October 2009

Special products for Penetrant Testing

Introduction

"Classic" PT materials are well-known. They are described in the ISO 3452-1:2008 and 3452-2:2006 standards as well as in the American SAE-AMS 2644E specification. All the manufacturers have them in their product range.

Some of the "classic" products are nevertheless rarely used, or less and less used. For example:
- Post-emulsifiable colour contrast penetrants.
- Water-washable Level ½ and Level 4 fluorescent penetrants.
- Lipophilic emulsifiers.
- Water soluble developers.
- Developers in suspension in water.

Special products for PT are less known and are the reason for this paper.

The products listed here:
- peelable developer,
- hydrocarbons-free penetrants comprising the water-based penetrants used for instance to detect leaks on liquid oxygen pipes/reservoirs,
have already been explained in papers published on our site.

1- Special products list

This paper deals with:
- Dual purpose penetrant.
- Thixotropic PT materials.
- Low temperatures PT materials.
- High temperatures PT materials.
- Reverse fluorescent PT materials.
- Volatile flammable oil-based penetrant.
- Fluorescent particles in suspension in hydrocarbons as a fluorescent penetrant.
- Felt-tip pens for PT.

2- How to design a "special product"

Till the end of the '70s some Laboratory Managers were free to do research and design new products without any concern about their marketability. That is a reason for several flops!

Other products just were no longer useful: the peelable developer(*) has been replaced by the digital camera to the users satisfaction.

Some products, though a technical step forward, failed after some time. That's the way of life!

This is true in very different areas. As an example let us write about the ice hockey skates.
In the '80s the skate blades got a surface treatment used to prevent wear of turbine blades used in aircraft engines. A titanium nitride layer was THE right answer! But it was soon understood that when an accident occurred the skin wounds were deeper and needed a longer time to heal. These blades were then forbidden during matches, and consequently this technique went nowhere.

Fortunately many special products meet specific, true needs. Nevertheless they are used on a small scale and some of them are even totally never heard of in standards or specifications.

The dual-purpose penetrants apart special products are manufactured in small quantities (thixotropic products, low- and high-temperatures products, etc) or as small series (felt-tip pens). Some very specific products have a 10 kg (22 lbs) per year market in France! Therefore you may understand that they do not come for cheap or that delivery time may be unusual.

3- Dual purpose penetrant

This penetrant was designed in 1969 to check welds of the Dreadnought, the first UK nuclear submarine. The Admiralty wanted the same product to check:
- The non-critical welds with a colour-contrast system whose sensitivity would duplicate that of then in-use colour contrast penetrants.
- The critical welds with a higher sensitivity using UV-A radiation.

The dual-purpose penetrant met both requirements.

The orange fluorescence of dual-purpose penetrants is far from giving the same brightness as the yellow, or yellow-green or green fluorescence from "classic" fluorescent penetrants.

That's why the dual-purpose penetrants are generally used nowadays as colour contrast penetrants.

The dyes of these penetrants are generally "diazo-free". But very often the dyes are from the rhodamine family. The rhodamines B and 6G are classified as R 40 (class 3; carcinogen): Limited evidence of carcinogenic effects- insufficient proof. Rhodamines are very soluble in water and are then more prone to enter the human body than diazo dyes used in the "classic" penetrants.

Due to their solubility in water the rhodamines are the dyes of choice for water-based dual-purpose penetrants. Nevertheless better to use fluorescent water-based penetrants if water-based and fluorescent are both required!

What do standards/specifications tell about dual-purpose penetrants?

From the standardization point of view:

- The dual purpose penetrant is classified Type 3 in the ISO 3452-2:2006 standard. This standard states that ''there are no sensitivity levels for dual purpose penetrants. Classification may be carried out as for colour contrast systems''.

- The American SAE-AMS 2644E specification states it does not cover special application products.

4- Thixotropic PT materials

Thixotropic PT materials were first marketed in 1973.

Thixotropic and classic products have a very important difference: the former's behaviour does not obey to the Newton's law. There is no relationship between the shear rate and the shearing forces. In other words a thixotropic material has a viscosity which is related to the shear time and decreases along the shear time.

Thixotropy is a physical phenomenon which may back-fire: if the product liquefied due to the mechanical action is let without any further action it comes back to its initial gel-like consistency.

4.1- When to use them

These thixotropic products are used in specific areas where using conventional products would lead to problems due to:
- Penetrants' and emulsifiers' low viscosity.

