DPCNews 019 - Tomorrow's penetrants

December 2009
Document updated in March 2012

Why this update?
This paper deals with a topical issue that draws many questions from our readers. These relevant questions make us think.
Nowadays, as tomorrow, we will update this article, as nothing is definitive: many technical points change.

The purpose of this paper is to give users pieces of information, and to increase the manufacturers’ awareness of the need to improve their PT products/equipment.

A technological watch cell's purpose is not only to check what competitors do, but, above all, is to think of what could be done within a 5-year span that competitors may not do!

Let us test this point of view about penetrants.

Our DPCNewsletter #18 issued in November 2009 (*) was about how to make products fluoresce and about standardization work-in-progress.

We may anticipate that the 450 nm wavelength (the blue light of the visible spectrum) will be chosen as the next means to excite the dyes of the fluorescent penetrants.

Fluorescent dyes currently used in penetrants formulations give a fairly good answer to this wavelength.

Nevertheless, there are at least three drawbacks as previously stated:

• No current standard allows for using any wavelength other than 365 nm to make penetrants fluoresce.

• Visible blue and violet lights ... are visible! They are detrimental to the operators' eyes ability to detect faint green or yellow-green indications.

• The fluorescent brightness is lower than when using 365 nm. For decades formulae have been optimized for the 365 nm radiation; therefore, it is easy to understand that they may not be optimized for a 450 nm radiation.

Three technical points shall be adjusted:

• The ISO 3059 shall be revised to allow for the 450 nm wavelength use.

• Blue light filtering goggles shall be worn by operators and inspectors all the time, whatever in the rinsing/washing booths or in the inspection booth. On the other hand, inspection may not be carried out in a dimmed visible light.

• Digital radiometers centred on the 450 nm wavelength shall be used. Some 460 nm radiometers already exist ... they measure the irradiance of the blue light used to treat hyperbilirubinemia - the jaundice - of newborns. Users will have to buy new radiometers! (The word radiometer shall be used, and not luxmeter, though the meter measures a visible light: in this situation only a small bandwidth of visible light is measured, and not the entire visible spectrum). This does not come for cheap and calibration laboratories will have to modify their installations for this purpose.

We have recorded very different reactions when talking about the fluorescent brightness of current penetrants under a 450 nm radiation. Some tests result in a brightness 60% to 80% only of the brightness gotten under a 365 nm radiation. Other people have noticed only marginal differences. Maybe these conflicting reactions are due to results got on different penetrants. It could be that, depending on the dyes/brighteners combinations, on the solvents (hydrocarbons and other additives, on the surfactants, etc.) used in each formula, the effect is somewhat different. In the laboratory of one of our companies, several years ago, we noticed that different hydrocarbons may have a detrimental effect on the fluorescence brightness. This point is probably one of the most difficult to overcome.

The only way is that PT materials manufacturers submit new formulae for approval by the US Air Force (this, for the aerospace industry).

The new formulae will come from a new dyes complex, mainly.

One of the basic means to make penetrants fluoresce is to use an optical brightener, which absorbs the 365 nm radiation. Thanks to the so-called "cascading effect", this brightener can transfer some energy to another dye. Provided that the emission spectrum of the brightener partially overlaps the absorption spectrum of this other dye, the latter is chosen among those that emit yellow, yellow-green or green photons - green being the best colour: 505 nm.

The other way is the synergy effect. Nobody has yet given an acceptable explanation of synergy. This effect is used, for instance, in cancer fighting. Antineoplastic drugs (drugs used to kill cancerous cells) are toxic for these cells, but the doses needed for a good result are also harmful to healthy cells. When combining two, three or four drugs showing a synergy effect (i.e. a kind of amplifying effect on each other), much lower doses of each drug may be used. The detrimental consequences on healthy cells are dramatically reduced (lower "collateral damages") while the efficiency against cancerous cells is similar. Then, the treatment is more easily acceptable to the person and as a whole, more efficient.

As for fluorescent penetrants, finding the right mix dyes/optical brighteners is of the utmost importance for sensitivity - and a well-kept industrial secret!

The optical brightener concentration is generally equal to or higher than those of the yellow dye in the fluorescent penetrant. If the latter is submitted to a too high UV-A irradiance, the indications become more and more bluish-whitish: the optical brightener emits more and more white photons. Indeed, when the UV-A irradiance is increased, the ratio between the whitish-bluish photons emitted by the optical brightener and the green photons from the fluorescent dye increases.

That is one of the justifications in the ISO 3059 standard for a maximum UV-A irradiance set at 50 W/m² (5,000 µW/cm²) on the parts' surface.

We were of the opinion that this phenomenon does not occur if the optical brightener concentration is significantly lower than that of the yellow dye, with a subsequent result: a substantial drop in the fluorescence brightness of the penetrant!

