French (Fr)English (United Kingdom)

DPC NEWS: a website dedicated to Penetrant Testing and Magnetic Testing



visits on site since April 2008

Log in


Receive HTML?

The specifications which changed the penetrant materials

Written by Administrator
Friday, 01 August 2008 17:38

August 2008
Document completed and updated in April 2012

I- Introduction

For many years, the American military specification MIL-I-25135, to which was annexed a list of approved materials, was the reference document regarding penetrant testing in the Aerospace industries.

As a matter of fact, the American aeronautical manufacturers have always exercised a part, more or less important, of their activities in the Military sector with the DOD (Department of Defence).

Further, the tied-up links, on both sides of the Atlantic Ocean, between the manufacturers, the subcontractors, the part manufacturers, the maintenance and repair shops made all of them taking into account this specification, either to apply it or to use it as a basis for writing their own specifications.

Considering the specificity of their manufacturing, the aircraft manufacturers indeed wrote their own instructions as soon there were liable vis-à-vis their customers for control procedures, maintenance procedures and recommended or qualified products.

Thus, the MIL-I-25135 specification became THE standard.

II- Evolution of the MIL-I-25135

William E.MOOZ (1) (2) reminded us of the evolution of this specification as per the following table:

Dates Revisions/Amendements Classifications
August 6, 1956 Titre : "Classification" • Type I - Inspection materials to be used with fluorescent inspection methods.
• Type II - Inspection materials to be used with non fluorescent (visible) inspection methods.
July 2, 1958 B revision No copy available of this document.
October 21, 1959 C revision Classifications of the penetrants in groups:
• Group I: colour contrast solvent- removable penetrant.
• Group II: colour contrast post-emulsifiaible penetrant.
• Group III: colour contrast water-washable penetrant.
• Group IV: fluorescent water-washable penetrant.
• Group V: high sensitivity, water-washable or post-emulsifiable fluorescent penetrant.
• Group VI: very high sensitivity, water-washable or post-emulsifiable fluorescent penetrant.
May 27, 1960 C revision,Amendment 1 Sharpened the washability requirements so that background fluorescence was reduced.
July 25, 1961 C revision, Amendment 2 Further sharpened the washability requirements.
June 1, 1964 Révision C, Amendement 3 Addition of the the Group VII: very high sensitivity solvent-removable fluorescent penetrant. Fluorescent penetrant materials kit consisting of spray cans of materials.
March 25, 1969 C revision, Amendment 4 Addition of valve leakage and other requirements for aerosol cans, and expansion of the toxicity requirements.
September 12, 1979 C revision, No amendment number Split of Group VI into Group VIA and Group VIB.
• Group VIA: very high sensitivity post-emulsifiable fluorescent penetrant.
• Group VIB: post-emulsifiable fluorescent penetrant, sensitivity higher than Group VIA.
June 29, 1984 D revision Replacement of the "Groups" Classification by "Type, Method, Sensitivity, Form and Class" classification.
June 26, 1989 E revision Addition of the sensitivity level ½.

The MIL-I-25135 as well as the relevant list of approved products (Qualified Products Lists) were updated through revisions and amendments over the years.

While the aeronautical technology quickly progressed, processing new alloys, in particular, those from titanium and from nickel, it is rather surprising to notice that the MIL-I-25135C has been in force for decades before being replaced by the MIL-I-25135D, on June 24th, 1984, then by MIL-I-25135E on June 26th, 1989. Indeed, it did not address the new requirements due to technological progress. The gaps of the MIL-I-25135C had been pointed out already in 1972 and 1973, when the Quality Direction of a French aeroengine manufacturer began to state required maximum allowable contents of chlorine and sulphur impurities in penetrant materials.

III- Gaps of the MIL-I-25135C

The following considerations had not been foreseen by the MIL-I-25135C.

III-1- Case of titanium alloys

The increasing use of titanium-based alloys, which led to lower weight and to interesting metallurgical characteristics, made aircraft manufacturers to ban using chlorinated solvents.
Indeed, under the effect of some physico-chemical phenomena, free radicals of chlorine, in the form of atomic chlorine, appear, for example, due to photolysis. The chlorine so released turned out to induce, under hot conditions, phenomena of stress cracking of the titanium alloys. General Electric (USA) suspected even methanol to exercise an adverse effect on these same alloys. This suspicion was due to the presence of OH radicals or, less likely, of H radicals. The problem is not a problem anymore as methanol, being classified toxic, is practically no longer used.

