## 4 The Influence of Surface Residual Stress on Corrosion Resistance  
### The Influence of Residual Stress on Material Corrosion Rate  
Figure 11 shows the curve of surface residual stress's effect on the specimens' electrolytic corrosion rate. It can be seen from Figure 11 that with the increase in corrosion time, the corrosion rate of specimens with different surface residual stress values has a decreasing trend, and the corrosion rate stabilizes after 3 h of corrosion. After 1 h of corrosion, the corrosion rate increases with the increase of surface residual stress, of which the specimens with a surface residual stress of \(-\)335.64 MPa, the corrosion rate of 245.1 mm/a, the corrosion rate is the largest. After 2 h of corrosion, the corrosion rate of different specimens changes, which appears the trend of the corrosion rate decreasing with the increase of surface residual stress. At this moment, the corrosion rate of the specimen with a surface residual stress of \(-\)335.6 MPa decreased to 139.5 mm/a, which decreases from the maximum to the minimum, compared to the corrosion rate measured after 1 h of corrosion decreased by 43.1%. Moreover, the corrosion rate of the specimen with surface residual stress of \(-\)120.2 MPa increases from the minimum to the maximum; at this time, the corrosion rate, which decreases to 170.3 mm/a compared to the corrosion rate measured after 1 h of corrosion, decreased by 19.1%. When the length of corrosion time is more than 2 h, the order of the corrosion rate of specimens with different residual stress remains the same, and the corrosion rate values remain stable. During the entire corrosion experiment process, the corrosion rate of specimens with surface residual stress of \(-\)335.6 MPa decreases by 49.9%, which is the largest decrease among the four specimens with different surface residual stress, and the stabilized corrosion rate is 122.752 mm/a. The corrosion rate of specimens with surface residual stress of \(-\)120.2 MPa decreased by 33.1%, the smallest decrease among the four specimens, and the stabilized corrosion rate is 140.881 mm/a. Figure 11: Electrolytic corrosion rate for different surface residual stresses. The reasons for the mentioned phenomenon are as follows: the increase in surface residual stress is due to the surface microstructure dislocation and slip caused by the increase in shot peening strength. In contrast, the electrochemical activity at the boundaries between displacement and slip is relatively high, where the corrosion reaction's location occurs preferentially [31; 32]. Meanwhile, with the increase in surface tensile stress and the decrease in displacement density, the internal crystal structure is damaged by internal residual tensile stress, which reduces the content of active spots at grain boundaries where pitting corrosion occurs. However, the residual compress stress could effectively reduce the oxidation susceptibility of grain boundaries, preventing stress corrosion from occurring via contact and ultimately reducing the corrosion rate as the surface residual compress stress increases. Shot peening is used to introduce residual compress stress during the experiment; although this method can effectively introduce residual compress stress, the surface roughness of the specimen will increase significantly. The surface roughness of the specimens could be close to that of the shot peening process through sandpaper polishing. However, the polished surface topography is still relatively complex. The destructive effect of sandpaper polishing can damage the surface oxide film on the surface of specimens. At this time, the contribution effect on corrosion produced by surface topography is stronger than the inhibition effect on corrosion produced by surface compress stress, which promotes corrosion of the surface material of the specimen.