The Liquid Penetrant (PT) method is a Surface Control that allows you to detect only surface discontinuities, i.e. with an orifice open to the surface.
The PT method is often used as a complementary test to other non-destructive methods in order to confirm hypotheses about the existence and actual extent of certain discontinuities. The principle on which the PT method is based consists in applying to the surfaces under examination a particular liquid with low surface tension (liquid with high wetting properties), capable of penetrating even very fine surface discontinuities and not directly visible to the naked eye; after suitable washing to remove the excess liquid from the surface of the piece and subsequent drying, the residual liquid, which has penetrated into said discontinuities, is released by means of a detector; finally, superficial indications are obtained that can be easily observed with the naked eye.
In order for the penetrating liquid to enter the discontinuities adequately, the surface of the pieces must be clean, i.e. free from oxides, grease stains, paint coatings, etc.
It is therefore understood the need for a good surface preparation before starting the application of the penetrant.
Similarly, at the end of the test, after observation, the surface of the pieces must be cleaned according to their subsequent use.
For example, for ground pieces, a final cleaning must be carried out, followed by the application of a rust inhibitor.
The observation of the indications must be done in suitable lighting conditions.
When using fluorescent penetrants, the observation must be made in a sufficiently darkened environment, irradiating the surface under examination with black light.