III-2- Case of nickel-based heat resisting alloys

The constant search for improvement of performances required to increase the thermodynamics yield of gas turbines. According to the second of Carnot’s principles, an increase of the temperature of the hot section of gas turbines required, on the one hand, to rely upon the technology of hollow air cooled gas-turbine  blades, and, on the other hand, to heat-resisting alloys, able to offset ever higher temperatures. These alloys are based, among other elements, on cobalt, chromium and nickel. At temperatures above 900°C (1650°F) approximately, nickel may undergo an "oxidation" by sulphur. This chemical reaction is known as sulphidation, which leads to the formation of black corrosion products that are nickel sulphide-based. This phenomenon is called “black plague".

The catalytic action of alkaline metals, such as sodium and potassium, was put in evidence in the sulphidation reaction:

Ni + S → NiS
or K

Sulphur is present in organic compounds and as impurities in oil products. Alkaline metals are present in surface active agents (surfactants), such as those of the anionic type, among which we can quote one of the most known: the sodium dodecylbenzenesulphonate (CAS N°27323-41-7).

It is necessary to point out that the organic synthesis of some non-ionic surface active agents (surfactants) may require the use of potassium sulphate as a catalyst. So, such surface active agents (surfactants) contain the unwanted impurities that sulphur and potassium are. As penetrants and hydrophilic penetrant removers contain surface-active agents (surfactants), it is necessary to carry out a rigorous selection of them. To get rid of the alkaline impurities, some manufacturers, at the beginning of the 80s, even made them undergo a long treatment on ion-exchange resins.

III- 3- The "Heat Fading" phenomenon

The drop of the fluorescent brightness of crack indications detected by penetrant testing is a phenomenon known as "heat fading". It mainly appears at the drying stage after water rinsing: because either the drying temperature is too high or the drying time in the air circulated oven is too long.

This topic was talked about by Pierre CHEMIN in a conference (3).

III-4- A higher flash point

The PMCC (Pensky-Martens closed cup) flash point of most of the colour contrast and fluorescent penetrants approved according to MIL-I-25135C was 70°C (circa 160°F). This quite low figure was due to low-boiling range aliphatic hydrocarbons in penetrants formulae. These hydrocarbons were very affordable.

Indeed, the higher the flash point, the higher the hydrocarbons price.

Because of this 70°C flash point, these penetrants had a major drawback when used by immersion. Indeed, when in a tank, there was a slow but true evaporation of the hydrocarbons, all the more important as the ambient temperature was high. This led also to the evaporation of the other less volatile ingredients of the penetrant. The same phenomenon could also occur by dipping parts into the penetrant tank, if parts were not allowed a sufficient time for cooling down to room temperature after trichloroethylene vapour phase degreasing. This led to an imbalance in the penetrant formula, causing, on the one hand, a viscosity increase and, on the other hand, an increase of the drag out losses along with an increased fluorescent background on the part surfaces.
That is why operators, who wanted to lower this fluorescent background, had a tendency to overwash the parts. The drawback was that, due to the poor overwash resistance of these penetrants, there was a drop of sensitivity; some crack indications were not seen and, in the end, their detection reliability was impaired.

Due to the imbalance in the formula, the add-ons needed to have the right level in the tank were performed by pouring a standard blend of hydrocarbons and other quite volatile ingredients, such as methylisobutylcarbinol, odourless kerosene. All of that, after an analysis was performed on the penetrant remaining in the tank, by distillation of flammable products! Nowadays, thanks to the high flash point of penetrants, there is no need for such add-ons. The loss compensation due to drag-out on parts is performed by topping the tank with brand new penetrant.

IV- The Pratt and Whitney specification

The first serious works on quantification of the penetrant process sensitivity were undertaken in the seventies. Among these, let us name the works of Norman H.HYAM † (4), then, those of E.O. LORMERSON Junior (5) (Two fold congruency test) of Pratt and Whitney, and a bit later, those of Jean VAERMAN † of Snecma (6) (7).

