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Magnetic Particle Inspection (MPI)

MPI (Magnetic Particle Inspection) is used for the detection of surface and near-surface flaws in ferromagnetic materials like iron, nickel, cobalt and gadolinium.

During inspection a permanent magnet, electromagnet or electromagnetic coil is used to apply a magnetic field to the test item, with flaws being revealed by the distortion of magnetised flux.

The component surface is first magnetised in at least two perpendicular directions at 90 degrees to each other, ensuring complete inspection in both planes. The process consists of spraying fine magnetic particles within a carrier fluid onto the component. Particles are then attracted to the area of flux leakage, creating a visible flaw indication.

To ensure clear contrast visibility, white paint is mixed with black magnetic particle ink. A further option uses dry powder application for hot surfaces where wet ink is less effective. The most sensitive process for revealing finer imperfections uses fluorescent particles viewed under UV light.

Types of electrical currents used for MPI

There are several types of electrical currents used in MPI. For a proper current to be selected considerations is given to the part geometry, material, the type of discontinuity being looked for, and how far the magnetic field needs to penetrate into the part.

  • Alternating current (AC) is commonly used to detect surface discontinuities. Using AC to detect subsurface discontinuities is limited due to what is known as the skin effect, where the current runs along the surface of the part.
  • Direct current (DC, full wave DC) is used to detect subsurface discontinuities where AC cannot penetrate deep enough to magnetize the part at the depth needed. The amount of magnetic penetration depends on the amount of current through the part. DC is also limited on very large cross-sectional parts in terms of how effectively it will magnetize the part.
  • Half wave DC (HWDC, pulsating DC) works similarly to full wave DC, but allows for detection of surface breaking indications and has more magnetic penetration into the part than FWDC. HWDC is advantageous for inspection process as it helps move the magnetic particles during the bathing of the test object. The aid in particle mobility is caused by the half-wave pulsating current waveform. In a Typical mag pulse of 0.5 seconds there are 15 pulses of current using HWDC. This gives the particle more of an opportunity to come in contact with areas of magnetic flux leakage.

Each method of magnetizing has its pros and cons. AC is generally the best for discontinuities on the surface, while some form of DC is better for subsurface defects.

See a complete range of the standards used by ITCL for Dye Penetrant Testing – here


We pride ourselves on the high quality of application and testing by our experienced technicians. Our workforce is fully qualified to current standards and work to approved procedures and control checks. Environmental and health & safety issues are of paramount concern to all staff working on your site, with expert consultation always available.

ITCL offers a comprehensive 24hr in-house and on-site service.

Contact a member of the ITCL team on 0151 356 7118 for more information.

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Magnetic Particle Inspection

Magnetic Particle Inspection

For information on any of our services please contact us on +44 151 356 7118