## 3 The Effect of Surface Roughness on Material Corrosion
### The Effect of Surface Roughness on the Degradation of Corrosion Material Properties  
Figure 9 shows the stress-strain curve of specimens with different initial surface roughness after 4 h of electrolytic corrosion. As can be seen from Figure 9, the mechanical properties of each surface roughness sample show different degrees of deterioration after 4 h of corrosion; the specimen with an initial surface roughness of Ra 0.052 \(\upmu\)m has the most obvious reduction in tensile strength after electrolytic corrosion, which is 33.86% lower than before electrolytic corrosion. Figure 10 shows the specimens' tensile strength and extensibility curve with different initial surface roughness after corrosion. It can be seen from Figure 10 that as the initial surface roughness of specimens increases, the tensile strength and elongation of each specimen after corrosion tend to increase first and then decrease. The tensile strength and elongation of the Ra 0.168 \(\upmu\)m specimen are the largest, which are 527.84 MPa and 19.8%, respectively. When the initial surface roughness is Ra 3.012 \(\upmu\)m, the tensile strength and elongation of the specimen are the smallest, which are 477.3 MPa and 18.12%, respectively.  
This phenomenon is because the specimens in the electrolytic corrosion process have surface defects, such as erosion, damage, material dissolution, and other surface defects on the material surface. The mentioned surface defects can trigger a certain degree of stress concentration, which leads to the decline of mechanical properties. The different electrochemical activities of different parts of surface material can also cause pitting corrosion. The dimple pits caused by pitting corrosion have the characteristics of small diameter and deep crater depth, which would produce serious stress concentration under the action of external tensile force, resulting in a substantial decrease in the material's mechanical properties. The incompleteness of the oxide film on the surface of the specimens changes with the change in surface roughness. When the surface roughness is small, the oxide film on the surface of the specimen is thinner due to the lower oxygen potential at the wave crest and trough, and only the peak of the oxygen potential produces the oxide film. When the surface roughness is larger, although the surface layer can form the oxide film, the tip effect leads to the rupture of the oxide film, and the different integrity of the oxide film on the surface of the specimen directly leads to the different corrosion resistance of the material. Different corrosion resistance would also lead to pitting corrosion on the surface of the workpiece adding to the difficulty and distribution of different situations; that is, when the oxide film is more complete and there is small initial surface roughness, pitting corrosion is more likely to occur.  
roughness, the degree of corrosion reduces and then increases, showing that the tensile strength and elongation first increase and then decrease. In comparison, we can see that the specimen with a surface roughness of Ra 0.168 \(\upmu\)m has the best surface corrosion resistance. Meanwhile, to obtain better corrosion resistance, the surface roughness should not be too low when processing the surface of the workpiece. It should be noticed that when pretreated, both sides of the specimen are polished, although the roughness of both sides of the specimen can be measured and corrected by polishing to ensure consistent roughness, but different polished areas due to uneven processing may lead to uneven corrosion of the specimen, which, in turn, produces stress concentration during the tensile process, thus affecting the accuracy of the test results and causing test errors.