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I-INTRODUCTION

 

Chemicals used for Penetrant Testing are: oil- or water-based penetrants, organic solvents (halogenated or not), emulsifiers (lipophilic or hydrophilic) and developers (dry powder, aqueous or non-aqueous wet).

 

As halogenated solvents, lipophilic emulsifiers and water-based developers are either rarely used or no longer used, they won't be dealt with in this paper.

 

 

II-PT MATERIALS COMPATIBILITY WITH METALLIC MATERIALS

 

Penetrant Testing is used on most of the metals and alloys. There are some exceptions we will not detail here. Only one specific example:

 

Obviously Penetrant Testing on sodium is ruled out.

 

The ISO 3452-2 standard is based upon the SAE-AMS 2644 American specification regarding the underneath corrosion tests to be met by Penetrant Testing materials:

 

• Moderate temperature corrosion on specified alloys: aluminium, magnesium, steel and any other alloy when required.
• High temperature titanium stress corrosion.
• High temperature corrosion of cast nickel alloys.

 

Unlike many other surface treatment materials used in aerospace industries (such as paint strippers, detergent cleaners, etc.) PT materials as well MT materials are not bound by specification to any test of hydrogen embrittlement on high tensile steel (ASTM F519 - 08 Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments) nor on cadmium plating (ASTM F1111 - 08b Standard Test Method for Corrosion of Low-Embrittling Cadmium Plate by Aircraft Maintenance Chemicals).

 

Some primes and some specifications require the penetrant material supplier to certify that their PT materials contain no mercury and that they have not, in any way, be in contact with mercury during manufacture and packaging.

 

In fact mercury gives rise to amalgams not only with gold but also with aluminium, tin, copper, silver, zinc, etc.

 

For instance should mercury or mercury chloride come in contact with aluminium a mercury/aluminium amalgam comes out. On the surface an oxide layer appears which poorly covers the metallic substrate; the oxide expands very quickly and kind of "beards" of oxides grow so fast you could see them. Within very few hours the aluminium is completely destroyed!

 

A serious concern for aerospace industries! Corrosion tests carried out on every batch of PT by the manufacturers give a further guarantee that the materials are mercury-free, along other items.

 

This concern has been so high that for many years there was a mercury spill treatment kit in airliners cabins-- and maybe this is still mandatory!

 

Some alloys such as: aluminium-zinc and aluminium-copper, are subject to corrosion in aqueous medium. PT of these alloys may be allowed provided that part thickness is enough when inspected or if excess penetrant removal is achieved using an appropriate clean rag moistened with solvent or water for Levels ½, 1 and 2 penetrants.

 

For ferritic steels and materials easily oxidised it is recommended (if needed according to tests) to add a corrosion inhibitor in washing water and to prevent any oxidation after inspection and final cleaning.

 

 

III-PT MATERIALS COMPATIBILITY WITH NON-METALLIC MATERIALS

 

Several non-metallic materials may be PTested: glass, enameled porcelain, ceramics and some synthetic macromolecules (so-called: plastics) with numerous limits for these latter materials.

 

In fact some of the chemicals used in PT materials may have a detrimental effect on these materials. As examples:

 

• Some of the hydrocarbons used in penetrant formula.
• Surface active agents (Surfactants) used mainly in penetrants and emulsifiers.
• Organic esters in penetrants.
• Acetone in some solvents and some non-aqueous wet developers.

 

 

III.1- LIKELY PROBLEMS DUE TO PT PROCESS

 

PT materials may degrade, soften or crack synthetic thermoplastic or thermosetting elastomers and macromolecules and then jeopardise their chemical and/or mechanical resistance.

 

Water-based penetrants are generally to be preferred as they are diluted up to 50% with water while containing far less hydrocarbons or organic materials.

 

Nevertheless so many different macromolecules are available with so a large range of polymerisation and cross-linking degrees that compatibility tests are almost mandatory before any use on a large scale. Even a water-based penetrant may have a detrimental effect on the substrate. It may also stain it in an irreversible way which could lead to an important background and subsequent trouble to detect discontinuities, or may have the parts unsightly coloured and then unsaleable!

 

That's why water-based fluorescent penetrants are usually used without any developer, or with a dry developer for flaws and porosities detection.

 

Two kinds of cracks may occur:

 

• Crazing: organic glasses, polysulfones, methyl polymethacrylate, polycarbonates, etc.
• Stress cracking: polyolefins, polyethylene, etc. It may be caused by some surface active agents (surfactants).

 

Furthermore, as stated previously, penetrants may stain plastics parts in such a way they are unsaleable. Dyes are then a concern; better to use fluorescent penetrants which contain far less dyes than colour contrast penetrants and which often leave no trace visible by the naked eyes.

