## 3 The Effect of Surface Roughness on Material Corrosion  
### The Effect of Surface Roughness on the Corrosion Rate of Material  
Figure 7 shows the corrosion rate of specimens with different initial surface roughness at different corrosion moments. It can be seen from Figure 7 that with the increase in corrosion time, the corrosion rate of specimens with different initial surface roughness shows a decreasing trend and stabilizes after 3 h of corrosion. With the specimen surface roughness decreasing, the corrosion rate decreases, while the change rate of the corrosion rate is first reduced and then increased. Among them, the specimen with an initial surface roughness of Ra 3.012 \(\upmu\)m shows the largest corrosion rate during the entire corrosion process. During the corrosion stage of 1-2 h, the corrosion rate of the Ra 3.012 \(\upmu\)m specimen decreases significantly, which reduces by 29.8%; after 3 h of corrosion, the corrosion rate is close to the corrosion rate of the specimens with a surface roughness of Ra 0.052 \(\upmu\)m and Ra 1.208 \(\upmu\)m. In addition, the four measured corrosion rates of Ra 3.012 \(\upmu\)m are 251.1 mm/a, 175.8 mm/a, 148.8 mm/a, and 138.8 mm/a, and the corrosion rate decreases by 45.3% after 4 h of corrosion. The specimen with an initial surface roughness of Ra 0.168 \(\upmu\)m shows the lowest corrosion rate during the entire corrosion process. The four measured corrosion rates are 145.1 mm/a, 135.3 mm/a, 120.9 mm/a, and 119.3 mm/a. The corrosion rate declines steadily during 1-3 h; the corrosion rate becomes stable after 3-4 h of corrosion; and after 4 h of corrosion, the corrosion rate decreased by 17.8%. However, for the specimens with the initial surface roughness of Ra 1.208 \(\upmu\)m and Ra 0.052 \(\upmu\)m, the corrosion rate and the rate of change in the corrosion rate are approximately consistent. Figure 6: Microstructure of surface layer after processing with different shot peening pressures. (**a**) 0 MPA; (**b**) 0.2 MPa; (**c**) 0.3 MPa; (**d**) 0.4 MPa. The reason for this phenomenon is that during the initial corrosion stage, the reaction between the specimen surface and corrosion medium is unstable due to the inhibition of substances, such as impurities and oxide films. As the corrosive medium continues to corrode the specimen material, the reaction between the corrosive medium and specimen material gradually stabilizes, which, in turn, leads to the reduction and stabilization of the corrosion rate. Furthermore, when the specimen surface topography is relatively large, the electrochemical activity at the surface peak of the specimen is large, making the specimen surface material react with the electrolyte for easier corrosion. With the increase in specimen surface roughness, the difference in the electron escape function increases at the peaks and valleys of the specimen surface, resulting in a decrease in the overall surface electron work and an increase in the fluctuation, which represents that the corrosion rate increases with the increase in surface roughness. Meanwhile, the surface roughness of the specimen is relatively large at the beginning of corrosion. The contact area of the corrosive medium is also relatively large, which is conducive to a corrosion reaction. Therefore, the corrosion rate and its change rate are relatively large at the beginning of corrosion. The specimen with an initial surface of Ra 0.052 \(\upmu\)m shows an increase in the corrosion rate when the specimen surface roughness is too low. The specimen surface oxide film appears uneven or missing, resulting in different parts of the material due to different oxygen potential and potential oxygen differences. When the surface roughness is lower than a specific value, the oxygen potential at the peak and trough of the material is lower, resulting in a thin oxide film or no oxide film formation. The oxide film only forms on the peaks of the specimen surface, which constitutes microcurrent corrosion and contributes to corrosion [30].