Magnetic flux leakage (MFL) detects anomalies in normal flux patterns created by discontinuities in ferrous material saturated by a magnetic field. This type of test can be used for piping and tubing inspection, tank floor inspection and other applications.
The testing head contains drive and sensor coils, This head is placed around the pipe or tube to be tested and the drive coil is energized, creating a magnetic field in the part. As the head travels along the length of the part, variations in the wall thickness due to corrosion, erosion, pitting etc., will cause a change in the magnetic flux density. This can be picked up by the sensor and sent back to the computer. This can be done without removing the insulation.
Tank floor inspection applies the same principle, but uses a series of drive coils ("bridges") and sensors located side by side across the front of a vacuum sweeper-like machine. The bridges generate a magnetic field that saturates the tank floor, and any reduction in thickness/loss of material due to pitting or corrosion will cause the field to "leak" upwards out of the floor material where it will be picked up by the sensors. On very basic machines, only an audio and/or a visual display will show that there is an indication; more advanced machines can have both visual displays and recording capability so that the results can be stored, analysed and compared to earlier results to monitor discontinuity growth.