 

 

III.2- COMPATIBILITY TESTS

 

III.2.1- QUICK TEST

 

It comes as follows:

 

• immerse in the penetrant a test part or some material to test for 24 hours.
• then process the part.
• check for any visual damage using a magnifier if needed.

 

Obviously that kind of test is unable to show likely change in physical parameters of the part and cannot warn against any adverse reaction of parts in service.

 

 

III.2.2- STANDARDISED TESTS

 

Some standards may be used for this specific application. Among the most useful:

 

III.2.2.1-IMMERSION TEST

 

According to the nature of plastics, one of the following methods is used:

 

• ISO 175:1999 ‘‘Plastics -- Methods of test for the determination of the effects of immersion in liquid chemicals’’ issued May 1999.
• ISO 4433-1:1997 ‘‘Thermoplastics pipes -- Resistance to liquid chemicals -- Classification -- Part 1: Immersion test method’’ issued December 1997.
• ISO 4433-2:1997 ‘‘Thermoplastics pipes -- Resistance to liquid chemicals -- Classification -- Part 2: Polyolefin pipes’’ issued December 1997.
• ISO 4433-3:1997 ‘‘Thermoplastics pipes -- Resistance to liquid chemicals -- Classification -- Part 3: Unplasticized poly(vinyl chloride) (PVC-U), high-impact poly (vinyl chloride) (PVC-HI) and chlorinated poly (vinyl chloride) (PVC-C) pipes’’ issued December 1997.
• ISO 4433-4:1997 ‘‘Thermoplastics pipes -- Resistance to liquid chemicals -- Classification -- Part 4: Poly(vinylidene fluoride) (PVDF) pipes’’ issued December 1997.

 

Depending on temperature and immersion time, some parameters are checked:

 

• Mass variation.
• Bulge rate and aspect.
• Mechanical properties variation.

 

Chemical resilience may be assessed as per the following table:

COMPATIBILITY OF PT CHEMICALS

• ISO 527-1:1993 ‘‘Plastics -- Determination of tensile properties -- Part 1: General principles’’ issued June 1993. Document amended by ISO 527-1 AMD 1, ISO 527-1/Amd1:2005. This standard has a corrigendum: ISO 527-1:1993/Cor1:1994.
• ISO 527-2:1993 ‘‘Plastics -- Determination of tensile properties -- Part 2: Test conditions for moulding and extrusion plastics’’ issued June 1993. This standard has a corrigendum: ISO 527-2:1993/Cor 1:1994.
• ISO 527-3:1995 ‘‘Plastics -- Determination of tensile properties -- Part 3: Test conditions for films and sheets ’’ issued August 1995. This standard has two corrigenda: ISO 527-3:1995/Cor 1:1998 and ISO 527-3:1995/Cor 2:2001.

 

Other ISO standards:

 

• ISO 527-4:1997 ‘‘Plastics -- Determination of tensile properties -- Part 4: Test conditions for isotropic and orthotropic fibre-reinforced plastic composites’’ issued April 1997.
• ISO 527-5:1997 ‘‘Plastics -- Determination of tensile properties -- Part 5: Test conditions for unidirectional fibre-reinforced plastic composites’’ issued April 1997.

 

 

III.2.2.2- STRESS CRACKING TEST

 

This test is carried out according to ASTM D 1693 – 08 method ‘‘Standard Test Method for Environmental Stress-Cracking of Ethylene Plastics’’.

 

Polymer samples are prestressed and put on holders then immersed in a thermostatic bath. Durability of the tested material is defined as the time just before it breaks.

 

 

III.3- EXAMPLE OF COMPATIBILITY TEST CARRIED OUT ON A WATER-BASED FLUORESCENT PENETRANT

 

These tests were carried out using the penetrant on the following materials: polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) and polyvinylidene fluoride (PVDF).

 

It is necessary to add some technical characteristics of these plastics:

 

• PE-HWU of SIMONA: High heat resistance, UV stabilized polyethylene, black.
• PP-DWU of SIMONA: Heat stabilized alpha-nucleated homopolymer, grey.
• PVC-CAW of SIMONA: Normal impact, chemicals resistant and flame resistant.
• PVDF of SIMONA: Highly crystalline thermoplastic material, excellent chemicals resistance and flame resistant.

 

Tests results are as shown in the underneath table:

Stress crazing tests were carried out as follows:

 

• Every test panel of every material was notched and kept bent in a holder.
• The holder is then immersed in penetrant in a test tube.
• Test tubes were kept at a constant temperature all the test long.
• Test panels were checked at stated intervals to look for any break.

 

Results: 500 hours later nothing to report.

 

These few tests show there is no compatibility concern for the materials under test when in contact with the tested water-based fluorescent penetrant.

 

 

I-PLASTIC SURFACE CLEANING PRIOR TO PT

 

Two types of chemicals may be used: solvents and detergent cleaners.

