Magnetic particle testing (MT) is a non-destructive testing technique utilized to detect defects or discontinuities (such as cracks) at or near the surface in ferromagnetic metals such as iron, steel, nickel, cobalt, etc. Magnetic particle testing (as well as other non-destructive testing techniques) is primarily utilized in the industrial sector to test metal materials such as oil & gas pipelines and various metal machinery components to prevent failures or accidents. Some of the most common industries that utilize magnetic particle testing include petrochemical, automotive, aerospace and structural steel – just to name a few. In addition, magnetic particle testing may be utilized to inspect underwater structures such as pipelines.

So exactly how does magnetic particle testing work? The concept is actually relatively simple. Any metal object that is magnetized will be surrounded by an invisible magnetic field. If there is a defect – such as a crack or hole in the metal object – the defect will cause a disruption in the magnetic field. Magnetic particle testing allows you to easily see that disruption – and thus identify the defect.

Here is how the actual process takes place. The first step is to magnetize the object being inspected (for example, a pipe weld). Electronic equipment – ranging from a hand-held magnetic yoke to a horizontal MT machine – is used to magnetize the object. Ferrous iron particles (similar to metal filings or shavings) are then applied to the object. The iron particles will be attracted to any area where the magnetic field is disrupted and will form clusters – thus plainly revealing the defect.

Because visual inspection is such an important part of the process, there are several different types of magnetic particles that may be utilized in this process. The choice of particles often depends upon the color or darkness of the metal being tested. The most commonly used particles include black iron particles and yellow or red iron oxide particles. When testing black or very dark metals, a thin coat of white paint is sometimes applied to the metal to provide contrast. In addition, magnetic particles may be coated with a fluorescent material. The test object is then viewed under an ultraviolet lamp in a darkened environment – causing any defects to glow brightly. Most magnetic particles are applied in a liquid suspension – which allows the particles to flow more freely over the surface of the object. Dry magnetic particle powders may also be used – and are especially useful when testing hot surfaces where using a liquid is not feasible.

Magnetic particle testing is one of the most common non-destructive testing techniques utilized by many industries. The advantages of this technique are that it is relatively fast and easy – plus there is usually little or no surface preparation needed. The disadvantages include the fact that magnetic particle testing will only detect defects at or near the surface of the object being tested – which means defects deep under the surface will not be detected. In addition, this technique may only be used on objects made of ferromagnetic materials – which does not include certain metals such as aluminum or copper.