What it finds
- Surface-breaking cracks from fatigue, grinding or heat treatment.
- Porosity, laps and seams that are open to the surface.
- Leaks through thin sections, in some applications.
How it works
- Clean. The surface must be free of oil, paint, scale and anything else that blocks openings.
- Apply penetrant. The liquid is sprayed, brushed or dipped on, then left for a set dwell time while capillary action draws it into openings.
- Remove the excess. Penetrant is taken off the surface while staying inside any flaw. Depending on the process, this is done with water, with an emulsifier and water, or with a solvent wipe.
- Develop. A thin developer layer draws the trapped penetrant back to the surface.
- Inspect. The inspector looks for marks, either visible dye in white light or fluorescent dye under ultraviolet light, which is more sensitive.
- Clean again, so residue does not harm the part.
Where it is used
PT is used on aluminum, titanium, nickel alloys, stainless steel, plastics and ceramics, and it is common for turbine parts, castings, welds and machined aerospace parts. Portable spray-can kits make it useful in the field too. See our guides to aerospace manufacturing, pharmaceutical manufacturing and food and beverage processing.
Limits
- Surface-breaking flaws only. A crack that stays beneath the surface will not show.
- Cleaning decides the result. Oil or residue in a crack blocks the penetrant. Machining or blasting can also smear metal over a crack, so soft metals may need chemical etching first.
- Not for porous materials, where the whole surface holds penetrant.
- Rough surfaces hold background penetrant that can hide or mimic real marks.
- Process control. Dwell time, temperature and over-washing all matter, and penetrant chemicals can matter too, since some alloys and services need low-contaminant products.
Who is allowed to do this work is set by a certification scheme. See How to Become an NDT Level I, II or III.
Frequently asked questions
Visible or fluorescent penetrant?
Fluorescent penetrant is more sensitive and is common in aerospace work, but it needs a darkened area and ultraviolet light. Visible dye needs less equipment and suits field work where the specification allows it.
What is dwell time?
The time the penetrant stays on the part so it can enter any openings. The procedure sets it, based on the material, the type of flaw expected and the temperature.
Can PT be used on painted parts?
Generally no. The coating must come off first, because paint fills or covers the openings the penetrant needs to enter.
How NDT LLC can help
Our team of Level IIIs helps with PT procedures, line setup and process control, and audit readiness, and we take on field NDT and inspection work. Contact us.
This article is general information for a business audience, not engineering, legal or regulatory advice. Always work to the current edition of the codes, standards, regulations and manufacturer data that apply to your equipment and contracts.
NDT LLC is an independent consulting firm. It is not affiliated with, endorsed by or sponsored by ASNT, ASME, ASTM International, or any other organization named in this article; names are used only to identify the organizations and documents referred to.