Magnetic Particle Inspection Explained

Like any construction industry material, metal must be solid and structurally sound. You can visually inspect the outer surface of metals for apparent cracks or deformities, but how can you ensure there aren’t defects, pores or lesions underneath the surface? The solution is magnetic particle inspection (MPI), a non-destructive inspection method that detects linear, surface and near-surface discontinuities.

Metal inspection: a brief history

Inspecting metal has been a challenge since the dawn of the Industrial Revolution. It used to be that the only way to check metal was with a magnifying glass or by tapping it with a hammer in various places and listening for sound differences. X-rays were a giant leap forward in the 1920s, but the method was too expensive for large-scale projects.

The idea of using magnetic particles to test industrial metals was a gamechanger. The method had already been used to inspect cannon barrels in the 1800s. Still, it wasn’t until the rail industry began using MPI in the 1930s that it gained widespread use.

How magnetic particle inspection works

MPI is an excellent way to inspect ferrous metals with magnetic properties like iron and steel. It uses tiny magnetic pieces ranging in size from microscopic to a grain of sand, immersed in a liquid solution or dry powder.

The particles are coloured fluorescent yellow or black to contrast with the test metal. The test metal is magnetized with an electric current, and the particles are sprayed onto it. Pores, cracks or other defects in the metal interrupt the magnetic field, and the particles will adhere to the metal in these areas differently, visually revealing potential problems.

Advantages of magnetic particle inspection

The most significant advantage of MPI is that it can quickly and affordably test large amounts of materials. The equipment is portable and easy to use. The method is sensitive, and the results are immediate. You can also use MPI on coated metals and irregularly shaped objects.

Magnetic particle inspection drawbacks

MPI isn’t perfect. The method can detect flaws only six millimetres below the surface. Inspecting deeper into metals requires more sophisticated and expensive techniques. Moreover, after the metal is tested, it must be demagnetized, requiring time and specialized equipment.

Another MPI drawback is that you can’t use it to test non-ferrous metals such as aluminum, copper or lead.

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