Magnetic Flux Leakage (MFL) is an advanced NDT technique which has the inspection capability in wall loss detection and measurement of sharp defects such as pitting, grooving and circumferential cracks in ferromagnetic materials. The working principle of MFL involves the induction of magnetic field in the part to be tested and the response or signal produced is analyzed to determine the presence and characteristics of defects.
MFL uses a powerful magnet to magnetize the conductive material under test. Where there are defects- corrosion or material loss- the magnetic field “leaks” from the material. 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.
Some of the Benefits of MFL are: 1) One of the few methods to inspect finned tubes 2) Can be used on all ferromagnetic materials 3) Good sensitivity to ID pitting and corrosion 4) High-speed inspection.