## 3 Discussion of Results
### Parametric Study in Flow Insensitive Region  
The results discussed in the previous section consistently shows two different regimes. A flow-sensitive region, where small changes to the flow or Reynolds number (Re) results in an exponential increase in corrosion rate and a flow insensitive region where the increase in flow rate does not affect the corrosion rate of the sample. This constant rate corrosion flow region is visible in most graphs, but not all. Figure 11 shows the flow insensitive region, the corrosion rates are shown at different Reynolds number for 1% NaCl concentration for pH of 7 and 5 at different temperatures. In all the cases shown in Figure 11, the flow insensitive region is visible, corrosion rates are increasing with Reynolds number, and when Re is above 5000, the corrosion rates reach a plateau. It is assumed that a corrosive oxide layer formed on the surface limits the flow to the metal surface, and it minimizes the diffusion of hydroxides from the steel surface to the flowing water, which reduces the ion concentrations in the interfaces. The corrosion rate in the corrosion insensitive flow region is where uniform corrosion is predominant, and that region can be utilized to derive correlations for corrosion behavior at different operating parameters. The identification and analysis of this flow insensitive region in pipelines will be valuable in corrosion prediction and control. Figure 10: Corrosion rates of API-5L-X65 QT metal are shown at different flow velocities maintaining the feed solution pH of 5 and 7 and varying NaCl concentrations at 25 \({}^{\circ}\)C.  
The corrosion rates in the flow-insensitive region are taken as constant when the Reynolds number are greater than 10,000, and the corrosion rates are plotted with temperature, salinity, and pH to find the correlation between the parameters. pH and NaCl concentrations have a significant effect on corrosion rate like temperature, and the relationship is derived using the corrosion rates obtained in the flow insensitive region. The analysis, shown in Figure 12, provides the relation between NaCl concentration and corrosion rates. The corrosion rate, influenced by the presence of chloride ion, can cause the acceleration of both the anodic and cathodic reactions. The plot is analyzed, and the corrosion rate seems to increase as the power of NaCl concentration (X = NaCl concentration) and from the figure, the experimental data shows a good match with the power equation. It can be seen from Figure 12 that there is a good fit between the experimental data and the power equation with the coefficient of determination (R\({}^{2}\)) above 0.9. To create one equation that can define the corrosion rate as a function of NaCl concentration (X), an average constant exponent value around 1.4 was chosen. The corrosion rates can be expressed as \(Ax^{1.4}\) and a constant, which depends on the other flow parameters. Thus, the following relations CR = A (NaCl Concentration)\({}^{1.4}\) can be obtained to correlate the corrosion rates with NaCl concentration. Figure 11: Corrosion rates are shown at different Reynolds number for feed solution maintained at pH–7 on the left, and at pH-5 on the right at 1% NaCl, at following temperatures (25 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, 35 \({}^{\circ}\)C, and 40 \({}^{\circ}\)C). The corrosion rates from the flow insensitive region are plotted against the temperature, and the corrosion rates at pH 7 and 5 are plotted for 1%, 2%, and 3.5% NaCl solution. The corrosion rate in the mild steel samples seems to increase with the temperature in the flow insensitive region. At pH 7 for 2% NaCl concentrations, the corrosion rates increase from 0.0484 mm/yr to 0.0660 mm/yr when the temperature increases from 25 \({}^{\circ}\) to 40 \({}^{\circ}\)C, whereas for the 3.5% NaCl concentration the corrosion rate changes from 0.1143 mm/yr to 0.1524 mm/yr. The data in the flow insensitive region is analyzed to predict the corrosion change with temperature.