## 4 The Influence of Surface Residual Stress on Corrosion Resistance
### The Influence of Residual Stress on Corrosion Appearance  
Figure 12 shows the roughness change curves after the specimens' electrolytic corrosion with different surface residual stresses. It can be seen from Figure 12 that the surface roughness of specimens with different surface residual stress shows a trend of first increasing and then decreasing. Among these, the surface roughness of the specimen with a surface residual stress of \(-\)120.2 MPa changes slightly during the corrosion process, and its surface roughness grows to a maximum after 1 h of corrosion and then decreases. For  
the remaining specimens with surface residual stresses after 1 h of corrosion, the surface roughness increases with the increase in surface residual compress stress, and after 2 h of corrosion, the surface roughness of each specimen decreases with the increase in residual compress stress on the surface. During the variation process, the specimen with a surface residual stress of \(-335.64\) MPa has the most pronounced surface roughness variation trend: after 1 h of corrosion, the surface roughness increased to Ra 6.112 \(\upmu\)m, while after 4 h of corrosion, the surface roughness eventually decreased to Ra 4.621 \(\upmu\)m. For the specimen with a surface residual stress of \(-215.47\) MPa, its corrosion surface roughness maximum value appears at the moment of 2 h of corrosion, and when the corrosion time extends, the specimen's surface roughness decreases significantly. The specimen with a surface residual stress of \(-293.27\) MPa has the same surface roughness after 1 h and 2 h of corrosion, and the surface roughness decreases significantly when the corrosion time extends. It is found that, except for the specimen with a surface residual stress of \(-120.2\) MPa, the surface roughness of the remaining specimens is close to Ra 4.7 \(\upmu\)m after the specimens reach the corrosion stabilization stage. Moreover, the variation pattern of the surface roughness of other samples with increasing corrosion time is relatively similar. Due to the similar trend of the specimens, except for the surface residual stress of \(-120.2\) MPa, the surface residual stress \(-335.64\) MPa specimen was selected to observe the variation in surface topography throughout the corrosion stage, and the variation in the electrolytic corrosion topography of the specimen with a surface residual stress of \(-335.64\) MPa is shown in Figure 13.  
Figure 12: Variation in surface roughness of specimens with different surface residual stresses. Figure 13: Variation in electrolytic corrosion topography of specimen with a surface residual stress of \(-336.64\) MPa. (**a**) 0 h; (**b**) 1 h; (**c**) 2 h; (**d**) 3 h; (**e**) 4 h.  
The reasons for this phenomenon are that during the first hour of corrosion, the uneven and incomplete anode film contributes to the pitting corrosion, increasing the surface defects. Meanwhile, during this stage, the corrosion rate of the specimen surface material is also relatively large, so the surface material seriously corrodes in a relatively short period; both the reasons mentioned above lead to an increase in material surface roughness. As the corrosion reaction proceeds, the oxygen precipitated from the anode decreases, and the current efficiency of the dissolved metal increases, resulting in the previous stage of corrosion produced by pitting defects being reduced or disappearing, and the surface roughness decreases again, which is shown in Figure 13a,b. As the surface residual compress stress increases, the degree of surface damage increases, which will increase the contact area of the corrosion reaction. In addition, the specimen's surface oxygen precipitation increases, which would further cause the incompleteness of the anodic film, increasing pitting corrosion defects and ultimately leading to the material surface roughness increasing with the increase in surface residual compress stress.