To check this hypothesis, a laboratory carried out test using a classic penetrant, and the same penetrant modified on a parameter only: the optical brightener concentration is half that of the yellow dye

So, the classic penetrant had an optical brightener/yellow dye weight ratio equal to 1.6. The modified penetrant had an optical brightener/yellow dye weight ratio equal to 0.5.

Under very high UV-A radiation, we found out that the indications became pale yellow and less bright with the classic penetrant, while this did not happen with the modified penetrant

However, the fluorescence brightness, determined in accordance with the Annex A of the ISO 3452-2 standard or ASTM E1135 standard, went down from 88 to 60, i.e. a 32% drop. This is a clear explanation of the role of the brightener in the cascading effect in fluorescent penetrants.

This leads to a loss of detection ability, hence a loss in sensitivity.

As the 450 nm wavelength seems not to be the best one to excite the optical brighteners used in current fluorescent penetrants that shall be used with 365 nm radiation, two questions arise:
• Is the cascading effect necessary to get the right fluorescent brightness with a 450 nm source? In other words, are two dyes to be used?
• Are there dyes able to absorb the 450 nm visible light and to transform it into a strong green, 505 nm fluorescence? Maybe only one dye would be enough.

The choice of the dye(s) or of the dyes/optical brightener complex is essential, and many tests are needed to get the right fluorescent brightness.

Another problem, due to the European REACH regulation, is to choose dyes and optical brighteners, which will have been tested as non-carcinogenic. Some of the currently used dyes have not been tested. As a matter of course, tests on human beings are forbidden. Tests on animals are costly and cannot give the evidence that no adverse effect would affect humans. Further, animal testing is more and more restricted, which one rejoices with. Only tests carried out on human cells could give a reasonable guarantee that no detrimental effects would occur on humans. Of course, these ideas deal not only with dyes for PT ... but with all the chemicals! Add to this the cost of tests relative to the involved quantity of chemicals as well as the number of people who could be at risk. Quite often, very simple precautions and some common sense at the work stations are a good means to prevent short-term or long-term ills.

By the way, just a philosophical question. Smokers nowadays get enough pieces of information about the all but certain risks for themselves and the likely risks for their family circle due to their habit. Thus, why do they smoke? Why are they especially scathing in the workshop when time comes to use potentially hazardous chemicals?

Let's come back to our today's matter of concern!

With the new dye(s) in the formula, the next point is to check:
• The fluorescent brightness under a 450 nm radiation.
• The fluorescence stability under a 450 nm radiation.
• The thermal stability of the fluorescence brightness.

These tests will need some time ... and money; manufacturers will have to spend time and money in Research and Development.
However, these last years saw a dramatic decrease in the R and D means of many PT/MT materials manufacturers. Hence, some trouble to design the required new products.

The American SAE-AMS 2644 specification and the ISO 3452 standard series will need to be revised accordingly.

Who will be the first manufacturer to get approval for its penetrants as per these revised documents? The first to market them? Probably, it is a good idea to think about it from now ... so that products are ready even before the new documents are published!

The manufacturers, which will be unable to have the new penetrants in their programme, will be wiped out from the market. Same situation for MT materials manufacturers.

The "pioneer" will stay ahead of the competitors if it complies with some precautions to prevent competitors to duplicate the new products without spending a euro or a dollar. Among the most important points allowing for the avoiding a me-too product (a copy!):

• Only very few people in the company shall know the new dyes. Further, they must sign a commitment not to divulge any information.

• The penetrants manufacturer shall not give its dyes supplier any information about the future of the dyes. Otherwise, the dyes supplier will inform the PT manufacturer's competitors ... only to increase its sales.

On the long-range, the information will be known. However, being two- to three-year ahead of competition allows for the taking of a fair share of the markets when competitors are unable to supply equivalent products. As a general rule, the first entrant on a new market keeps its lead for a long time.

Another problem, not the least one! Users will have to throw away huge quantities of penetrants (at least, those users who have tanks.)They will have to pay for disposal of these rapidly obsolete products ... though these products would be technically valid yet!

What will be the "timing" for this switch, being given that "365 penetrants" and "450 penetrants" shall not be mixed?

The most simple would be that, for a 5/6 year-span, "365 PT process lines" and "450 PT process lines" are used in parallel. Keep in mind that the lighting sources (365 nm or 450 nm), radiometers (365 or 450 nm), goggles worn by operators/inspectors, the required (365 nm or 450 nm) irradiance, the acceptable ambient visible-light illuminance, would be specific for each process line. As already written, auditors will have a fancy time, non-conformity reports will pour over companies, costs will go to the sky.

