Magnetic Particle Inspection (MPI)

MPI is a method of detecting both surface and subsurface flaws in ferromagnetic materials (carbon based steels/mild steel). The principle of MPI is that when a current is applied to a conductor, a magnetic field will occur. The induced magnetic field will produce a flux leakage field around any discontinuities in the item – this leakage field is revealed by applying ferrous iron particles (usually from an aerosol) to the surface of the item. These ferrous particles cluster at areas of flux leakage to give indications of flaw in the item.

MPI is a stock-in-trade technique that may be used as the primary means of inspection (on bare or rough surfaces) or as a means of further exploring flaws detected by Eddy Current Inspection.

Typical applications are primary method NDT, post CVI, of welded fabrications and statutory inspections of in-service welded items.

Pre- upgrade MPI on the Beryl AlphaAxiom NDT have developed approved procedures for inspection of galvanised (conductive) and non-conductive coated weldments using Eddy Current and this is strongly recommended as an alternative for in-service items, detecting fatigue induced cracking, on such as;- padeye welds, racking board welds, powder coated steel on stadiums etc..

Note: MPI aerosol products are considered ‘dangerous goods’ for shipping purposes and are supplied by Axiom complete with dangerous goods paperwork, certificates of conformity and COSHH data sheets.

Visit our Resource Area to find out about or buy British Standards applicable to Magnetic Particle Inspection

Applications are:

  • first app
  • second app
  • third app

Advantages are:

  • MPI can provide detailed information about both surface and internal flaws is a relatively simple/robust and inexpensive technique.


  • The surface coating of the item to be inspected must be removed, usually by needle gunning, hence introducing the need for coating repair.

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