Only Ferromagnetic materials can be inspected by the MT method. This is because Ferromagnetic materials develop strong internal magnetic fields when an electrical current is passed through them. An electric current can be introduced in to the test part in several ways. It can be wrapped in encircling coils and rods or the current can be applied directly with the use of the yoke producing a magnetic field perpendicular to the current flow. When these internal magnetic fields encounter a change in permeability (i.e. an open fissure/crack) the magnetic field is forced outside of the materials surface, and produces flux leakage. This leakage will attract any other Ferromagnetic materials that may be close to the leakage site.
Prior to any MT being carried out the part is cleaned of any loose scale, oil/grease, and then covered with a very thin layer of background contrast paint (this is applied by painting or by aerosol). The aim of the contrast paint is to make any defects or anomalies stand out, and help the Inspector in locating the defect. Once the contrast paint is dry, particles with an affinity for leakage fields are passed over the part, these Ferromagnetic particles are applied by aerosol i.e. wet or dry powder form depending on the temperature or the part. These particles are highly visible against the contrast paint. When the particles are attracted to the leakage field around the surface flaw, they take the shape of the anomaly that has broken the magnetic field. The pattern of the particles clearly shows the shape and contours of the anomaly, allowing for easy monitoring and recording by the inspector.