## 3 Discussion of Results  
### Experimental Observations and Analysis  
Prediction of corrosion rate in the flow loop is a challenging task as various parameters affect the rates depending upon the material properties. The dissolved oxygen in the system is an important factor that controls the corrosion rate, since the corrosion mechanism is different whether the system is aerated or deaerated. The temperature of the fluid in contact, the flow rate of the fluid, the concentration of the chloride ions, dissolved salts, and pH are parameters that can enhance and reduce the corrosion rate of the mild steel in a flow system. In this study, we investigated the effects of flow rate, temperature, pH, and NaCl concentration on corrosion rate by changing one of the parameters and keeping the other parameters constant. The study is conducted in two different pH levels, namely, pH-7 and pH-5. The pH is maintained by adding sodium acetate buffer solution and is monitored using a well-calibrated pH meter. The temperature of the feed solution is changed from 25-40 \({}^{\circ}\)C every 5 \({}^{\circ}\)C degrees, the flow rate of the solution in the loop is varied from 0-0.8 m/s (Reynolds number up to 22,000), and the salt concentration of the solution is varied from 1% to 3.5% by adding NaCl. The effect of velocity on corrosion rate has been studied previously, and the application of these findings in practical flow systems is very difficult. The corrosion rate and its relationship with flow velocity varies differently for different metals and alloys; thus, it is very challenging to form a specific conclusion. The changes in velocity alter the oxygen supply to the material and remove corrosion products from the specimen surface, thus resulting in different corrosion rates. Generally, at low flow rates, the corrosion rate will be uniform at the surface, and at higher flow rates, corrosion will be more localized, resulting from the formation of oxygen concentration cells. At higher flow rates, corrosion rates can sometimes increase because of increased oxygen supply and can decrease due to the destruction of thermal and concentration gradients. Here are the corrosion rate results, obtained from the flow loop of Figure 4.  
Figure 8 presents the corrosion rate of API-5L-X65 steel in millimeter per year (1 mm = 0.0393 inches = 39.3 mils) versus flow velocity (between 0 and 0.8 m/s), as NaCl concentration varies from 1% to 3.5%, and temperature varies from 25 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, 35 \({}^{\circ}\)C to 40 \({}^{\circ}\)C with the feed solution maintained at pH-7. The change in corrosion rate with the parameters, such as flow velocity, temperature, and NaCl concentrations is discussed in  
Figure 7: Potential is plotted against time for 1%, 2%, and 3.5% NaCl concentrations at 25 \({}^{\circ}\)C when the flow velocity is kept at zero. the figure. For flow velocities up to 0.2 m/s, the corrosion rate increases as the oxygen supply to the material increases, and at higher flow velocities (0.2 to 0.8 m/s), the corrosion rate appears to reach a steady value. It is assumed a stable oxide film is formed on the material surface, which provides a constant corrosion rate. At zero velocity, the corrosion rate is significant and shows an increasing trend with an increase in NaCl concentration and temperature. The formation of the black iron oxide layer is also observed on the metal surface when there is no flow. It can be observed from Figure 8, that at pH-7, the corrosion rate varies from 0.0254 mm/yr to about 0.2032 mm/yr when the NaCl concentrations vary from 1% to 3.5%. At a temperature of 25 \({}^{\circ}\)C, the corrosion rate is less than 0.0254 mm/yr for 1% NaCl concentration and the corrosion rates reach 0.0508 mm/yr and 0.1143 mm/yr at 2% and 3.5% NaCl, respectively. A corrosion rate of 0.0558 mm/yr is observed for 2% NaCl solution at 30 \({}^{\circ}\)C, and 0.0635 mm/yr at 35 \({}^{\circ}\)C. At 40 \({}^{\circ}\)C, the corrosion rates increase from 0.0508 mm/yr to 0.08636 mm/yr when NaCl concentration changes from 1% to 2%.