Penetrant Testing

Penetrant testing (PT) is a non-destructive testing method which is easy to perform. Penetrants  which have a low surface tension and a high capillary effect are used. So they can easily penetrate into openings at the surface of the test specimen such as surface pores and thin cracks with widths in the micrometer range (≥25 µm).

Typical indications which can be detected with penetrant testing are:

  • Cold cracks
  • Hot cracks
  • Grinding cracks
  • Gas pores and clustered porosity
  • Spongy microstructure
  • Stress corrosion

Three different methods are distinguished depending on the penetrant applied:

  • Colour penetrant testing (penetrant type II)
  • Fluorescent penetrant testing (penetrant type I)
  • Fluorescent colour penetrant testing  (penetrant type III)

General principles for application of the different testing product systems are described in detail in the basic standard for penetrant testing EN ISO 3452 part 1.

The essential work steps of application of colour penetrant testing using the red-white method based on wet developer solvent-based (form e) are briefly presented Below.


Mechanical precleaning


Precleaning  ensures that the surface of the test specimen is free from residues such as grid, grease or scale, so that possibly existing indications are accessible for operation of the penetrant. Precleaning can take place:

  • mechanically
    for example: by brushing,  grinding, blasting
  • chemically
    by cleaning agent which must be removed from the surface after precleaning

Application of the penetrant

Application of the penetrant


The penetrant is sprayed onto the surface of the test specimen applied by a brush, or the object is poured with or immersed into the penetrant. The penetration time depends on the test conditions and the material (between 5-60 minutes).

Excess penetrant removal

Removal with a sponge


After the penetration period (according to a standard or specification; generally 5 - 60 minutes) the excess of the penetrant is  carefully removed from the surface, e.g. using a lint-free cloth or a sponge. The surface is carefully checked for residues.


Drying of the surface


Before application of the developer the surface of the test specimen has to be dried by

  • wiping with a lint-free cloth
  • transpiration
  • forced ventilation (e.g. hair dryer, compresseg air)

Application of developer

Spraying of the developer


The developer isapplied homogenously onto the test area.


Inspection of the tested surface


Immediately after application of the developer the testing surface is examined according to given inspection intervals. The test result is recorded in the test protocol by the test personnel.  Depending on the test procedure or specification, documentation by fotos or video recordings are made.

Classification of penetrant systems according to EN ISO 3452 part 2

Testing products for penetrant testing are classified by

  • Type of penetrant
  • Method of excess penetrant removal
  • Mode of developing

(according to EN ISO 3452-2 Table 1)

Fluorescent penetrant systems

are classified additionally  according to sensitivity levels  starting with ½ (very low) up tp 4 (ultra-high) .

For  colour contrast  und  fluorescent colour contrast product families  two sensitivity levels  apply:

Sensitivity level 1       normal 
Sensitivity level 2   high

Welcome! - Helling GmbH