The SLOFEC (Saturated Low Frequency Eddy Current) inspection technique uses the eddy current principle in combination with a magnetic field. Utilising superimposed DC-magnetisation, the depth of penetration of the eddy current field lines in the ferromagnetic material is increased. In case of a defect, the magnetic field lines have a higher density in the remaining wall thickness, which consequently changes the relative permeability in the area, which again changes the eddy current field lines.