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 Liquid Penetrants 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 dwell 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 spreads throughout the developer surface magnifying the size of the indication. The contrast in colour between the red penetrant and the white developer plus the magnifying effect caused by the spreading of the dye leads to a clearly visible indication.