MFL (Magnetic flux leakage) uses a powerful magnet to magnetize the conductive material under test (usually steel). Where there are defects - corrosion or loss of material - the magnetic field "escapes" from the steel. MFL probes incorporate a magnetic detector positioned between the poles of the magnet where it can detect the leakage field. During inspection, a kind of magnetic circuit is formed between the workpiece and the probe. The magnetic field induced in the workpiece saturates it until it can no longer hold the flow. The flow overflows and escapes from the pipe wall and sensors can accurately measure the leakage field.