Liquid Penetrant Inspection

Principles

Liquid penetrant inspection  is a non-destructive method that’s used to inspect for surface breaking flaws using either a fluorescent or coloured dye. It can be used for a variety of materials such as metallic vs non-metallic, ferrous and non-ferrous materials, conductive and non-conductive materials.  

Penetrant inspection makes defects easier to see because the indications that form from the dye, is much larger than the flaw itself and they’re formulated to glow brightly so that the inspector is able to identify the flaw itself.

The process includes a pre-cleaning to remove any dirt, soil, water form the examination surface - penetrant application - penetration dwell time - removal of excess penetrant - developer application - inspection and/or examination - post-cleaning of part surface. 

What’s the process of Liquid Penetrant Inspection?

Surface Preparation 
  1. Test surface is cleaned using vapour degreasing, media blasting, solvent cleaning, alkaline chemical removal

  2. Media blasting or mechanical removal may smear indications, etching may be required if these cleaning procedures are used

  3. The inspection surface must be clean to remove any dirt, grease, oil, paint, coatings, water and anything else that could prevent the penetrant from entering defects. Coatings are important to remove because they may make false or non-relevant indications. 

Penetrant Application & Dwell
  1. Penetrant application is usually performed by spray application, immersion (dipping in a penetrant bath) or can be brushed on the test surface.

  2. Penetrant is allowed to be drawn and seep into any defects, this is called the “dwell time.” This could vary depending on the type of penetrant used, manufacturer or the specification being followed.

  3. Dwell times are usually from 10 to 60 minutes, and significantly longer dwell times could be used for extremely tight or fine defects.  

Excess Penetrant Removal
  1. Removal methods of excess penetrant vary depending on the type of penetrant being used. The removal methods are water washable, solvent removable or post-emulsifiable.

  2. Solvent removers are used in conjunction with a lint-free cloth and used to wipe off excess surface penetrant. Solvent removers are not sprayed directly onto the test surface as this will remove trapped penetrant from the defect surface.

  3. Post-emulsifiable penetrant requires emulsifiers that chemically react with the penetrant to allow the removal of excess penetrant by using a water spray.  

Developer application
  1. A developer is then applied to the test surface to help draw and reveal any surface breaking defects to the surface to make them visible. Developers can come in a variety of forms and applications including dusting (with dry powder), dipping or spraying (wet developers).

Indication development
  1. After the developer is applied, the test surface is given another dwell period to allow for the developer to pull trapped penetrant back to the surface of any flaws. The minimum development time is 10 minutes, longer development times could be used for tighter flaws. 

Inspection
  1. Inspection is performed under appropriate lighting conditions to detect any flaws that could be present.

  2.  For Fluorescent Penetrant, the inspection is performed under ultraviolet light. For Visible Dye Penetrant, the inspection is performed under visible white light.

Post-clean
  1. The final step in the inspection process is to thoroughly clean the test surface to remove any remaining developer on parts that were found to be acceptable.  

Conclusion

Liquid Penetrant Inspection is an excellent non-destructive testing method to process and inspect large areas or large volumes of parts and materials, it has excellent sensitivity for very fine and small surface defects, and it can be used in a variety of different materials provided the surface is non-porous. 

For more information, or if you have questions, contact us today, and our technicians would be glad to speak with you. 

Have a question about Liquid Penetrant Inspection or about our services? We look forward to hearing from you. 

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