Magnetic particle inspection is a non-destructive testing process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The magnetic lines of force are perpendicular to the direction of the electric current.

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak. Ferrous iron particles are then applied to the part. If an area of flux leakage is present, the particles will be attracted to this area. The particles will build up at the area of leakage. The leakages can then be evaluated to determine what it is, what may have caused it, and what action should be taken.