Liquid Penetrant Inspection (PT) is another common method of NDT, and is solely used to detect surface breaking discontinuities, which are free from debris that can limit the entrance of the dye (oil, grease and paint).
This method of Inspection can be applied to Ferromagnetic and non-ferromagnetic materials, however it is most commonly used in non-ferromagnetic components, for example when MT is not practical. The LiquidPenetrants used have a low viscosity and a high affinity for metallic surfaces. The dyes are applied to the test part by aerosol spray or by being submerged in a tank were the dye penetrates any surface breaking flaws. After a required soak time the dye can be removed so no excess fluid remains on the surface and a thin coating of a highly absorbent developer is sprayed over the test area. The developer draws any entrapped dye out of any cracks of fissures by capillary action and the dye.