MFL probes incorporate a magnetic detector placed between the poles of the magnet where it can detect the leakage field. During inspection, a magnetic circuit of sorts forms between the part and the probe. The magnetic field induced in the part saturates it until it can no longer hold any more flux. The flux overflows and leaks out of the pipe wall and strategically placed sensors can accurately measure the three-dimensional vector of the leakage field. Because magnetic flux leakage is a vector and that a sensor can only measure one direction, any given probe must have three sensors to accurately measure the axial, radial, and circumferential components of an MFL signal.

Advantages of Magnetic flux leakage Inspection

Sizing of the defect is very limited in MFL inspection.
Fill factor should be very high compared to other technique.

Limitations of Magnetic flux leakage Inspection

One of the few methods used to inspect finned tubes.
Can be used on all ferromagnetic materials
Good sensitivity to pitting
High-speed inspection.