Finding surface defects in ferromagnetic metals

Magnetic Particle Inspection (MPI) or Magnetic Testing (MT) is an NDT method for checking the surface integrity of ferromagnetic materials. The material is magnetized using a handheld yoke. Defects in the surface and shallow subsurface cause magnetic field fluxes to "leak". When a liquid containing tiny magnetic particles is applied, these particles accumulate around those discontinuities making them visible to the naked eye. If necessary a white contrast agent is applied to the material before testing.

Advantages of MPI

  • Low cost

  • Portable equipment (if any)

  • Immediate results

  • Minimum special skills required

  • Minimum part preparation

Limitations of MPI

  • Only suitable for surfaces than can be viewed

  • Usually only larger defects can be detected

  • Scratches can be misinterpreted for cracks

  • Quality of inspection influenced by surface condition, physical conditions, environmental factors and physiological factors