## 4 Discussion
### Effect of Flow Velocity on the Corrosion Mechanism of API 5L X65  
The corrosion rate for the API 5L X65 has been observed to be higher for the dynamic condition than the static condition from the potentiodynamic polarization data and the EIS data. It is obvious that the mass transfer rate increases under the dynamic condition because the fluid flow causes shear stress on the sample surface, leading to the hindering of the protective oxide layer formation [6,52]. This is observed from the EIS data given in Table 3, where the R\({}_{\rm f}\) values are very high for the static condition and very low for the dynamic condition. The Nyquist plots under condition A1, given in Figure 5a, show the largest diameter capacitance semicircle obtained: an R\({}_{\rm ct}\) value of 3310 \(\Omega\)cm\({}^{2}\) and an R\({}_{\rm f}\) value of 490.3 \(\Omega\)cm\({}^{2}\). As soon as flow is induced in the system, the diameter of the capacitance semicircle decreases, as shown in Figure 5b,c, giving an R\({}_{\rm ct}\) value of 1420 \(\Omega\)cm\({}^{2}\) and an R\({}_{\rm f}\) value of 4.295 \(\Omega\)cm\({}^{2}\) for condition B1, and an R\({}_{\rm ct}\) value of 723.9 \(\Omega\)cm\({}^{2}\) and an R\({}_{\rm f}\) value of 4.778 \(\Omega\)cm\({}^{2}\) for condition C1.  
Depending upon oxygenated and deoxygenated systems, the corrosion behavior of metals and alloys can change, which makes predicting the effect of hydrodynamics on corrosion rates and the corrosion mechanism difficult. The effect of flow velocities on the corrosion rate with a magnetic field and without a magnetic field is shown in Figure 9. In this oxygenated flow system, the corrosion rates tend to increase with flow velocities in all the conditions studied. When there is no magnetic field present, the corrosion rate is 0.28 mmpy at zero velocity, and, when the flow velocity is increased to 1 m/s (Reynolds Number \(\approx\) 19,000), the corrosion rate increases to 0.70 mmpy, which is one and a half times higher than zero flow. At 2 m/s (Reynolds Number \(\approx\) 38,000), the corrosion rate reaches 0.78 mmpy, showing an 11% increase as compared to a velocity of 1m/s and two times higher as compared to zero flow. When a magnetic field is introduced, similar trends are observed with an increase in flow velocity. In the N-S magnetic orientation, at zero flow, the corrosion rate is 0.33 mmpy, and a 230% to 350% increase can be observed for flow velocities of 1 m/s and 2 m/s, respectively. Similarly, for the N-N magnetic orientation, at zero flow, the corrosion rate is 1.00 mmpy, and a 53% to 140% increase can be observed for flow velocities of 1 m/s and 2 m/s, respectively. When the flow velocity is increased, the hydrodynamic force removes the corrosion products formed, but it can increase the dissolved oxygen diffusion to the material surface. This combined effect alters the corrosion rates with flow. The EDX data from Table 7 show that, as the flow rate increases, the elemental concentration of the oxygen rate on the surface for A3, B3, and C3 conditions increases, as expected. However, the action of hydrodynamic forces to reduce corrosion product deposits on the metal surface at higher fluid velocities could not be observed. The authors believe that this may be due to the magnetic field applied to the material, which resists the removal of the corrosion product from the metal surface. From the SEM and surface roughness images given in Tables 4 and 7, we can see that, at zero velocity, we observe predominantly uniform corrosion across the material, and, at higher velocities of 1 m/s and 2 m/s, oxygen concentration cells are formed on the surface, resulting in localized corrosion, causing small pits.