# Salt-Fog Corrosion Behavior of GCr15 Steels Treated by Ultrasonic Strengthening Grinding Process
## 3 Results and Discussion

Table 3: Corrosion potential (E\({}_{\text{corr}}\)) and the corrosion current density (I\({}_{\text{corr}}\)) obtained in electrochemical corrosion test for each sample. Figure 6 shows the surface macro-morphology of the samples. It is found that most regions on the matrix surface of Sample polished are corroded (See Figure 6a), and the width of corrosion cracks is larger than in other samples. The corroded area of Sample USP, DUSGP, and WUSGP is relatively smaller (See Figure 6b-d). Especially in Sample WUSGP, the uncorded area occupied approximately half of the whole matrix. The white substance is CaCO\({}_{3}\) formed in the corrosion process because the OH\({}^{-}\) produced by cathodic reacts with HCO\({}_{3}\)\({}^{-}\) and generates the CO\({}_{3}\)\({}^{2-}\), which further reacted with the Ca on the sample surface [39]. Corrosion products were further analyzed by XRD, as depicted in Figure 7. In Sample polished, the corrosion products mainly consist of Fe, Fe\({}_{2}\)O\({}_{3}\), Fe(OH)\({}_{3}\), and Fe\({}_{3}\)O\({}_{4}\) (See Figure 7a), but in other samples, FeOOH appeared (See Figure 7b-d). This could be attributed to the chemical reaction between Fe(OH)\({}_{2}\) and O\({}_{2}\). In the initial phase of corrosion, the passive film Fe(OH)\({}_{2}\) on the sample surface will undergo a redox reaction with O\({}_{2}\) to form FeOOH. Over time, Fe(OH)\({}_{2}\) will absorb O\({}_{2}\) to produce reddish-brown Fe(OH)\({}_{3}\). At the same time, Cl\({}^{-}\) will intrude into the vulnerable regions of the matrix, resulting in the further oxygen absorption reaction of Fe(OH)\({}_{3}\) to form the black Fe\({}_{3}\)O\({}_{4}\). It is also noted that the diffraction peak intensity of Fe is the strongest in Sample WUSGP, indicating that there are fewer corrosion areas, and the content of the formed FeOOH and Fe\({}_{3}\)O\({}_{4}\) is relatively low. Figure 5: (**a**) The X-ray diffraction pattern characteristics on sample surface; (**b**) dislocation density and grain size of samples. Figure 6: Macroscopic corrosion morphology of each sample: (**a**) the corrosion macro-morphology of Sample polished; (**b**) the corrosion macro-morphology of Sample USP; (**c**) the corrosion macro-morphology of Sample DUSGP; (**d**) the corrosion macro-morphology of Sample WUSGP. The corrosion micromorphology of Sample polished, USP, DUSGP, and WUSGP are shown in Figure 8a-d, respectively. Corrosion holes in Sample polished have a diameter of approximately 6.4 \(\upmu\)m, while sample WUSGP has a diameter of only approximately 1.4 \(\upmu\)m, which degrades 78%. This can be explained by the surface plastic deformation of WUSGP-processed samples, resulting in grain refinement and a denser surface layer. Corrosion holes in these samples exhibit different shapes, such as island, cotton, and lath-like. This is because Cl\({}^{-}\) gradually erodes the matrix's weak regions during the corrosion process, forming countless corrosion micro-batteries and reacting with the corrosion solution. The internal stress of the matrix material and the accumulation of reaction energy push corrosion micro-bubbles to rupture, form \"island-like\" and \"cotton-like\" structures, and then further form pitting holes. As the corrosion process intensifies, the initial local pitting gradually develops into corrosion holes, which then connect with each other and finally form the cracks. Figure 9a-d report the micromorphology of corrosion cracks for Sample polished, USP, DUSGP, and WUSGP, respectively. Different widths of corrosion cracks are observed. The polished sample shows a width of approximately 8.4 \(\upmu\)m, while samples that experienced surface treatment have a relatively narrow width. For instance, Sample WUSGP realizes a crack width of only 2.1 \(\upmu\)m, which degrades 75% compared to the polishing. The energy spectrum of the corrosion crack area has been analyzed using EDS. The element distribution and atomic percentage of the samples are shown in Figure 10 and Table 4, respectively.