## 3 Results and Discussion
### Metallography and Surface Roughness Observations  
Numbers of eroded surfaces were investigated by an optical microscope to gain more insight of the material removal mechanisms. The test conditions were a slurry of 3.5 wt.% sodium (NaCl) and solid particles of silica sand (SiO\({}_{2}\)) with an average size of 250-500 \(\upmu\)m, 20% concentration as erodent and a 45\({}^{\circ}\) impacting angle. Typical position near the center on the test specimens was selected. Figures 12 and 13 show the eroded surface of copper samples before and after ECAP. As it can be seen in Figure 12a; for low velocity (1.4 m/s), cutting marks represent the material removal and indicates a ductile fracture of the material. Micrograph in Figure 12b shows the eroded copper surface after four passes of ECAP. Fewer cutting marks were noticed. The E-C is less as compared with the previous one, which is supported by the results. At higher velocity (5.4 m/s), pitting was observed on the surface. In Figure 13, the shapes of eroded zone are rectangular and elliptical similar to sand particles suspended to the slurry. This occurred because of high particle impact on the copper surface at high velocity. Figure 11: The variation of weight loss with ECAP passes at different sand concentrations (\(v\) = 3.8 m/s, impacting angle = 45\({}^{\circ}\), 3.5% NaCl, and \(t\) = 48 h). Figure 12: Optical micrograph of the E-C samples. (**a**) before ECAP; (**b**) after four passes of ECAP (impact velocity = 1.4 m/s, sand concentration = 20 wt.%, time = 48 h and impact angle = 45\({}^{\circ}\)). Figures 14-17 show the surface texture images (obtained by Bruker profiler, Billerica, MA, USA) of copper samples subjected to four ECAP passes and different E-C conditions. Figure 14 shows the surface texture of annealed copper subjected to corrosion before ECAP. Figure 15a-d reveals the variation of surface texture of eroded copper surface after ECAP passes. Figure 16 presents the surface texture of as-annealed copper samples (before ECAP) subjected to E-C. Figure 17a-d shows the surface texture of copper samples subjected to E-C after four ECAP passes. It is observed that the surface texture enhances with increasing the number of ECAP passes. This can be seen clearly through the red color zones that reduce with increasing ECAP passes. In addition, the combined effect of E-C on surface texture is more profound than the contribution of corrosion only. In this part of the research, the surface roughness parameters R\({}_{a}\), R\({}_{t}\), and R\({}_{q}\) were studied as a function of ECAP passes at two different slurry velocities; \(v=3.8\) m/s and 5.4 m/s. Other experimental parameters were kept at constant values of: \(t=48\) h, impact angle = 45\({}^{\circ}\), and erodent (sand) concentration = 20 wt.%. Figure 18 shows the variation in \(R_{a}\) values with respect to number of ECAP passes. It is noticed that the slurry flow velocity significantly affects the surface roughness parameter \(R_{a}\). For annealed copper (before ECAP), the \(R_{a}\) value increased from 15.62 \(\upmu\)m to 54.34 \(\upmu\)m at \(v=3.8\) m/s and \(v=5.4\) m/s, respectively. After four passes of ECAP it is observed that the \(R_{a}\) value decreased by 7.8% and 4.1% with \(v=3.8\) m/s and 5.4 m/s, respectively. This means that increment of ECAP passes leads to smoother surface. Figure 19 revealed that slurry flow velocity has a considerable effect on the surface roughness parameter \(R_{t}\). For velocities of 3.8 m/s and 5.4 m/s, the parameter \(R_{t}\) of ECAPed copper increased by 2.6% and 13.25%, respectively, as compared to annealed copper. Figure 20 show that parameters \(R_{q}\) is sensitive to slurry flow velocity, similar behavior to parameter \(R_{a}\) have been observed for parameters \(R_{q}\) along the four passes of ECAP. Figure 14: Surface texture of annealed copper subjected to corrosion before ECAP (time = 48 h, velocity = 3.8 m/s, impact angle = 45\({}^{\circ}\), and sand = 0 wt.%).