Magnetic Flux Leakage (MFL) Testing is an NDT technique used to detect pitting, circumferential cracks, and wall losses in ferromagnetic tubes such carbon steel, nickel, and ferrous stainless steel. This technique is typically used for the inspection of air cooler and heat exchanger tubes.

 The defect sizing capacity of MFL has some limitations; the signal amplitude is affected by pulling speed variations and the signal has no phase component. This makes MFL more of a detection method than a sizing one. Inspection is quite fast; the pulling speed can go up to 1 m/s. As for ECT, this technique requires a good fill factor as well as a good probe centering position to maintain a fair sensitivity to small volume defects. 

The principle of operation is based on magnetization. Two permanent magnets coupled to a steel core are used to magnetize the tube wall until complete saturation is reach. To that, three coils are added; each one being sensitive to specific defects.

For wall loss detection, we use the coil operating in the absolute mode. That coil is wound around the steel core between the two magnets. It measures the strength of the magnetic field produced by the magnets. So where there is general thinning such as erosion, even if no flux leakage occurs, the strength of the magnetic field decreases and this variation is detected.

To detect pitting, we use the lead coil. That coil is wound over the absolute coil, between the two magnets. When the magnetic field comes across pitting, the flux lines are diverted and theses distortions cannot be contained in the tube wall. Then flux leakage occurs on the inside or outside surface of the tube depending on the pitting location. The lead coil will detect this leakage but it is not possible to deduce the location (ID or OD) of the defect. To find the pitting location, you need the response from a third coil, the trail one.