 h & 60 Si2 Mn-B & 120.9 & 826.7 & 292.1 \\
60 Si2 Mn-A & 131.1 & 710.3 & 185.1 \\ \hline \end{tabular}
\end{table} 
Table 5: Values of EIS fitting data of experimental steel with rust. The anodic reaction is that FeSO\({}_{4}\) in the rust layer will undergo hydrolysis reaction to form FeOOH:  
\[\text{FeSO}_{4}+2\text{H}_{2}\text{O}\rightarrow\text{FeOOH}+\text{SO}_{4}{}^{2- }+3\text{H}^{+}+\text{e} \tag{5}\]  
When the rust layer is formed on the steel surface, the corrosion electrochemical reaction is different from the initial stage of corrosion. [PERSON] [31] proposed the corrosion model of steel in the atmospheric environment, as shown in Figure 11.  
The anodic reaction is between matrix Fe and Fe\({}_{3}\)O\({}_{4}\):  
\[\text{Fe}\rightarrow\text{Fe}^{2+}+2\text{e} \tag{6}\]  
The cathode reaction is between Fe\({}_{3}\)O\({}_{4}\) and FeOOH:  
\[6\text{FeOOH}+2\text{e}\rightarrow\text{2 Fe}_{3}\text{O}_{4}+2\text{H}_{2} \text{O}+2\text{OH}^{-} \tag{7}\]  
The rust layer participates in the cathodic reaction. Fe\({}^{3+}\) in the rust layer is reduced to Fe\({}^{2+}\), and after the rust layer is dried, the Fe/Fe\({}_{3}\)O\({}_{4}\) interface changes to open circuit, and the oxygen in the air oxidizes Fe\({}^{2+}\) in the rust layer to Fe\({}^{3+}\):  
\[4\text{Fe}_{3}\text{O}_{4}+\text{O}_{2}+6\text{H}_{2}\text{O}\rightarrow\text {6 FeOOH} \tag{8}\]  
With the extension of corrosion time, the corrosion in the rust layer in the later stage of corrosion \(\alpha\)- FeOOH can effectively improve the density of the rust layer, and the isolation effect of the rust layer on the corrosive medium in water and atmosphere becomes stronger and stronger, thus reducing the area where the corrosion reaction can occur. At the same time, due to the thickening of the rust layer and the accumulation of corrosion-resistant elements in the rust layer near the substrate, the impedance value of the whole corrosion electrochemical-reaction system increases, and corrosion electrochemical reaction becomes more and more difficult.