Penetrants and emulsifiers come from the surface under test and goes to close areas where they are not welcome, such as:

• Painted areas.
On painted areas colour-contrast penetrants give rise to indelible red traces.
As a matter of fact reapplying a new layer of paint will not hide these traces. The solvents of the paint dissolve again the dyes, and the new layer becomes reddish.

• Areas with rubber, synthetic elastomers, plastics. These materials absorb the conventional penetrant which leads to: part dimensional changes such as: swelling, shrinking, and to an indelible staining. Mechanical and leak-proof properties may be affected.

• Remote areas from which removing the excess of penetrant may be arduous.
In situ check of mechanical or structure parts of aircraft, such as the wing lower surfaces, compressor blades, etc. makes it almost impossible to use conventional products.

- No water available for penetrant removing, or water use forbidden due to a risk of corrosion.

- No solvent allowed to remove the excess of penetrant from the surface and/or solvent drying almost impossible.

4.2- What are the available products

It is possible to design a thixotropic version of almost every penetrant, be it colour contrast, dual-purpose or fluorescent.

Some lipophilic emulsifiers exist as thixotropic, when no hydrophilic emulsifier may come in a thixotropic form. In fact as hydrophilic emulsifiers are supplied as concentrates to be diluted in water, it is impossible to design a product able to take up to 95% of water or even more and still be thixotropic. Further if rinsing water is a concern for corrosion all the more for the water in the emulsifier which would be a similar concern. This a case not to make a technological miracle ... without any market!

The most "common" products:
- Colour contrast or dual purpose penetrant.
- Water-washable Level 2 fluorescent penetrant.
- Post-emulsifiable Level 4 fluorescent penetrant in conjunction with a lipophilic emulsifier or a solvent remover.

These thixotropic penetrants' sensitivity favorably compare with the similar non-thixotropic penetrants. But their sensitivity for wide or low depth discontinuities is higher.

What do standards/specifications tell about thixotropic penetrants?
- The ISO 3452-1:2008 and 3452-2:2006 standards say nothing about these products.
- The American SAE-AMS 2644E specification states it does not cover special applications products such as the thixotropic materials.

In fact thixotropic PT materials are rarely used; aerospace and railways industries are their "largest" users.

More and more felt-tip pens are a more convenient means.

5-  Low temperatures PT materials

Low temperatures (low-temp) PT are carried out in-situ in cold areas.

Many reasons explain why using conventional products/parameters lead to poor results: moisture on the part will prevent penetrant entering the flaw; long time for the developer to dry; some people think that the higher viscosity due to low temperature is also a concern, etc.

The low-temp penetrants formula is specific as moisture, frost or ice are almost a constant.

Pay special attention to the procedure given by the manufacturer.

When at less than 5 to 6°C (41/43°F) propellant's pressure in spray cans goes down: spray does not have the quality required by PT.

A recommended procedure is to keep the spray cans in a warm area (office, a pocket in clothes very close to the operator's body) till their use. If the spray can is to be heated do it ONLY in a water bath set at 50 °C (122 °F) maximum; a spray can shall never be heated directly with a flame or any heater, as there is then NO WAY to know the temperature inside the can (risk of explosion if the pressure in the can goes up quickly above the acceptable maximum pressure).

The first specific low temp PT materials have been designed in the '70s by a French manufacturer.

For many years the ASME aluminium Test Block has been used to qualify low temp PT materials while it is not the right tool to discriminate between systems performances.

The ISO 3452-2:2006 does not deal with low temp PT. On the other hand the American SAE-AMS 2644E requires a test at -18°C (-0.4°F) for 7 hours. Hence the penetrants listed in the QPL-SAE-AMS 2644E could be used at -10°C (14°F) probably while doubling the penetration time.

The classic dry powder developers may be used with fluorescent penetrants. But the non-aqueous wet developers (NAWD) designed for use at room temperature do not evaporate fast enough due to the ubiquitous solvent used (isopropanol or isopropanol/acetone).

In Canada an hexane-based NAWD has been used.

Nowadays specific PT materials allow for performing PT in the range -10°C/-30°C (14°F/-22°F). The right propellant for the spray can is of the utmost importance to get a suitable spraying.

The low temp PT materials comprise a solvent, a penetrant and a developer. The excess of penetrant removal is wiped off using a cotton rag lightly moistened with a suitable nonhalogenated solvent.

