Magnetic Particle Inspection (MPI)

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 or a horizontal MPI bench setup. 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. For high sensitivity, a fluorescent liquid and blacklight can be used. This requires a dark testing environment.

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MPI advantages:

  • Rapid inspection of large surfaces
  • Relatively low equipment cost 
  • Detection of surface and subsurface defects
  • Immediate interpretation of indications
  • Can be used in workshop and on site
  • Surface preparation is less critical than with Penetrant Testing (PT)

MPI limitations:

  • Restricted to ferromagnetic materials
  • Requires supply of electricity
  • It is sometimes unclear whether the magnetic field is sufficiently strong to give good indications
  • Cannot be used if thick paint coating is present
  • Some of the paints and particle suspension fluids can cause fire risk, particularly in a confined spaces
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  • How do I know if my part can be tested using magnetic particle inspection (MPI)?

    A simple way to establish if a part can be tested using magnetic particle inspection is by using a permanent magnet to see if it is ferromagnetic. If it is, the magnet will "stick" to the part and we can inspect it using MPI.

  • What kind of particle liquids do you use for Magnetic Particle Inspection (MPI)?

    Depending on the material and location requirements, we can choose from the following liquids:

    • Oil-based fluorescent ink
    • Water-based fluorescent ink
    • Oil-based black ink
    • Water-based black ink

  • What are the main differences between magnetic particle and dye penetrant testing?

    MT: fast inspection technique
    PT: relatively slow process

    MT: can only be applied in examination of ferromagnetic materials
    PT: provides optimal results in all metals and alloys

    MT: detects not only surface cracks but also imperfections that are very near to the surface
    PT: only detects discontinuities which are on surface and open for the dye to penetrate

    Surface conditions
    MT: no pre-cleaning and post-cleaning processes are necessary because surface pollutants don’t have adverse effect on inspection results
    PT: Strict pre-cleaning is required 

    MT: is applicable up to extreme temperatures (normally below Curie temperature)
    PT: limited temperature conditions (5-60 degree Celsius)

    MT: power supply is required to operate yoke magnet
    PT: no power supply needed

Henk Schroots

Area Sales Manager NDT & Laboratory

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