When a magnet is in close proximity to a steel plate magnetic lines of force (flux) are created within the plate. These lines of flux prefer to travel within the plate than the air. If the magnet is of suitable power it can create a near saturated flux in the plate.
A Corrosion pit and wall thinning will force the magnetic flux 'out' of the material and thus be detected using a Hall Effect sensor. The scanner has 36 no. of Hall Effect sensor; these 36 sensors are arranged in pairs forming 18 numbers.
A Hall Effect sensor is solid-state device, which when placed within an appropriate electrical circuit generates a voltage signal dependant on flux density. MFL system is designed to detect and size under floor corrosion for above ground level storage tank.