When the ISO 3452-2:2006 standard is for PT between +10°C and +50°C (+14°F/122°F) there is now an ISO standard, ISO 3452-6:2008 titled. "Non-destructive testing - Penetrant testing - Part 6: Penetrant testing at temperatures lower than 10°C" in which one may find the requirements for PT at temperatures lower than +10°C (+14°F) as well as the method to qualify products with suitable tests. It is written only for products that the manufacturer classifies as "For low temp use" and as far as the manufacturer recommendations are followed.

The American SAE-AMS 2644E specification does not cover low temp products, considered as products for special applications.

6-  High-temperatures PT materials

High-temperatures (so called "high-temp) PT materials were designed in 1973

For many years the ASME aluminium test block has been used to qualify low-temp and high-temp PT materials, though it is far from the perfect tool!

The ISO 3452-2 standard requires a test of heat stability of the fluorescence brightness at 115+/-2°C (239+/-3.6°F) for an hour. On the other hand the SAE-AMS 2644 specification asks for a 7 hour test at 66°C (150°F).Therefore penetrants listed in the QPL-SAE-AMS 2644 could be used at 50°C (122°F) or even at 80°C (176°F) while shortening contact time.

The "standard" dry powder developers may be used with fluorescent penetrants. Some non-aqueous wet developers (NAWD), though not all of them, may be used up to 200°C (392°F).

Nevertheless it is more advisable to use specific developers for temperatures above 80°C (176°F).

Most of procedures require PT to be performed at 40°C (104°F) or 51°C (ca 125°F) maximum, these being the temperature of parts and/or materials.

The maximum temperature as specified in specifications is due to concerns about the ability of penetrants to counter detrimental effects at high temperature:
- Evaporation of the most volatile ingredients of the formula basis, hence a higher viscosity.
- Deterioration of some ingredients, most probably the dyes. This may also lead to vapours harmful or toxic for the inspector.

Further high temperature may lower sensitivity due to:
- Penetrant's drying on the part's surface leading to a more difficult removing off the surface.
- A risk of clogging of discontinuities which may be detrimental for any further PT inspection. Nevertheless the penetrant remaining in the discontinuities will be dissolved and "liquefied" by the NAWD solvent thus allowing for flaw detection.
- Fast penetrant bleeding out of the flaw: heat lowers the capillary effect from the flaw's walls. In other words less penetrant will remain in the discontinuity and after removal of the excess of penetrant and developer application some indications may not be seeable.

These physical effects of temperature may lead to a more or less complete fading of indications with dramatic consequences on sensitivity and reliability.

A final point is that as temperature is higher capillary ingredients comprised in the NAWD do not adhere any more on surfaces: the lightest particles take their way out of the surface under inspection!

Designing PT materials for the 140 °C-200 °C (284 °F-392 °F) range was an answer to the following needs:
- Check multipass welding (boiling manufacturing).
- Inspection of running utilities/equipments when any stop or temperature lowering would be detrimental for safety or would lead to costly production losses.

High temp PT materials range comprise: a colour contrast penetrant, a remover to wipe off the excess of penetrant and a NAWD.

Contact time may be as short as one minute, up to 5 or 10 minutes depending on the surface temperature.

While the ISO3452-2:2006 standard does not deal with high temp inspection there is now a specific standard: ISO 3452-5 titled "Non-destructive testing - Penetrant testing - Part 5: penetrant testing at temperatures higher than   50°C" (122°F) .  This standard gives clues for PT testing at temperatures higher than 50 °C (122F) as well as for the qualification of products through suitable tests. It is applicable only for products their manufacturer states as usable in the temperature range specified for the inspection and only when the manufacturer's instructions are followed.

The American SAE-AMS 2644 specification does not cover high temp products, considered as products for special applications.

7- Reverse fluorescent PT materials

Reverse PT materials were designed in 1984 and were the topic of a paper(*).

It is known that fluorescent substances may lose their fluorescence capability when in contact with substances which absorb the wavelength which excites the dyes of the fluorescent penetrants.

As an example a substance A lighted by a radiation with a λe wavelength may give birth to a longer wavelength λf. This effect is called the bathochrome effect (or shift). This concept is the basis of all the PT and MT materials which use a UV-A radiation at 365 nm to excite the fluorescent dyes which then emit a visible radiation in the green or yellow green range (ca 550 nm) which happens to be the wavelength of the maximum sensitivity of the human eye in photopic conditions.

But guess this substance A is mixed with a substance B which absorbs the lambda λe exciting radiation. The loss of fluorescence of A is enhanced if the substance B absorbs also the visible λf emission.

A very simple test will show the dramatic result of such a mix: pour some few drops of a colour contrast penetrant in a beaker full of a fluorescent penetrant: under UV-A radiation the loss of fluorescence is impressive!

