## 4 Discussion
### Effect of Magnetic Field on the Corrosion Mechanism of API 5L X65  
The magnetic properties of a material depend on different factors, such as microstructure, chemical composition, heat treatment, and mechanical conditions [47]. The study conducted by [PERSON] in 1893 shows that the non-magnetic inclusion, mainly carbide, plays an important role in the metal magnetic properties [48]. Iron derivatives, such as carbon steel and stainless steel, are differentiated based on the presence of these non-magnetic inclusions. In this case from Table 1, it is understood that the amount of carbon is only 0.11 and, hence, shows very high magnetic properties. Metals exhibiting very good magnetic properties are categorized as ferromagnetic materials. Ferromagnetic materials possess a permanent magnetic moment in the absence of an external magnetic field, which helps to attain very high magnetization. Unlike other materials, ferromagnetic materials have their magnetic moment aligned in the same direction, which results in an uncancelled magnetic moment among themselves. For ferromagnetic material, the magnetic dipole moments, which are the electrically charged particles in a material, orient themselves in the same direction in a particular domain, as shown in Figure 7. Domains are the regions where the magnetic dipole moments arrange themselves in the same direction within the domain wall. When a magnetic field is applied, the magnetic dipoles present in different domains orient in the same direction that is favorable to the applied magnetic field. When a magnetic field is applied to a ferromagnetic material in the presence of an aqueous medium, the electrochemical behavior of these materials is driven by a magnetohydrodynamic mechanism. Consistent with a magnetohydrodynamic mechanism, when a magnetic field is applied to an electrochemical reaction, two kinds of body forces are introduced to the system: the Lorentz force and the magnetic field gradient force (MFGF). The MFGF comes into existence when an external magnetic field is imposed on a ferromagnetic material, which results in the deviation of magnetic flux lines. This force drives the paramagnetic particles to high magnetic flux density areas, causing a gradient concentration of paramagnetic ions, and acts on the regions where the magnetic field gradient exists [34; 50]. The magnetic field force, \(\vec{F_{\
abla B}}\) is given in Equation (16) where \(\chi_{\text{m}}\) is molar susceptibility, B the magnetic flux density, \(c\) the concentration, \(\vec{\
abla B}\) the gradient magnetic flux density, and \(\mu_{o}\). the magnetic permeability of vacuum. \[\vec{F_{\
abla B}}=\ \frac{\chi_{\text{m}}c\underline{B}\frac{\partial}{\
abla B}}{ \mu_{o}} \tag{16}\]  
Lorentz force originates when there is movement of particles caused by the combined effect of the electric current and magnetic field. Lorentz force \(\vec{F_{L}}\). is given in Equation (17) where \(\vec{J}\). is the current density and \(\vec{B}\) is the magnetic flux density. \[\vec{F_{L}}=\ \vec{J}\ \times\ \vec{B} \tag{17}\]  
Lorentz force is maximal when the \(\vec{J}\) is perpendicular to \(\vec{B}\) and zero when \(\vec{J}\). is parallel to \(\vec{B}\). From Table 2 and Figure 8, it can be observed that the magnetic field increases the corrosion rate of the API 5L X65 samples. At 0 m/s, the corrosion rate for the no magnetic field condition is measured as 0.28 mmpy. As soon as a magnetic field of a N-S orientation is applied over the material, the corrosion rate increased by approximately 20%, resulting in 0.33 mmpy, and, when a N-N magnetic field is applied, the corrosion  
Figure 7: Schematic depiction of domains in a ferromagnetic material [49]. rate increased by more than 250%, resulting in 1.00 mmpy. As the velocity increases to 1 m/s, the corrosion rate is measured as 0.70 mmpy for a no magnetic field condition.