# Corrosion Behavior of Corrosion-Resistant Spring Steel Used in High Speed Railway
## 3 Results and Discussion
### Polarization Curve of the Sample after Corrosion  
Figure 7 shows the polarization curve of 60 Si2 Mn-B and 60 Si2 Mn-A samples after various times of the corrosion. The polarization curve represents the relationship between the potential of self-etching electrode and the current density. As can be seen from the figure, the corrosion potential of 60 Si2 Mn-B is higher than that of 60 Si2 Mn-A in any period. With the extension of the corrosion period, the corrosion potential of the experimental steel also keeps increasing, which is due to the increase of the thickness of the rust layer and the improvement of the structure, which further hinders the electrochemical corrosion; the resistance to corrosion becomes higher, and the corrosion tendency decreases. In addition, it can be seen from the figure that both polarization curves violate the linear law in the strongly polarized region. The reason is that charge transfer is not the only control step in the electrochemical reaction. When the polarization current density is high, the solution between the reference electrode and the metal electrode will produce a large ohmic potential drop [29].  
Figure 5: Cross-section morphology and element mapping distribution of rust layer section of 60 Si2 Mn-A sample in the period of 360 h: (**a**) cross-section of rust layer; (**b–d**) element-mapping distribution of Cu, Ni, and Cr.  
Figure 6: Chemical analysis of rust section of 60 Si2 Mn-B sample in the period of 360 h: (**a**) cross-section of rust layer; (**b**) Cu, (**c**) Ni, and (**d**) Cr.  
The polarization curve was fitted using C view software and traditional Tafel method, and the fitting results were shown in Table 3. It can be seen from the table that the range of corrosion potential of the experimental steel is \(-0.3\) V\(-\)\(-\)0.04 V, and the variation rule is the same as the polarization curve. The corrosion current density at 24 h is on the order of \(10^{-5}\) A/cm\({}^{2}\), and at 360 h, it is on the order of \(10^{-6}\) A/cm\({}^{2}\). The corrosion current decreases with the extension of the corrosion cycle. The corrosion rate of 60 Si2 Mn-B in the same period is lower than the 60 Si2 Mn-A, which is consistent with the change rule of weight loss rate. As mentioned in the preceding section, corrosion-resistant elements are enriched in the rust layer on the side near the matrix to form a protective layer between the matrix and the rust layer, which hinders the corrosion electrochemical reaction between the matrix and electrolyte solution, reduces the reaction rate, and then reduces the corrosion current. The corrosion-resistant element also promotes the production of \(\alpha\)-FeOOH, which is an insulator and increases the resistance of the rust layer.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Corrosion Time/h** & **Steel** & **Corrosion Current/(A/cm\({}^{2}\))** & **Corrosion Potential/V** & **Corrosion Rate/(mm/a)** \\ \hline
24 h & 60 Si2 Mn-A & \(1.6571\times 10^{-5}\) & \(-0.3391\) & 0.19367 \\
60 Si2 Mn-B & \(1.3934\times 10^{-5}\) & \(-0.30091\) & 0.16285 \\ \hline
72 h & 60 Si2 Mn-A & \(1.0056\times 10^{-5}\) & \(-0.24608\) & 0.13839 \\
60 Si2 Mn-B & \(9.8076\times 10^{-6}\) & \(-0.20574\) & 0.11462 \\ \hline
360 h & 60 Si2 Mn-A & \(7.5821\times 10^{-6}\) & \(-0.12557\) & 0.088613 \\
60 Si2 Mn-B & \(5.3492\times 10^{-6}\) & \(-0.04382\) & 0.062518 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Values of parameters fitted from polarization curves of steels.  
Figure 7: Polarization curves of experimental steels with different corrosion cycles: (**a**) 24 h, (**b**) 72 h, and (**c**) 360 h.