Published on 11-Aug-2023

Liquid Penetrant Testing: A Complete Guide

Liquid Penetrant Testing: A Complete Guide

The importance of perfection in the engineering industry can never be taken for granted. The quality of builds, welds, and even the most minute of components can cause a difference between the longevity of the Iron Pillar of Delhi, which dates to the 5th century, and the capsizing of the Flotel disaster of 1980 in Norway. The latter caused 123 people to perish and is remembered as one of the worst engineering disasters of all time. The reason for the massive incident was a defect in a mere 6mm weld. The oil rig was designed to handle winds of up to 21 m/s and waves of a height of 12 m, yet an error of a few millimeters took it down.

Non-Destructive testing enables the detection of cracks/crack propagation in a structure, material, or mechanism. The structural integrity of welds holds great importance in various industries. Liquid Particle Inspection is used to detect such welds and materials that show surface defects. The liquid impinges on the surface of a weld or material; the capillary action hence causes the liquid used to enter the minor cracks, pits (air gaps in welds), porosity, and similar defects.

New manufacturing techniques, like additive manufacturing, and the existence of older processes produce numerous defects due to the mere nature of the techniques and are formed even before defects are caused due to handling and post-production processes. To counter these, non-destructive techniques like liquid penetrant techniques are used.

The first Liquid Penetrant Testing machine was built in the 1940s(Image credit- American society of Non-Destructive testing)


Liquid Penetrant inspection is a non-destructive technique that can be used on metallic and non-metallic components. It’s used extensively to test weld quality and integrity due to the cost-effectiveness and simplicity of the testing process. Liquid Penetrant testing utilizes physical and chemical methods to enter surface defects in non-porous materials and help visually perceive the locations and types of deformities.

The testing liquid is a penetrant dye that enters any discontinuities in the surface and can be identified by the naked eye or under specific lighting (white light of specific luminosity or black light).

The Liquid Penetrant test is a multi-step procedure that is carried out as follows:

  •  It should be ensured that the surface is entirely free of dust, dirt, debris, and grease, which can coat the material's surface and shield the discontinuities, making the test inaccurate. It could also contaminate the liquid penetrant and affect its fluorescence properties if fluorescent. Pickle liquor or etchants are used to clear the adulterants on the surface of the material in some situations.
  • The liquid dye is then spread on the metal surface. A waiting period, called the dwell time, is hence carried out to ensure the dye infiltrates areas of defects.
  • Excess dye should be rinsed with caution (sometimes with an emulsifier), guaranteeing that none of the dye within the defects is removed.
  • A developer is utilized post-removal of excess penetrant, which draws out the dye trapped within the defects to the surface. This developer provides contrast to the dye; in the case of fluorescent dyes, it boosts the fluorescence to improve the visibility of defects.
  • The test subject is inspected for any surface defects under the intended lighting setup for the dye used. An experienced operator can then detect the suitability of the component or structure for continued use.
  • Post-cleansing of the surface is then carried out, often with measures taken to prevent corrosion and ensure that the test subject is safe to use.

Depiction of the Liquid Penetrant testing process (Image credit- American society of Non-Destructive testing)


Cleansing of the surface and ensuring no contamination is essential, and the surface should be observed prior to testing to check for any manufacturing-related issues that may obstruct the liquid penetrant from thorough performance.

Penetrant materials are classified into three types, such as:

  • Fluorescent penetrant dye
  • Contrast dye (usually red in color)
  • Dual-function dye (both visible and fluorescent)

 The dye removal method is also classified into the following types:

  • Water washable method
  • Lipophilic method
  • Hydrophilic method
  • Solvent removal method

The dwell time is often gauged by testing experience. Following are the dwell times for steel parts (based on standard specifications):

The usage of developers was derived from an age-old liquid penetrant inspection technique called Oil and Whitting. The Oil and Whitting processes involved the immersion of the test subject in oil and then coating it with chalk (whiting) that stuck to the oil at points of discontinuity. Developers have come a long way since then. The developer used to draw out the penetrant dye is now available in a variety of forms, mainly:

  • Powder form (dry in nature)
  • Water soluble
  • Suspendable in water
  • Non-aqueous or solvent form, used for fluorescent penetrant dye.
  • Non-aqueous form used for color contrast dye.

The penetrant liquid is cleansed following the inspection processes. These are available in a halogenated, non-halogenated, and case-specific solvent remover variety.

Various components during operation and post Liquid Penetration Testing procedure (Image credit:


Liquid Penetrant Testing, also called Dye Penetrant Testing (DPT), can be used for various applications and in diverse industries, such as :

  • Liquid particle inspection can be used to carry out leak testing of components.
  • It can be used in primary metal production in foundries, in the testing of Ferrous metals, and in testing in Light Alloy foundries.
  • Liquid Penetrant Inspection has electric power applications and can be used in electric and  Nuclear power plants to indicate discontinuities in nuclear reactor piping.
  • Liquid Penetrant Inspection has applications in the aerospace industries as well, being used in aircraft fleet maintenance and to test radioisotope heat sources for deep space missions.
  • Apart from these, Liquid Penetrant Testing procedures are also used in testing metal cutting tools, the marine industry, automotive parts, and plastic materials.


Liquid Penetrant Testing and Magnetic Particle testing both involve the application of a material on the surface of a material meant to infiltrate discontinuities and defects, followed by an indication process. However, there are multiple stark differences between the above non-destructive testing techniques, such as:

  • Magnetic Particle Inspection also involves liquids as the indicating material in certain situations, yet the principal difference is that the magnetic particles carried by the liquid are the indicator, whereas in Liquid Particle Inspection, the liquid itself is the indicator.
  • Magnetic Particle Inspection can only be performed on ferromagnetic materials, whereas Liquid Particle Inspection can be carried out on a variety of materials like plastics, metals, and non-metals.
  • Magnetic Particle Inspection and Liquid Particle Inspection both involve the use of chemicals. However, Magnetic Particle Inspection requires magnetization of the test surface, which adds more hazards to the test process.