Nevertheless, we think about several means to overcome some of the hindrances:

• The current penetrants, which display a lower fluorescent brightness under a 450 nm radiation than under a 365 nm one may be "requalified" at a lower Level, after tests for sure. Say a Level 4 penetrant as per the current SAE-AMS 2644 under 365 nm might be qualified as Level 3 as per a future SAE-AMS 2644/450 (450 comes for 450 nm).

• Some people think that, to overcome the lower fluorescence brightness of penetrants under a 450 nm radiation, the answer is to increase the 450 nm irradiance. This is debatable: read the January 2010 Mail Inbox on this topic.

• Manufacturers design new penetrants for qualification at Level 4 under a 450 nm radiation. This is technically possible; it will be written about later.

• By doing this way, well-known formulae could be used for Level 1, 2 and 3. Only new Level 4 penetrants would be designed, tested and approved. Costs would be more reasonable both in R and D and for the disposal/replacement of in-service products.

This is what we see as the most likely.

Let us anticipate on the next years:

• Very important: to be the first entrant, or among the very first ones on a new market with new penetrants. Who will be #1? Could we think the race has already begun?

• PT/MT materials manufacturers must be ahead of what will be a kind of revolution, must anticipate on the standards - better for them to be active members of the standardization organisations!

• PT/MT materials manufacturers are generally very small companies, comprising 7 to 20 people. Very few exceptions. Small companies are more able to innovate, to have ideas, to be the first to market them (take as an example the generic drugs: large, very large drugs companies even invented the "pay for delay" process. They pay the small company ready to market a low-cost copy of a "blockbuster" - a one billion of dollars a year drug -- just for having it wait a bit before marketing the product! Similar situation in the genetic research).

True, we cannot compare the biological research with the design of new penetrants ... even though ... PT materials are a major contribution, and growing - whatever some people may think (refer to our November 2009 Editorial (**) and to the Readers' mail--Inbox / mail--induced by this Editorial) - for the safety of airplanes, cars, trains, power plants, refineries, prosthesis, etc.

Whatever happens, users will need new lighting sources: in the rinsing/washing booths and in the inspection booth, new overhead sources comprising many LEDs will be used -- quite expensive but long-lasting and needing almost no maintenance. New goggles; new radiometers calibrated at 450 nm ... and, on the other hand UV-A sources, UV-A blocking goggles, radiometers/luxmeters that shall be disposed of!
Another problem is arriving: how to measure the ambient visible light? Even if it is possible and allowed to inspect with a higher illuminance than with 365 nm penetrants, there will be a maximum illuminance allowed: the lower, the higher sensitivity and reliability.
The minimum 450 nm irradiance for inspection will have to be defined.
Luxmeters will not be usable, as the source of energy for fluorescence will emit a huge quantity of blue photons! Therefore, physiologists, physicists specialized in optics, but also people using fluorescent products will have to work together to decide how to measure the "parasitic" ambient visible light, which name give to the measuring unit, what would be the figure acceptable by most users, how to calibrate/verify the meters ... how to design new calibration equipment. Stacks to do, an interesting job ahead ... and maybe, for the first time, identical specifications/standards from our American and European friends!

Reference

(*) Patrick DUBOSC and Pierre CHEMIN: "How to make penetrants fluoresce": On our Website.

(**) Patrick DUBOSC and Pierre CHEMIN: Editorial - November 2009: "Once again the end of penetrant testing is announced!": On our Website.

Normative references

• ISO 3059:2001 Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions, International Organization for Standardization, Geneva, Switzerland, 2001.

• ISO 3452-1:2008 Non-destructive testing - Penetrant testing - Part 1: General principles, International Organization for Standardization, Geneva, Switzerland, 2008.

• ISO 3452-2:2006 Non-destructive testing - Penetrant testing - Part 2: Testing of penetrant materials, International Organization for Standardization, Geneva, Switzerland, 2006.

• ASTM E1135 - 97(2008)e1 Standard Test Method for Comparing the Brightness of Fluorescent Penetrants, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA,2008.

• SAE-AMS 2644E: Inspection Material, Penetrant, Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvanie 15096, USA, 2006.

We, Pierre CHEMIN and Patrick DUBOSC, welcome any comment, any idea. If you have some examples you would like to see discussed here, please give us all the useful indications. If you require confidentially, we would modify locations, names and some parameters to prevent any traceability.
Nevertheless, we are convinced that our site may be a kind of surge-valve: the topic is NOT to target this company, or that auditor; but it is always to make users think, to make them ask themselves, or others, the right questions.
We may also give advice, once again on a confidential basis if needed: please, feel free to ask questions, to document our data basis: about Material Safety Data Sheets (MSDS), about environment, a chemical name you don't understand, a Penetrant process you have heard about, etc.
We have plenty of examples, some being out of all the specifications/standards, which led to the discontinuities detection, when the "current, normal, processes" prevented discontinuity finding.