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Penetrant film retraction on surface during penetrant contact

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Written by Administrator
Tuesday, 01 July 2008 17:05

July 2008

One of the requested qualities for a penetrant is to be able to spread out as a very thin film on the surface of the part and not to show any film retraction so as to obtain the maximum coverage power.

This criterion is naturally met with good quality penetants, the composition of which is optimized to meet the other criteria aiming to have an ideal compromise which is the key factor to obtain a good penetrant.

In the sixties, the quality control of penetrant was limited to a relatively reduced number of tests while nowadays their number has been increased.

One of the tests concerned the spreading of the penetrant on a square glass plate. This plate was mounted in a frame fitted with 4 feet, the height of which was adjustable. The frame was fitted with two spirit levels mounted on two adjacent sides of the frame. The plate was as flat as possible and set as much as possible horizontally by checking the two spirit levels. The glass plate was roughly degreased then immersed in a solution of sulphuric acid and of chromic acid (sulphochromic mixture), then water rinsed and dried. The plate was handled with gloves to avoid to leave on it fingerprints since the epidermis is covered with a lipo-acidic coating containing, among others, sebacic acid.

One or more drops of penetrant were settled down at the centre of the glass plate using a pipette. After 30 minutes, the penetrant was spread and formed a practically circular spot of ca 6 cm diameter.

The reproducibilty of this test was quite good.

Because the red dye penetrants in the sixties contained hydrocarbons with a distillation range which was lower than nowadays, care was to be taken that the ambient temperature was not too high in order to avoid drying of the penetrant during test.

At the beginning of the seventies, this test disappeared as it was not introduced into any standard, without really knowing the reason why.

At the same period, The Quality Manager of a French manufacturer of aircraft landing gears reported to us that he inspected hard chromium coatings by fluorescent inspection. On that type of coating, stress cracks which appear are called ‘‘chicken wire’’. As a matter of fact, on that type of coating, this Manager observed a penetrant film retraction on surface during penetrant contact time. This phenomenon was noticed using certain Type 1, Method A, Level 2 fluorescent penetrants.

What is it about?

The parameter which is to be taken into account is the wetting power of the penetrant which depends on:
• The surface condition of the part.
• The penetrant physical properties and especially its surface tension.
• The cleanliness of the part.

When we apply a drop of penetrant on a surface, two types of force inter-react:
• The cohesive forces which glue the penetrant molecules to each other.
• The adhesion strength of the penetrant on the surface.

These two types of forces determine a connecting angle (θ) formed by the surface and the tangent to the surface of the liquid to the intersected point penetrant-surface of the part.

The angle that forms the tangent to the meniscus with the surface of the component characterizes the wettability.

Angle θ < 90° The penetrant wets well the surface.

For penetrants, the angle θ is practically equal to 0.

Angle θ >90° The penetrant badly wets the surface.

Apart of the surface condition, if a penetrant wets well the surface and an other one wets it less well, it depends on the surfactant system which is part of the penetrant formula.

There are four families of surfactants (synonym: surface active agents): anionic, cationic, non ionic and amphoteric (synonym: ampholytic).

In penetrants, the non ionic surfactants offer the most interesting characteristics. Furthermore they have an excellent biodegradability. There are a very large number of non ionic surfactants and the best ones are the condensates of ethylene oxide moles on long chained primary or secondary alcohols. Their appropriate combination is very complex to get an optimal HLB (Hydrophilic/Lipophilic Balance) value giving the penetrant: a good wettability, a good water washability and a good capillary retention of penetrant in the defects at the water rinse stage.

In conclusion, an unappropriate surfactant sytem may give a bad penetrant.

Nevertheless the surface condition of the surface may influence the wettability of the penetrant.

So, the phenomenon of penetrant film retraction on surface during penetrant contact time was observed with red dye penetrants approved to the American specification AMS 2644 E (type 2 Methods A & C) and meeting EN ISO 3452-2 (sensitivity level 2 of red dye penetrants) requirements.

As a matter of fact, they were red dye penetrants widely known for their excellent quality and their sensitivity to crack detection.

It is good to remind here that penetrant testing is a reliable method provided that an adequate operating process is applied. Sometimes, for reasons of delay, it can be attractive ‘‘to speed up the process’’, and this, after all, may cause many surprises.

Miscellaneous investigations were carried out and brought to light the cause of this retraction This phenomenon appears when parts are degreased using hydrocarbons-based solvents cleaners and when the penetrant is applied before the complete evaporation of the solvent. In the same way when processing reference test pieces type 1 of the norm EN ISO 3452-3, we observed the same phenomenon especially on the edges of these reference test pieces.

To avoid this retraction phenomenon, we found out that, under normal conditions, it is necessary to let evaporate the solvent for 5 minutes at least before application of the penetrant. In the same way, if this period of 5 minutes is allowed, it is sufficient enough to apply penetrant on these reference test pieces, which were previously handled with fingers soiled by some hydrocarbons, to observe the same phenomenon.

This was not observed in the past when 1,1,1 TRICHLOROETHANE-based solvents or TRICHLORETHYLENE-based solvents, much more volatile, were used as degreasing solvent cleaners.

Other remark: Silicones cause the same retraction phenomenom. Among the chemicals which  can contain silicones we could quote: some oils, some greases, some plastic mold release agents, some antispatter chemicals (to avoid adherence of  solder drops), etc ...

We can say that there are silicone free antispatter compounds, such as these based on ox-foot oil.

Last Updated ( Monday, 23 May 2011 17:25 )