## 3 The Effect of Surface Roughness on Material Corrosion
### The Effect of Surface Roughness on Surface Corrosion Topography of the Material  
Figure 8 shows the results of surface roughness change after the electrolytic corrosion of specimens with different initial roughness surfaces. It can be seen from Figure 8 that after 4 h of corrosion, the surface roughness of specimens with an initial surface roughness of Ra 0.168 \(\upmu\)m and Ra 0.052 \(\upmu\)m are higher than the surface roughness before corrosion. The surface roughness increased from Ra 0.168 \(\upmu\)m to Ra 1.02 \(\upmu\)m and from Ra 0.052 \(\upmu\)m to Ra 1.203 \(\upmu\)m, respectively. The surface roughness of specimens with the initial surface roughness of Ra 3.012 \(\upmu\)m and Ra 1.208 \(\upmu\)m all decreased after 4 h of corrosion, and the surface roughness of the specimens decreased from Ra 3.012 \(\upmu\)m to Ra 2.81 \(\upmu\)m and from Ra 1.208 \(\upmu\)m to Ra 1.02 \(\upmu\)m, respectively. Meanwhile, in addition to the specimen with the initial surface roughness of Ra 0.168 \(\upmu\)m, the surface roughness of the other three different initial surface roughness specimens increases first and decreases obviously during the corrosion process. All three groups of specimens show a significant increase in surface roughness: the surface roughness of the specimen with the initial surface roughness of Ra 3.012 \(\upmu\)m increased to Ra 5.01 \(\upmu\)m, and the surface roughness of the specimen with the initial surface roughness of Ra 0.168 \(\upmu\)m also has an obvious growth, and after 1 h of corrosion, its surface roughness increased to Ra 1.55 \(\upmu\)m. After 2 h of corrosion, the surface roughness of the three groups of specimens is reduced compared to the surface roughness after corrosion for 1 h, and the specimen with the initial surface roughness of Ra 3.012 \(\upmu\)m reduced from Ra 5.01 \(\upmu\)m to 3.71 \(\upmu\)m, which decreased significantly in the three sets of specimens. Meanwhile, the surface roughness of the specimens with an initial surface roughness of Ra 1.208 \(\upmu\)m and Ra 0.052 \(\upmu\)m also reduced to Ra 1.673 \(\upmu\)m and Ra 1.312 \(\upmu\)m, respectively. After 3 h of corrosion, the surface roughness variation and surface roughness of the specimens tends to be stable, and the surface topography changes tend to be smooth. Among these, the surface roughness of specimens with the initial surface roughness of Ra 0.052 \(\upmu\)m and Ra 1.208 \(\upmu\)m are relatively close. The changing trend in Figure 8 is that with the change in specimen surface roughness, the material surface oxide film appears a certain degree uneven or missing, which causes the oxygen potential difference, and the oxygen potential difference between the various parts of the material surface is one of the main reasons for the occurrence of electrochemical corrosion of the surface. When the surface roughness of the specimen is low, the oxygen potential at the peak and valley of the surface is low and causes a thin or missing oxide film, while the rough surface and the tip effect make the surface oxide film easy to rupture. Both situations mentioned above lead to an uneven distribution of the oxide film on the material surface, which contributes to the formation of corrosion current and promotes the  
Figure 8: The variation in surface roughness after electrolytic corrosion with different surface roughness. electrolyte corrosion of the material surface. Meanwhile, from the initial stage of corrosion, the corrosion rate is affected by surface roughness and the contact area between the material and corrosion medium; after 1 h of corrosion, the surface roughness of specimens with the initial surface roughness of Ra 3.012 \(\upmu\)m, Ra 1.208 \(\upmu\)m, and Ra 0.052 \(\upmu\)m increase due to a large amount of surface material corrosion removal. After 3 h of corrosion, the material corrosion rate also tends to be stable; the part of the specimen surface removed by material corrosion is dominated by the surface roughness peak, which causes the change in surface roughness to tend to be stable.