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First, Magnetic flux leakage testing (MFL) refers to the nondestructive testing technology that the surface and near surface defects form a magnetic leakage field on the surface of the material after the ferromagnetic material is magnetized, and the magnetic leakage field is detected to find defects.

When magnetizing the ferromagnetic material under test with a magnetic saturator, the magnetic induction line in the material will be constrained in the material if the material is continuous and uniform, and the magnetic flux is parallel to the surface of the material. There is almost no magnetic induction line passing through the surface, and there is no magnetic field on the tested surface.

However, when there are defects of cutting magnetic lines of force in the material, the change of defects or structure state on the surface of the material will change the permeability. Because the permeability of the defect is very small and the magnetic resistance is very large, the magnetic flux in the magnetic circuit will be distorted, and the magnetic induction line will change the way. In addition to a part of the magnetic flux passing through the defect directly or bypassing the defect inside the material, some of the magnetic flux will leave the surface of the material, bypass the defect through air and enter the material again, forming a leakage magnetic field at the surface defect of the material. We can detect the distribution and size of the leakage magnetic field through the magnetic sensor, so as to achieve the purpose of non-destructive testing.

Second, the magnetization method of MFL detection system plays an important role in MFL detection, which affects the magnetic field signal of the detected object. From the perspective of magnetization range, it can be divided into local magnetization and overall magnetization; from the perspective of magnetizing magnetic source, it can be divided into alternating magnetic field magnetization method, direct current magnetic field magnetization method and permanent magnet magnetization method.

The method of alternating magnetic field magnetization uses alternating current to excite the electromagnet to magnetize. With the increase of current frequency, the depth of magnetization decreases. After magnetization, the ferromagnetic material does not produce remanence and does not need demagnetization.

In the method of DC magnetic field magnetization, the electromagnet is excited by DC current to produce magnetic field for magnetization, and the magnetization intensity can be achieved by controlling the current; in the method of permanent magnet magnetization, the permanent magnet is used as the excitation magnetic source, and its effect is equivalent to the fixed DC magnetization.

Permanent magnet can use rare earth permanent magnet, aluminum nickel cobalt permanent magnet and so on. Generally, it uses rare earth permanent magnet, which has high magnetic energy and small volume. Remanence will be produced by using DC magnetization and permanent magnetization. Demagnetization depends on the specific requirements, and there is no specific requirements for the detection speed parameters. The selection of magnetization is generally aimed at ensuring the detection sensitivity and reducing the magnetizer so that the magnetic field generated by defects or structural features can be detected.

Third,main application fields of magnetic leakage detection:
In addition to the appearance and near appearance crack defects, the biggest advantage of MFL detection is that it can find the corrosion pit and other defects inside the tank from the outside, and it does not need to polish the surface of the tank, which is suitable for online detection of the tank. At present, online detection of the tank is the most urgent method for users, which can reduce unnecessary downtime and reduce the cost of inspection. In addition, MFL detection can also determine some characteristic dimensions of defects, such as depth and length. Magnetic flux leakage detection is widely used in tank online detection. The magnetic flux leakage detection technology of the outer wall of the vessel is used to reflect the defects of the inner wall of the vessel, especially the severity and distribution of the corrosion on the inner and outer surfaces.

Besides, magnetic flux leakage sensor can also be used to detect the defects of mobile pressure vessels that can not be inspected internally when they are in parking inspection such as unmanned holes. At present, there is no good inspection method for the equipment that can not be inspected internally except by endoscope, and it may not be able to effectively observe the condition of the internal and external surfaces due to the reasons, for example, the cover of the internal and external surface media can not be removed. Magnetic flux leakage detection can be used for scanning detection when the outer surface is covered with paint layer, and the size and depth of corrosion pit on the inner wall can be detected from the outside, so as to reduce the increased inspection cost due to unnecessary parking and grinding, and also reduce the labor intensity. The on-line magnetic flux leakage detection can generally understand the internal and external surface states of the tank body and the distribution of defects. The magnetic flux leakage detection results of key equipment and important parts can be archived as a reference for regular spot check, so as to predict the thinning rate of the tank car wall thickness and ensure that the equipment can be safely transported to the next inspection cycle.

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