Published on 19-Jan-2023

Liquid Penetrant Testing: Basics, Usage, and Benefits

Liquid Penetrant Testing: Basics, Usage, and Benefits

What is Liquid Penetrant Testing?

Liquid penetrant testing or liquid penetrant inspection is a widely used non-destructive testing method and the oldest one. It is also known as dye penetrant inspection. The liquid penetrant testing takes advantage of capillary action, that is the capacity of the liquid to run into breaks or holes without any external aid and also, in opposition to external forces like gravity, to identify any kind of defect. After a while, an excess amount of surface penetrant is removed and a developer is applied. The resting period is called the dwell time. The developer pulls the penetrant from the surface-breaking defects and exposing it.

There are six types of methods used in the liquid penetrant technique. They differ based on the kind of penetrant and process of cleaning before applying a developer. The two types of penetrants are fluorescent or colour contrast/dye penetrant, and the three processes of cleaning are, water washable, solvent removable, and post-emulsifying. The most common method is dye penetrant that is solvent removable.

The dye penetrant solvent removable technique is most common as it is reasonable and very adaptable. Typically, it is available in three aerosol cans that are cleaner, penetrant, and developer. The aerosol cans are very flexible, that enables them to be taken up ladders, inside boilers, down into wells, and into very compact places.

Application of Liquid Penetrant Testing

The liquid penetrant testing is adaptable, cost-effective, and demands the least amount of training as compared to the other non-destructive testing methods. The test is efficient in identifying laps, cracks, permeability, layers, fractures, and other types of defects that are open to the surface of the test item. These defects can be due to bursts, overuse, shrinkage, grinding, overload, and machining. Consequently, it is frequently used in welds, castings, manufactured products, forgings, bars, plates, and pipes.

The most successful use of liquid penetrant testing is on non-porous and relatively smooth materials such as metals, glass, plastics, and fired ceramics. The common nonporous materials that are examined using liquid penetrant testing are steel, rubber, stainless steel, plastics, cast iron, brass, aluminium, bronze, titanium, and glass. Porous materials like concrete, cloth, wood, paper, and some types of fibreglass if the fibres are visible on the surface should not be tested using this technique.

Steps of Liquid Penetrant Testing

The proper methods for conducting liquid penetrant testing differ across many factors, like the size and type of the material, the type of penetrant system, the type of discontinuities anticipated in the component, and the state and setting under which the inspection is being conducted. Nevertheless, the following are the common steps taken in liquid penetrant testing:

1. Surface preparation

The first step of liquid penetrant testing is surface preparation that is one of the most important steps. The requirements for the testing are that the surface should not have any water, grease, oil, or any other type of impurities that can lead to obstruction in the flow of the liquid into the defects. The object of penetrant testing might be required to be marked if mechanical operations such as sanding, grit blasting, or machining are being carried out. These and other mechanical operations may smudge metal over the defect opening and stop the penetrant from entering.

2. Penetrant application 

After the surface is prepared and cleaned, the penetrant material is applied using a brush, spray, or dipping the part in a penetrant solution. The application of the penetrate can be done by both visible and fluorescent dye penetrants. The process of application depends on the personal choice but can also be affected by the shape and size of the object being tested, the availability of the equipment for conduction of the test, and the requirements of the test description appropriate for the samples.

3. Penetrant dwell time

The penetrant is given some time to stay on the surface to be pulled down into the cracks or areas of defect. The total amount of time the penetrant is on the surface is called the penetrant dwell time. The dwell time is often mentioned in the penetrant procedure being followed or by the penetrant producers.

The dwell time differs based on the type of penetrant material, its application, the type of discontinuity being inspected, and the form of the material being inspected. Typically, the dwell time can be between 5 to 60 minutes. Unless the penetrant dries, there is no side effect of applying penetrant for a long period of time. Most of the time, the dwell time is established by experimentation and may be exact for a certain application.

4. Excess Penetrant Removal

The next step, that is the removal of excess penetrant is fragile as it requires the excess penetrant to be taken off of the surface without removing a minimal amount of penetrant from the defects. This step may include additional work depending on the system of penetrant, like direct cleaning with water, cleaning with a solvent, or first treating the part of the object using an emulsifier and then cleaning with water. If the liquid used is a visible dye, then the excess penetrant is taken off using a solvent. With excess fluorescent dye, it may be rinsed with water or may be emulsified before and later it is rinsed.

5. Developer application

To extract the penetrant stuck in the cracks or flaws back to the surface visibility, a thin layer of developer is put on the surface. There are a wide variety of developers available in the market that can be applied with the help of spray for wet developers, dipping the part into the solution, or dusting with a dry powder. The subsequent indications are larger than the real flaw. They have a great range of contrast between the penetrant and developer, thus, making them more observable to help in the inspection.

6. Indication Development

Like the penetrant dwell time, the developer is also required to stay on the surface of the object to allow the removal of the trapped penetrant out of any surface defects. The minimum time required for a developer to remain on the surface is about 10 minutes and might be more, based on the characteristics of the cracks.

7. Inspection

Inspection is conducted by trained or certified personnel using a visual examination. It is done with proper lighting to identify indications from any of the defects that may be present. In the case of fluorescent penetrants, the indications are detected in a dark condition with the help of a high-intensity UV lamp or black light, whereas, with visible dye, they require adequate white light. The obtained sample will be accepted or rejected based on the requirements or acceptance criteria followed for the order.

8. Clean surface

The final step in the process is to clean the surface of the part of the object to clean the developer from the parts that were discovered to be acceptable.

Benefits and Limitations

The liquid penetrant testing has various benefits and limitations:

Benefits of liquid penetrant testing

Liquid penetrant testing is a relatively quick, easy, and cost-effective method of surface inspection. The technique is non-destructive, so the test samples are not damaged during testing. It can be conducted on various kinds of equipment regardless of its shape or size, although the volume of equipment and the facility used for testing can produce some limitations. They possess great sensitivity to small surface discontinuities.

The liquid penetrant testing can be used with a wide range of materials, from metallic to non-metallic, magnetic to non-magnetic, and conductive to non-conductive. A visual representation of the defect is shown directly on the part surface. The process is convenient, reasonable, and portable due to the use of aerosol sprays. Indications can expose the relative size, shape, and depth of the flaw. It is an easy technique and involves the least amount of training.

Limitations of liquid penetrant testing

The major drawback of liquid penetrant testing is that the defect should be open to the surface. The surface finish also affects the accuracy of the testing like the materials with porous surfaces cannot be examined using this technique. The surface is required to be clean and smooth. Any impurities like dirt, paint, oil, rust, and grease are to be removed before the inspection. Therefore, cleaning the part before the testing is a vital part of this technique.

The liquid penetrant examiner should have accessibility to the surface being examined. The metal smudge from shot blasting, grit blasting, or wire brushing should be cleaned before the liquid penetrant testing. There are various steps required to perform and control the testing process. Thus, the chemicals used need to be properly handled and disposed of. Fumes can prove to be dangerous and combustible without proper ventilation.


In conclusion, liquid penetrant testing is one of the easiest and valuable non-destructive testing methods used in the inspection of a variety of materials. This testing technique has its benefits and limitations. However, liquid penetrant testing is a flexible, cost-friendly examination and one of the best non-destructive testing techniques. 

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