 

Solvents containing acetone shall be banned; those isopropanol- and/or hydrocarbons-based may be used after thorough compatibility tests.

 

As for the detergents we suggest to choose among those approved as per the American specification SAE-AMS 1526C titled ‘‘Cleaner for Aircraft Exterior Surfaces, Water-Miscible, Pressure-Spraying Type’’ issued March 2008. We think a Qualified Products List (QPL) is attached to it. This specification requires compatibility tests on acrylic materials such as the polymethyl methacrylate (PMMA) which aircraft windows are made of. Tests are carried out according to ASTM F 484 ‘‘Standard Test Method for Stress Crazing of Acrylic Plastics in Contact with Liquid or Semi-Liquid Compounds’’ issued April 1, 2008.

 

This test method shows the crazing effect a liquid or semi-liquid material may have on transparent acrylic materials under bending stress. At the end of the test, the samples of acrylic materials must exhibit no crazing, tarnishing or scratch.

 

 

II-COMPATIBILITY OF PT CHEMICALS WITH LIQUID OXYGEN

 

Liquid oxygen is used for instance for cryogenic engines of rockets.

 

Even water-based penetrants have some organic (i.e. carbon-based) materials in their formula: dyes, surface active agents (surfactants). So residual traces of oil based penetrant, in contact with oxygen, a highly oxidising substance may cause accidents. It's the same if a piece rag is inconveniently forgotten in a LOX circuit.

 

If "classic penetrants", those oil-based, are used for inspection of parts designed for running out-of-earth space, a slow degassing of hydrocarbons may occur, polluting telescopes, for instance. After a penetrant inspection with water-based penetrant, it is easier (not easy, easier) to have water evaporate before assembling inspected parts.

 

 

III-COMPATIBILITY WITH COMPOSITE MATERIALS

 

One of us gave a conference on this topic in 1983 (*).

 

Tests confirmed the hypothesis that the hydrocarbons-based penetrants were detrimental as regard to the mechanical properties of composites. Water-based water-washable penetrants, free of hydrocarbons, organic solvents and organic esters were also a problem due to the very strong background left on parts, making it almost impossible to detect discontinuities.

 

A specific penetrant, specially designed for this purpose and used with a synthetic dry developer led to satisfactorily results.

 

Anyway keep in mind that nothing may be done without preliminary compatibility tests.

 

 

IV-COMPATIBILITY WITH OTHER MATERIALS (WOOD, ETC.)

 

VII.1- WOOD

 

Penetrants are absorbed by wood and stain it irreversibly.

 

Penetrants contain "third-solvents", i.e. chemicals which allow the formula to be stable even at -20 °C (-4 °F). These chemicals have also some part in surface wettability by penetrants.

 

Their effect on wood of course is detrimental as they soften wood fibres, spread in them and never evaporate!

 

Furthermore, penetrants contain hydrocarbons which in contact with wood may only embrittle it: they will penetrate into the cellulosic fibres and their mechanical resistance will be strongly altered.

 

Note that colour contrast penetrant traces on wood eventually may be altered in daylight.

 

VII.2- CONCRETE AND SIMILAR MATERIALS

 

Penetrant Testing is NOT the right NDT method to choose: the background left by the penetrant makes it impossible to have reliable results; this comes from the surface condition which makes it highly absorbent. Tests were carried out using a water-based fluorescent penetrant and a hydrophilic emulsifier without developer. But they fail.

 

 

VII.3- CERAMICS

 

Sintered ceramics chemically resist to penetrant testing materials, as moreover to many chemicals. They nevertheless have a high retention power: it is necessary to think of it for the subsequent use of the inspected parts.

 

Note that, considering this characteristic and the type of discontinuity that could appear on ceramics, a special penetrant testing process will be generally used: no degreasing, 2 or 3 minutes penetration time, water washing, no developer. A Level 2 water-washable fluorescent penetrant will be often the ideal.

 

 

V-CONCLUSION

 

 

PT chemicals are designed to be compatible with metals and metallic alloys.

 

Using them on non-metallic materials is possible after satisfactory compatibility tests have been carried out.

 

Some users failed to check the compatibility of the chosen penetrant with the non-metallic parts they manufacture. Sometimes the problem was detected after several batches of parts which then had to be discarded. What a shambles!

 

 

(*) Jean-Claude HUGUES and Pierre CHEMIN: ‘‘Détection des délaminages superficiels sur matériaux composites après perçage par une nouvelle génération de pénétrant fluorescent’’. 3rd National Congress of the Italian Society for Nondestructive Testing- May 12/21, 1983- Lido, Venice (Italy). Paper issued in Revue Pratique de Contrôle Industriel N°124 December 1983, pages 64 to 66.