Dye Penetrant Inspection (DPI)

This is a widely used technique for identifying surface breaking flaws in all non-porous materials, either ferrous or non-ferrous, although mainly used on austenitic stainless steel assemblies or parts.

As the name suggdye penetrantests, DPI involves applying a dye to the cleaned surface of a material (2) which is able to penetrate into surface breaki ng discontinuities. After a period of time to allow penetration (dwell time), the excess surface dye is removed (3) and a develope r is applied to the surface (4) – this draws the dye back out of any discontinuities (due to capillary action), where it can be seen by the inspector & subject to interpretation.


Typical uses are detection of fine defects in aircraft components, clad welded parts for the oil and gas industry, etc.

Note: DPI dyes are usually aerosol products, which are considered ‘dangerous goods’ for shipping purposes and are supplied by Axiom complete with dangerous goods paperwork, certificates of conformity and COSHH data sheets.

Visit our Resource Area to find out about or buy British Standards applicable to Dye Penetrant Inspection

Advantages are:

  • DPI can be used on all clean ferritic and non-ferritic materials.
  • It is a simple, effective and inexpensive technique.
  • Systems have been developed in the aerospace industry which are sensitive to fine defects in the various austenitic alloys under test.
  • A good aid to visual inspection.


  • DPI only detects surface flaws, is labour intensive in the pre cleaning, performs poorly on rough surfaces and the various dyes/developers used can be harmful.

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