Magnetic Particle Inspection is used to detect surface and slightly sub-surface defects of ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. Magnetic particle inspection uses magnetic field and Ferrous iron particles to detect flaws in materials.

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Direct current (DC) and alternating current (AC) are both suitable for magnetizing parts for MPI. The primary difference between the two currents is, the fields generated by DC penetrate the cross section of the part, and the fields generated by the AC are confined to the metal at or near the surface of the part. Therefore, AC should not be used for subsurface discontinuities.