All these important and remarkable works allowed for the determining of the effect of variation of the operating parameters of the penetrant processes on the sensitivity of crack detection.

These works made Pratt and Whitney, in the absence of the revision of the MIL-I-25135C specification, write a new Penetrant Testing specification, which came into effect at the end of the 70s.

Pratt and Whitney was particularly concerned by the flash point figure. Did one of their plants go on fire then? However, Pratt and Whitney deleted the qualification of any chemical and refused any chemical, whatever it was designed for, with a PMMC flash point under 200°F (93°C).
As an example, let us quote the compressor washing fluids (designed to restore the thrust of aeroengines and industrial gas turbines) although these fluids are diluted at 20% by volume in water.

The Pratt and Whitney specification also stated:
• Maximum allowable contents in impurities: chlorine, sulphur, sodium and potassium.
• Resistance to the thermal effects (heat fading).

These new requirements made the penetrants manufacturers design new penetrants, as almost none of the then-existing ones complied with the specification requirements. Therefore, Pratt and Whitney distanced itself and made some manufacturers scratch their heads to have new products quickly approved. Too fast, for some.

While new penetrants were approved according to this specification, some gave rise to surprises; in particular, problems of water washability, and other problems linked to penetrant removal when using a hydrophilic emulsifier, even at the too high concentration of 20% by volume in water instead of 10%, as recommended for the immersion technique by the manufacturer!

Thus, the suppliers and the subcontractors working for this aeroengine manufacturer had then to double, at that time, the number of their penetrant process lines, ones to meet Pratt and Whitney’s requirements, the others to meet the requirements of the other aeroengines manufacturers, which had not concurred with Pratt and Whitney. Same situation in the Repair and Maintenance shops worldwide.

The future praised Pratt and Whitney ideas because its specification became the core of the MIL-I-25135D. Nobody can dispute the relevance of all the requirements that Pratt and Whitney pushed for some years before, and, which were introduced in the so long waited for MIL-I-25135D specification on June 29th, 1984. Everyone involved in PT in aerospace industries was relieved, as this specification has been largely endorsed by most of the primes.

V- Main differences of products between the MIL-I-25135C and the MIL-I-25135D/E

Let us point out here that the differences between the MIL-I-25135D and E are small.

With regard to penetrant materials listed in the QPL of the MIL-I-25135C, those listed in the QPL of MIL-I-25135D/E have the following advantages:
• Guarantee that products were checked about hygiene and safety (in 3.3.1) for operators. Complete absence of asbestos required in developers.
• Higher flash point 200°F (93°C) minimum (in 3.3.3), higher safety. Furthermore, there is an empirical rule of thumb establishing a relationship between the flash point and the maximal temperature of penetrant application. The rule is: PMCC flash point less 20°C is roughly equal to the maximum temperature of penetrant application. For instance, if the flash point is 93°C, the maximum temperature for penetrant application on the part surface is 73°C. As consequence it is possible to reduce the cooling time after parts hot degreasing before penetrant application.
• Thermal stability (in which requires the evaluation of the penetrant resistance to thermal effects (heat fading).
• Penetrant stability when in a tank (in 3.4.7).
• High temperature titanium stress corrosion (in
• High temperature (1,850°F ± 50°F) or (1,010 ± 28°C) corrosion of cast nickel alloys (in
• Fluorescent brightness test in accordance with ASTM E 1135.
• Ultraviolet stability.
• Etc.

VI- Assessment

The American specification MIL-I-25135 D/E has been since replaced by the SAE-AMS-2644 specification. The AMS-2644E, issued on October 30, 2006, is the current revision. The latest edition on paper of the qualified products list is the QPL-SAE-AMS-2644-4 dated October 1st, 2004.

Now, the QPL-SAE-AMS 2644E is published in a database on the Internet, and is regularly and more frequently than before updated.

Each qualified product is listed as follows:
Governmental designation according to the SAE-AMS-2644E classification.
• Manufacturer designation, i.e. the commercial name given by the manufacturer.
• Source name, i.e. the manufacturer’s name and full address, together with the test reference of the relevant product.
• A 5-digit alphanumeric CAGE code (Commercial And Government Entity Code) together with a "traffic light" indicating the status of approval of the relevant product:
> Green: Source is certified.
> Yellow: Source is due for certification.
> Red: Source is overdue for certification.
• Related links.

