Benefits of liquid penetrant inspection

The method is simple to use, and can be performed on large components or in large batches of parts.


Dye penetrant inspection is one of the most widely used non-destructive inspection techniques used. It is useful for a wide variety of product applications and industries. This method is used in metal working shops, aerospace, power generation, and in any process that uses any sort of metal joining method such as welding. It is used in the petrochemical industry to verify weldments and to inspect piping.

There are many advantages to dye penetrant testing. This method is readily portable to the job site, or to a specific location. It is a very inexpensive method to use. It is sensitive to detecting many types of flaws and is useful in a production environment. It is also readily documented by photography.

Disadvantages or limitations of dye penetrant testing include the possibility that the inspection is limited to surface flaws; the process can be messy, so post-inspection clean-up must be performed; and prior to testing, the surface must be properly prepared for testing.

Applications of Liquid Penetrant Inspection

There are many proven applications for liquid penetrant inspection. Some of these are [1]:

  • Inspection of tools and dies during manufacture and during use.
  • Nonferrous part inspection, such as connecting rods, pistons, and cylinder blocks.
  • Forgings and castings for laps, seams, and hot tears.
  • Inspection of weldments or leak testing of pipe.
  • Fatigue cracks in aerospace components.

Liquid penetrant inspection is limited to surface defects by capillary action. It is not an appropriate method for porous materials or the detection of subsurface defects.

The method is simple. First, the material is prepared by proper cleaning and degreasing. A liquid penetrant containing a dye is applied to the surface of a part and is allowed to sit on the part for a defined period of time. This is called the dwell time. The penetrant is drawn into the surface defect by capillary action. After the dwell time, the penetrant is removed from the part surface. Once the penetrant has been removed from the surface, a developer is applied to the surface. This developer draws the penetrant and dye to the surface, making the indication visible. The dyes and developer are chosen to provide a strong contrast to make the indication readily visible.

Figure 1: Location and morphology of cracking in a weldment by dye penetrant inspection [3].

Preparation of the surface

Prior to applying the liquid penetrant, the surface of the part must be free from paint, scale, or grime. Parts can be chemically cleaned, or brushing with a wire brush is usually all that is required. Use of grit or sandblasting is not recommended because it can deform the metal and mask any defects present. Vapor degreasing is usually used to remove oils and greases. Parts should be dried thoroughly, usually with hot air, as water can prevent the penetrant from entering any discontinuities. [2]

After the part has been properly cleaned, the liquid dye penetrant is applied. This can be done in any number of methods, such as dipping, spraying, or brushing. A spray can containing the liquid penetrant is a very common method of applying the penetrant. Adequate results can be achieved provided a continuous film of the penetrant is applied to the test surface.

After application of the liquid penetrant, the parts can sit, so that the dye is drawn into any surface discontinuities. The length of the dwell time, which is the time the penetrant will sit on the part, can vary from a few minutes to an hour, depending on the liquid penetrant used.

Once the penetrant has been applied, and an appropriate dwell time has elapsed, the penetrant must be removed. The removal process depends on the type of penetrant applied. Water is a common method for removing excess penetrant, as are solvents or emulsifiers. The type used depends on the penetrant used. The manufacturer can provide information regarding the proper removal fluid. Often, removal of the penetrant is performed under an ultraviolet light to make sure that all the excess penetrant has been removed.

The developer is applied after the removal of the excess penetrant. There are four types of developers: dry, non-aqueous, aqueous-soluble, and suspended aqueous. A thin, uniform coating of the developer is applied over the surface of the part. A thick film will mask any discontinuities. It usually has a white appearance. This helps in providing a proper contrast between the discontinuity and the background. The developer can dwell on the part from a few minutes to an hour, depending on specification. Typically, the part is examined immediately after application of the developer to assist examination of the part.

After the appropriate dwell time, the indications must be evaluated. These indications are evaluated on the size, shape, and intensity of bleed-out.

The size of the indication is proportional to the amount of penetrant that was captured by the discontinuity by capillary action. If the indication is a crack, it will often show as a linear indication. If the penetrant bleed-out is narrow, the crack is tight or shallow.

Once the discontinuities have been identified, and determined to be non-acceptable, they must be removed. This is often accomplished by grinding. Often the process is repeated to verify that the grinding has removed the unacceptable discontinuity.

Documenting the inspection process is often mandatory and is always good practice. Use of photographs or hand sketches is invaluable. Most digital (and smartphone) cameras do an excellent job of photographing under black light conditions. This will aid importing the documentation of the discontinuity into a formal report.


In this short article, the method of dye penetrant inspection was discussed. This method is simple to use, and can be used in either large components or in large batches of parts.

In the next article, we will discuss the use of magnetic particle inspection. Should there be any comments regarding this article, or suggestions for further articles, please contact the editor or myself. 


  1. Mix, P.E., Introduction to Nondestructive Testing — A Training Guide, Hoboken, N.J.: John Wiley & Sons, 2005.
  2. Hellier, C. J., Handbook of Nondestructive Evaluation, New York, NY: McGraw-Hill, 2003.
  3. Naz, N.; Tariq, F; Baloch, R.A., “Failure Analysis of HAZ Cracking in Low C-CrMoV Steel Weldment,” Journal of Failure Analysis and Prevention. 2009.