# Salt-Fog Corrosion Behavior of GCr15 Steels Treated by Ultrasonic Strengthening Grinding Process
## 3 Results and Discussion  
Figure 3 reports the Nyquist diagram of all the samples immersed in 3.5% NaCl solution made in the laboratory for 96 h. The capacitive loop was utilized to characterize the charge transfer resistance at the interface of corrosion products. It can be seen that the capacitive loop of Sample polished is much smaller than the other samples, which can be attributed to the accumulation of corrosion products. The capacitive loops of the samples with surface treatment are significantly improved. In particular, the biggest capacitive arc radius was obtained in Sample WUSGP. The magnitudes of the charge transfer resistance of all the samples are ranked as follows: WUSGP > DUSGP > USP > Polished. The corrosion potential (E\({}_{\text{corr}}\)) and the corrosion current density (I\({}_{\text{corr}}\)) are critical parameters to evaluate the performance of corrosion resistance, which are calculated through Tafel extrapolation [36]. The polarization curves of the samples and the obtained results are reported in Figure 4 and Table 3. It can be found that the Sample WUSGP has the largest corrosion potential (i.e., \(-\)0.3999), while the corrosion potentials of Sample DUSGP and USP are relatively close, realizing a value of \(-\)0.5926 V and \(-\)0.5122 V, respectively. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Vibration frequency (kHz) & 20 \\ Peening distance (mm) & 30 \\ Processing time (min) & 3 \\ Diameter of ceramic balls (mm) & 1 \\ Grain size of brown corundum (\(\mu\)m) & 15 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Working parameters of ultrasonic processing. Figure 3: Nyquist plots of the samples immersed in seawater for 96 h.  
This can be by explained considering the effect of the strengthening liquid in surface treatment, which degrades the corrosion potential of Sample WUSGP. Generally, the lower the corrosion potential, the worse the corrosion resistance. Furthermore, the corrosion current of sample WUSGP is two times lower than sample Polished, exhibiting a value of 9.435 mA/cm\({}^{2}\). A smaller corrosion current means a lower corrosion rate. Therefore, sample WUSGP possesses the best properties of corrosion resistance. The X-ray diffraction patterns of all the samples are shown in Figure 5a. It is noted that the new diffraction peaks occurred in the surface-treated samples, such as (200). The new diffraction peak is particularly obvious in Sample WUSGP. To further understand the possible mechanism, the grain size and dislocation density of these samples have been calculated (See Figure 5b) through Williamson-Hall equations [37]. Sample polished has the maximum grain size and the minimal dislocation density, realizing a value of 28.7 \(\upmu\)m and 0.3658 \(\times\) 10\({}^{16}\) m\({}^{-2}\), respectively. After surface treatment, the grain size decreases, and the dislocation density increases. In sample WUSGP, the grain size is refined to 6.45 \(\upmu\)m, and the dislocation density increased to 2.639 \(\times\) 10\({}^{16}\) m\({}^{-2}\), which is mainly due to the plastic deformation of the surface tissue after intensive grinding and the phase transforms, resulting in an increase in the martensite content (See Figure 5a). The martensite structures typically show a shape of needle and lath, making the surface structure denser. Previous studies demonstrated that the dense structure of high dislocation density and small grain size can significantly improve the corrosion resistance of the material [38]. This evidence further illustrates the benefits that USGP may bring. Figure 4: The polarization curves of all the samples. \begin{table}
\begin{tabular}{c c c} \hline
**Serial Number** & **E\({}_{\text{corr}}\)/V** & **I\({}_{\text{corr}}\)/(mA\(\cdot\)cm\({}^{-2}\))** \\ \hline Polished & \(-\)0.7309 & 18.15 \\ \hline USP & \(-\)0.5926 & 15.28 \\ \hline DUSGP & \(-\)0.5122 & 12.79 \\ \hline WUSGP & \(-\)0.3999 & 9.435 \\ \hline \end{tabular}
\end{table}