What Is Dye Penetrant Testing?

Dye penetrant is a dye used by inspectors in dye penetrant inspections, an inspection method in which inspectors apply a dye or liquid to a surface to identify defects in it.

Dye penetrant is also commonly called liquid penetrant or liquid dye penetrant.

Also, dye penetrant testing is also commonly called dye penetrant inspection (DPI), liquid penetrant inspection (LPI), liquid dye penetrant testing, liquid dye penetrant inspection, liquid penetrant testing (LPT), or simply penetrant testing (PT).

In this guide, we will use the terms dye penetrant and liquid dye penetrant interchangeably, following the alternate terms listed above.

Dye penetrant is one of the most commonly used inspection methods. It falls into the non-destructive testing category of inspection methods, because inspectors can use it without permanently altering or damaging the object they’re inspecting.

[Dye penetrant NDT is just one of the non-destructive testing (NDT) methods that inspectors use. Learn more about NDT in this in-depth guide.]


What Is Dye Penetrant Testing?

Inspectors use liquid dye penetrant testing to look for cracks on the surface of assets and materials.

After flowing the dye penetrant onto the surface they want to inspect, inspectors will then draw the liquid out using a chalk-like developer, and thus reveal any material defects that might be present on the surface.

The procedure for a dye penetrant test relies on the principle of capillary action, which describes how fluids penetrate into cracks (or discontinuities) on the surface of a material.

Liquid dye penetrant testing is commonly used to inspect the surfaces of non-porous assets made out of ceramics, plastics, and metals, where inspectors will be looking at:

  • Welds

  • Castings

  • Forgings

  • Plates

  • Bars

  • Pipes

In these materials, inspectors will be using dye penetrant to look for defects like:

  • Leaks

  • Joint flaws

  • Fractures

  • Porosity on the surface of the materials

  • Cracks (cracks from fatigue, hairline cracks, or grinding and quenching cracks)

  • Incomplete fusion

Dye penetration inspections were first performed in the railroad industry in the early 20th century.

These early tests followed the oil and whiting method, in which a person first cleans a surface with an oil solvent and then applies chalk to the area (or another “whiting” material). The whiting would absorb oil from cracks present in the material, revealing their presence so that railroad personnel could identify and fix them.

Other industries began to adopt the liquid dye penetrant inspection method for inspections, eventually adding dye to make cracks more visible.


The Pros and Cons of Dye Penetrant Examination

Penetrant testing is a popular NDT method for inspectors because it is inexpensive and fairly easy to learn how to do. 

But those are just two of its benefits—here’s a list of pros and cons for dye penetrant testing:


  • Easy to perform, even with complicated surfaces/shapes

  • Inexpensive—no expensive cameras or equipment are required to perform DPI

  • Can be used to inspect large areas quickly

  • Findings (i.e., defects identified by this method) can be seen visually on the surface of the materials and can show the dimensions of the defect

  • Material flexibility—can be used on a variety of materials, including ferrous/non-ferrous, conductive/non-conductive, and magnetic/non-magnetic


  • Limited findings—only detects cracks on the surface (or “open” to the surface)

  • Porous materials can’t be inspected with dye penetrant

  • Dirty surfaces can’t be inspected with dye penetrant—DP won’t work on surfaces that contain paint, oil, dirt, rust, or any other similar kind of obstruction 

  • Direct access to the material is required

  • There are several steps in the inspection process, each of which could impact the quality of the findings

  • Cleaning is required both before and after the inspection (before to prepare the surface for the penetrant and after to clean the penetrant off the surface)

  • Chemicals are involved—inspectors must follow protocol to handle and get rid of them, and these chemicals could produce hazardous or flammable fumes 

Procedure for a Dye Penetrant Test

In dye penetrant testing, inspectors generally follow these six steps:

1. Clean the surface

First, inspectors clean the surface they plan to test so that the surface is open and any defects it contains will be exposed, instead of remaining hidden underneath dirt or other foreign elements. 

Cleaning processes inspectors commonly follow could include less invasive methods, like vapor degreasing, the use of solvents, or just wiping it with a wet rag, or more invasive methods, like grinding or wire brushing.

2. Apply the dye penetrant 

The penetrant that inspectors use is made just for this purpose, and it’s typically sprayed or wiped onto the surface with a brush. After applying the penetrant, inspectors wait for a “dwell period” of five to twenty minutes to allow it to dry. (The right amount of time should be indicated on the label of the specific penetrant being used.)

3. Remove extra penetrant and apply remover

Remove any excess penetrant with a dry rag.

After cleaning off extra penetrant, apply a remover to the surface and rub it dry with a fresh clean, dry rag.

4. Apply developer

After cleaning and removing the dye penetrant, apply a white developer to the surface. The developer will draw the penetrant from the flaws or cracks on the surface of the material and make them visible.

5. Inspection

At this point, cracks and other types of defects will be visible either to the naked eye or using white or ultraviolet light, depending on the type of penetrant that was used.

Now that the defects have been made visible, inspectors can conduct a visual inspection to identify any flaws that are present.

The difference between visible dye penetrant and fluorescent dye penetrant:

  • For visible dye penetrants, regular lighting conditions will allow defects to be visible.

  • For fluorescent penetrants, U.V. lights at low ambient lighting will allow defects to be visible.

6. Clean the surface

After the inspection inspectors typically clean the surface that was inspected to return it to its original condition.


Choosing Penetrants, Removers, and Developers

Inspectors have a few options to choose from for the penetrant, remover, and developer in the penetrant testing process.

Each of these options can be matched with any other (except for dry powder, which can’t be used with color contrast penetrant) allowing inspectors to create their own combination to suit their needs.

Here are the different options for each step:

Penetrant options

  • Color contrast

  • Fluorescent

  • Combination (both color contrast and fluorescent)

Remover options

  • Solvent

  • Post-emulsifiable remover

  • Water-soluble

  • Water-suspendable

Developer options

  • Dry powder

  • Aqueous

  • Non-aqueous developer

Inspectors choose between each of these options based on factors like the type of surface they’re inspecting, the size of the surface, and complexity of the surface.

Dye Penetrant Standards and Codes

Given how inexpensive it is and how relatively easy it is to do, dye penetrant is often used by inspectors for informational purposes—that is, for inspections that do not have to comply with a specific code or set of standards.

But penetrant testing is also commonly used for code-based inspections.

For these inspections, inspectors must follow specific steps in how they conduct the test, including the requirement that they follow a written procedure and that the person conducting the test is certified to do so by the relevant standards body.

Here are some of the more widely used dye penetrant testing codes:

ASTM (American Society of Testing and Materials)

ISO (International Standards Organization)

CEN (European Committee for Standardization)

Follow these links to read more about penetrant testing on the websites of internationally recognized standards organizations:

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