Thus, a manufacturer/supplier, a prime or a user may check if this or that product is qualified and what is its status.

The French aircraft (airframes and aeroengines) manufacturers more or less adapted their own specifications to the SAE-AMS 2644, and the penetrant materials they approved are listed in the QPL-SAE-AMS-2644-4.

The ISO 3452-1 and 3452-2 standards took into account the SAE-AMS 2644E specification.

However, it is interesting to note that the ISO 3452-2 standard of May, 2000 did not include some requirements of the SAE-AMS 2644E such as: thermal stability, high-temperature titanium stress corrosion, high-temperature corrosion of cast nickel alloys, etc.

Nevertheless, considering the fact that the Americans did not take into account this ISO standard, it was decided that the ISO 3452-2 standard would almost duplicate the SAE-AMS 2644E. That is the main reason for the 2006 edition of the ISO 3452-2 standard.

VII- The future

Nowadays, there is almost no Research and Development works on Penetrant Testing. Further, very few communications on the topic were given in the latest National, European or International conferences: the situation again seems to have come to a standby.

Nevertheless, works of optimization should be pursued to improve: sensitivity of detection, penetrant washability along with overwash resistance and with detection reliability.

When the SAE-AMS 2644E is revised, it is desirable that it states:
• Banning use of halogenated solvents in penetrant materials.
• A standard test method for resistance of penetrant to overwashing.
• Maximum allowable contents of: chlorine, fluorine, sodium, potassium and sodium impurities.

Finally, since the beginning of the 70s, very few tests and theorization have made to design an automatic reading equipment able to accept or reject parts, or send them for repair. There are some prototypes, but an operator’s expertise in deciding, within a very short span of time, what is acceptable or not, cannot yet be duplicated. No penetrant process line comprises such a system! That’s why one should think Penetrant Testing lags behind in ability and reliability when compared to other NDT methods.

Keep in mind that Penetrant Testing was probably the first of the "nanotechnologies", as only 25 to 75 nanograms of dye come out of the tiniest cracks accepted on engine blades. This very small quantity is easily detected and processed by this very old equipment: human eye (sensors) + human brain (signal processor equipment) without any electronics attached, No computer bug risk.


(1) Patrick DUBOSC and Pierre CHEMIN, Un Américain À Paris… qui aime Paris au mois de mai, Editorial, June 2010: On our Website.

(2) The February 2011 Penetrant Professor issue, on this Website.

(3) Pierre CHEMIN, Thermal effects in Penetrant Testing processes. Proceedings of the 2nd European Conference on Non-destructive Testing, Hall of Ceremonies (Zeremoniensaal) of the imperial Palace of the Hofburg, Vienna (Austria), September 14-16, 1981.

(4) Norman H. HYAM, "Quantitative Evaluation of Factors Affecting the Sensitivity of Penetrant Systems". Materials Evaluation, pages 31-38, February 1972. Materials Evaluation, 1711 Arlingate Lane, PO Box 28518, Columbus, OH 43228-0518, USA.

(5) LOMERSON Jr., Statistical Method for Evaluating Penetrant Sensitivity and Reproduceability. Material Evaluation Volume 27, pages 67-70. - March 1969. Materials Evaluation, 1711 Arlingate Lane, PO Box 28518, Columbus, OH 43228-0518, USA.

(6) Jean VAERMAN, Fluorescent Penetrant Inspection Process, Automatic Method for Sensitivity Quantification. Proceedings of the 11th World Conference on Nondestructive Testing, Volume III, Las Vegas, NV, November 1985, pp. 1920-1927.

(7) Jean VAERMAN, Fluorescent Penetrant Inspection, Quantified Evolution of the Sensitivity Versus Process Deviations. Proceedings of the 4th European Conference on Non-Destructive Testing, London (United Kingdom), Pergamon Press, Maxwell House, Fairview Park, Elmsford, New York, Volume 4, September 1987, pp. 2814-2823.

Normative references

• American Military Specification MIL-I-25135C (ASG), Inspection Materials, Penetrants, October 21, 1959.

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

• 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.

Last Updated ( Sunday, 16 September 2012 16:59 )