Magnetic Flux Leakage Testing Magnetic Flux Leakage testing (MFL) is a technique used for the inspection of tubes made of ferrous

materials. This technique will normally be applied as a fast screening technique if small diameter pitting is

expected. Because of limitations to its sizing abilities the MFL technique is not often used as a stand-

alone technique, and verification by other techniques is recommended. An advantage of the Magnetic

Flux Leakage inspection is that it can also be used on airfin cooler tubes.

Magnetic Flux Leakage is sensitive to sharp type defects like pits and grooving. Internal and external pits

can be detected. Depending on probe configuration MFL can distinguish between internal and external

defects and can detect gradual wall-loss. For ID/OD discrimination the probe needs to be equipped with a

second coil and to detect gradual defects and a Hall-effect sensor in the probe is needed.

The probe in this technique contains permanent magnets which are utilized to form a magnetic flux field

in the tube wall. Defects will influence the path of the magnetic field and will cause some of the flux to

leak out of the tube wall. This leakage field will be picked up by the coils and the Hall-effect sensors in the

probe. Size of the leakage field is determined by pull speed of the probe and by the shape, the

dimensions and the location of defects. Signals that represent the size of the leakage field and thus the

condition of the tube are presented on a computer screen.

MFL technique has the following advantages and disadvantages:

Sensitive to pits in carbon steel

Can be used on fin tubes

Sizing of defects is very limited

Very poor for detecting axial cracks

No single inspection technique is suitable for all tube inspection and Magnetic Flux leakage is no

exception. We would welcome your enquiry or questions so that we can help you to assess the best

method for your tube inspection requirements.

General

MFL uses a powerful magnet to magnetize the conductive material under test (usually steel). Where

there are defects — corrosion or material loss — the magnetic field “leaks” from the steel.

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.