As with every Non-destructive testing process, liquid penetrant inspection has industry standards that need to be diligently followed to maintain consistency and ensure safety; they are:

  • American Society of Mechanical Engineers, Section V, Article 6- Liquid Penetrant Examination
  • American Society of Mechanical Engineers, Section V, Article 24-Standard Test method for LPI
  • American Society of Testing and Materials, E 165 – Standard Practices for LPI (General Industry)
  • American Society of Testing and Materials, E 1417- Standard Practise for LPI
  • European Standards, 1371-1 and 2 – Liquid Penetrant Inspection
  • European Standards, 10228-2, NDT of steel forgings, Penetrant testing
  • European Standards,10246-11, NDT of steel tubes- Liquid Penetrant Testing of steel tubes for the detection of surface imperfections.\
  • ISO 3452-1, 2, 3, 4, 5, & 6 - Penetrant testing.
  • ISO 3059- Penetrant testing- Viewing conditions.
  • ISO 12706- Non-destructive testing – Penetrant testing – Vocabulary
  • ISO 23277- Non-destructive testing of welds – Penetrant testing of welds – Acceptance levels


Liquid Penetrant testing has lasted through the times because of its various merits, some of which are:

  • Liquid penetrant inspection is easy to carry out even on test subjects with complex geometries.\
  • It can be used to inspect the larger surface area and is hence time efficient.
  • Metallic, non-metallic, conductive, and otherwise materials are all eligible for this process.
  • Liquid Penetrant Testing is a sensitive procedure and can be used to detect minuscule defects as well.\
  • It provides a visible depiction of the discontinuities on the surface.
  • The visible detections also provide information on the dimensions of the defects. (Based on the operator’s knowledge and experience)
  • Modern variants of this inspection method have portable options as well (aerosol cans).

Liquid Penetrant Testing is subject to a variety of shortcomings, including:

  • The penetration capability of the dye. This can be affected by various factors, including contamination and post-manufacturing or processing flaws that may cover the defects.
  • Care should be taken in cases of coatings on test subjects as the coating material may retain the penetrant liquid and prevent it from entering discontinuities.
  • The pre-cleaning should be thorough enough to clean within discontinuities as grease, moistures, or corrosives may react or repel the dye liquid and further obstruct testing.
  • Certain test subjects become ineligible for Liquid Particle Inspection after use or mechanical operation as the surface of the metal may be affected and might close the openings of the discontinuities.
  • Liquid Penetrant testing is better at testing flaws under tensile loading or no loading rather than compression loading.
  • Liquid Penetrant Testing is better at detecting more profound flaws than shallow flaws.
  • Liquid Penetrant Testing is also better at detecting minor, rounded defects rather than linear defects.
  • Porous material may cause the dye to seep entirely through the material and make the process unsuitable. It will also be arduous to cleanse the dye post-application.
  • The penetrants, developers, and emulsifiers used in this process may be hazardous for humans, and precautions need to be carried out while performing these tests. They can also prove to be corrosive to the material and corrosion inhibitors need to be used to prevent deterioration of the material.
  • The chemicals used in this process should be disposed of responsibly to not cause harm to the environment or life.


Acceptance criteria refer to the types of discontinuities that can be potentially found in the testing process and the extent to which they are permitted. The nature of the test subject affects the Liquid Penetration testing procedure, as well as the criteria for the test. In the case of pressured vessels, welds, process piping, etc as test subjects, the American Society of Mechanical Engineers code, section VIII, Division 1, appendix 8 is referred to.

For casting used in rotating mechanisms and pressure vessels, ASME code, section VIII, Division1, appendix 7 is used as a reference.

In the case of steel structures, the codes in the American Welding Society, D1.1-Structural welding, steel should be used.

These standards provide detailed criteria for subject design, assembly, inspection, certification, and testing. According to the ASME standards, the surfaces should be devoid of the following:

  • Relevant linear indications, wherein linear indications refer to any indication whose length exceeds three times its width.
  • Relevant rounded indications greater than 5mm. Here the indication is circular or elliptical in nature, with its length equal to or less than three times its width.
  • Four or more relevant rounded indications separated by 1.5 mm.

As per the criteria in Appendix 8, page 417 of ASME code Section VIII, Division 1 (2017 edition), indications with major dimensions greater than 1.5mm are considered relevant indications.

It should be noted that the temperature of the liquid dye and the test subject need to be considered as well. Temperatures ranging between 27°C to 49°C are considered optimal for testing. Penetrant materials that meet the specifications under AMS 2644 are preferred.

The AWS D1.1 criteria for visual inspection of Liquid Penetrants Inspection is as follows:


Non-destructive techniques such as Liquid Penetrant Testing, with strict adoration to the industry standards stated in the previous sections, have not only helped in preventing catastrophe but further the life of structures, the functionality of machines and technology far beyond the scope of what was previously thought possible, since it doesn’t hinder the functionality of the structures tested and helps check any maintenance issues.

The industry standard allows liquid penetration to provide accurate detection of defects and deformities, saving Economical and human capital, which is crucial with the developing and ever-changing technologies faced in the industry today.

Liquid Penetrant Test is an excellent Quality Control tool and helps organizations keep up with safety and industry standards.

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