The reverse fluorescent PT materials range comprises a so-called biodegradable colour-contrast penetrant and an NAWD including dyes which fluoresce under UV-A radiation.

The NAWD fluorescence is "killed" by the tiniest traces of colour-contrast penetrant bleeding out of the open-to-the-surface discontinuities.

Hence very clear black indications appear against a fluorescent background when seen under UV-A radiation. That's why it is called the "reverse fluorescent PT".

The ISO 3452-1:2008 and 3452-2:2006 standards are silent about this process. The American SAE-AMS 2644E specification does not cover this process considered as a special products process.

As far as we know this PT process is not used. It may be useful to know what it is.

We nevertheless think this inspection method is very tiring for the eyes: trying to see a black indication against a very shiny background is by far more difficult than having the eyes drawn by a yellow-green indication against a dark background. We may call it a "technical tour de force" ... without any market.

8- Volatile flamable oil-based penetrant

This penetrant has a light blue fluorescence. But when applied on a part and after evaporation (within 30 seconds) it leaves a dry layer which gives a strong green fluorescence under UV-A radiation.

Its main use is to detect: porosities, leaks (through PT) and flaws. Additionally welded containers are more and more used and leaking welds are easily detected with this material.

When penetrant has entered open-to-the-surface discontinuities the fluorescence brightness of the bleeding out penetrant is far higher than that of the background; the contrast ratio between the indications and the background is enough to make it easy to detect the indications.

When using it as through penetrant on transformers tanks it is possible to detect a quantity as small as one hundredth of a cubic centimetre (ca 1/5 of a drop, or 1/500th of a teaspoon!). The exact location of the tiny leak is known as the volatile solvent evaporates from the penetrant coming out of the leak. Then there is a piling up of the fluorescent material. A more important leak gives birth to a spot as the penetrant spreads out on the surface before evaporation. Only by comparison a qualitative assessment of the size of the porosity or of the leak may be done after the speed of bleeding out; in no way a quantitative assessment shall be given.

This penetrant is not very efficient when looking for discontinuities; it is more kind of help than a test method such as the normal fluorescent PT.

Points of interest:
- An halogenated solvent-based penetrant has been used to check cars engines valves.
- No standard/specification deals with this penetrant.

9- Fluorescent particles in suspension in hydrocarbons as a fluorescent penetrant

This penetrant is almost unknown in France. In Anglo-Saxon countries it is referred to as "particle filtration process" or "dye precipitation penetrant system".

This penetrant is specifically designed to detect open-to-the-surface discontinuities such as flaws, cracks, porosities, etc on ceramics, sintered metals, concrete and other absorbing materials which generally prevent using "classic" penetrants which would leave a background too high to allow for a reliable detection of discontinuities.

This penetrant is shaken before use and shall be poured or sprayed onto the surface under inspection, or applied with a brush. Never immerse the part in this penetrant.

The excess of penetrant drains from the surface during the short contact-time. No developer is needed. Inspection is carried out under UV-A radiation.

Any flaw, crack, porosity and other open-to-the-surface discontinuity is seen as fine yellow-green indications against a faint background.

The ISO 3452-1:2008 and 3452-2:2006 standards do not know of this process.

The American SAE-AMS 2644 states it does not cover special products such as a "dye precipitation penetrant system".

10- Felt-tip pens for PT

They were first marketed ca 2000. They are used for on-spot inspection on small areas of parts, newly manufactured or reworked, or for maintenance so as to lower waste.

They are used mainly in aerospace industries, automotive industry, in services companies, etc.

Felt-tip pens are available for:
- Colour contrast or dual purpose penetrant.
- Water-washable or solvent-removable fluorescent penetrant.
- Post-emulsifiable/solvent-removable fluorescent penetrant.
- Non-aqueous wet developer.

Though these felt-tip pens are not special products strictly speaking, their application is not described in standards/specifications-- not yet, by the way.

11- Other special PT materials

Let us mention two materials: TEMOR® and TEGLO® which were designed by BURMAH CASTROL. We have no available information on these products.
Conclusion

As evident as per this paper there are a lot of special PT materials; we do hope we have not forgotten any.

(*) Jean-Claude C.HUGUES, Pierre CHEMIN, David J. HUTCHNGS - L'avènement d'une nouvelle ère dans le domaine du ressuage coloré - Revue Pratique du Contrôle Industriel, Numéro 130, décembre 1984, pages 59 et 60.

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.