## 3 Results and Discussion
### Effect of ECAP on Erosion-Corrosion at Different Flow Velocities  
Figure 7 shows the variation of weight loss with ECAP passes as a function of slurry velocity. 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 was used in the tests. The test duration was 48 h with impact angel of 45\({}^{\circ}\). According to the results, the weight loss due to E-C increases with increasing the impact velocities. At low velocities the weight loss is minimal. However, higher increase in weight loss occurred at the velocities of 3.8 m/s and 5.4 m/s, respectively. The largest weight losses for all ECAP passes occurred at a velocity of 5.4 m/s. These results can be attributed to the effect of flow field in the container of the slurry. At low velocities, the erodent is not completely suspended in the fluid; therefore, the erosion is unable to occur. It is recognized that erosion is a function of the fluid velocity (\(v\)) and it and be calculated by the nonlinear formula: Erosion = K-(\(v\))\({}^{n}\), where K is a constant depends upon particle size and impacting angle, and n is the velocity exponent [41; 42]. Comparing the weight losses for pass zero (annealed copper) and four pass ECAP at different velocities it is observed that E-C resistances increased by 0%, 14%, 31.8%, and 13% at 1.4 m/s, 2.7 m/s, 3.8 m/s, and 5.4 m/s, respectively. The improvement increased to 31.8% at the speed of 3.8 m/s and then it has dropped to 13% at the speed of 5.4 m/s. One possible reason can be due to the decrease of hardness at a higher velocity.  
After both ECAP and E-C experiments, the hardness (HV) of the specimens were measured as presented previously in Figure 5. It can be observed that the weight loss at a higher slurry velocity is more than at lower velocities. The decrease of the hardness at higher velocity can be attributed to the increase of the degradation of the surface. Other researchers investigating the erosion resistance,  
Figure 7: Variation of weight loss with ECAP passes at different velocities. (\(t\) = 48 h, impacting angle = 45\({}^{\circ}\), 3.5% NaCl, and sand concentration = 20 wt.%).  
with alumina, silicon carbide, and quartz particles [44] reported similar results. It is believed that work hardening due to ECAP enhances the E-C resistance of copper. It is also concluded that, at higher velocity, the elastic strain energy would exceed the strain energy of the tested material and the plastic deformation leads to micro cutting zones on the surface. Furthermore, optical microscopy to study the effect of flow velocity was employed to examine the surface of selected E-C samples. The obtained results will be presented and discussed in (Section 3.7) later in this work. The increasing of the slurry velocity resulted in increasing the kinetic energy of the erosive solid particles. This yields to more weight loss appears as a pitting on the surface. The micrographs of optical microscope showed the same findings (Section 3.7). For the ECAPed samples that were subjected to higher impact velocities, the damage on the impact areas are more clearly compared with the same sample subjected to lower velocities.