WO1991007654A1

WO1991007654A1 - Penetrant composition and process for use

Info

Publication number: WO1991007654A1
Application number: PCT/GB1990/001761
Authority: WO
Inventors: Philip Ian Brittain
Original assignee: Brent Chemicals International Plc
Priority date: 1989-11-16
Filing date: 1990-11-15
Publication date: 1991-05-30
Also published as: GB8925887D0; AU6735690A

Abstract

A process for detecting surface flaws in metal parts comprises applying a dye penetrant composition to the surface, rinsing the penetrant and developing the surface. The dye is usually fluorescent and development usually involves application of a suspension of pigment particles to the surface. The penetrant is usually water washable. The observation of the flaws in the process is improved by carrying it out in the presence of N-C5-18alkyl-2-pyrrolidone, usually N-cyclohexyl-2-pyrrolidone which increases the brightness ratio.

Description

PENETRANT COMPOSITION AND PROCESS FOR USE

The present invention relates to processes for revealing cracks and other surface-opening defects in metal parts and compositions for use in such processes.

The method of use of dye penetrant compositions for revealing surface-opening defects in engineering parts is well known. Such penetrants consist essentially of an oily non-water miscible solvent in which is dissolved a coloured dye or, in the case of a fluorescent penetrant, a dye or dyes which fluoresce when their solution is exposed to ultra-violet (black) light of wavelengths of the order of 365 nm. The penetrant is applied to the metal surface whereupon cracks or other surface defects are penetrated by the composition. The defects can be observed after washing and development.

There are two main types of penetrant composition, the post-emulεifiable system and the water-washable system. In the post emulsifiable system the penetrant composition contains little or no surfactant and so in order for the penetrant to be washed from the surface

(whilst being retained in the defects) a surfactant, which enables the formation of an emulsion of the penetrant in water, must be applied. The surfactant can be applied as a solution in the wash water or can be applied prior to washing in the form of a concentrated surfactant remover composition eg by dipping or spraying. In the case when an aqueous solution of remover is used it is common to remove a proportion of the penetrant first by spraying with water in a pre-rinse stage, which physically removes the excess penetrant. This acts to extend the life of the remover bath and also improves the remover's uniformity of action over the surface. The penetrant composition may be gellable on contact with water, this property being intended to reduce the tendency of the penetrant to be removed from the defects during washing.

In the water washable system, the surfactant is incorporated directly in the penetrant composition so as to render it self-emulsifiable in water. Removal is by application of water alone. Although this system is simpler to use it has the disadvantage that it is more susceptible to overwashing and tends to be less sensitive than the post-emulsifiable system. Rarely, the penetrant may be washed off the surface using a non-aqueous rinse liquid.

These penetrants have to fulfil a number of sometimes conflicting requirements. They must work well, they need to be stable, relatively non-volatile, non-toxic, non-flammable and non-corrosive. They are preferably of reasonable cost.

To work effectively, the penetrant should have a low viscosity in order that the time taken to flow and penetrate defects is not excessive. It should readily wet smooth surfaces without showing any signs of retraction or dewetting, which is generally the case with materials having a lower, rather than a higher surface tension. In contact with^" water, penetrants should not hydrolyse or precipitate solid matter such as dyes or inhibitors.

In the case of post-emulsifiable penetrants it is advantageous for the density to be significantly lower than that of water so as to enable separation of the penetrant from water collected after a pre-rinse stage, which is often applied to remove excess penetrant from the surface of the part before final cleaning in a hydrophillic (water base) remover.

Sensitivity to small defects is a factor of great importance in penetrant technology. This requires the use of penetrant materials of such a kind and in such a way that the excess penetrant is readily removed from the surface of the component, thus avoiding confusing coloured or fluorescent background, without any appreciable removal of the penetrant-oil from the cracks. These residual, usually small, amounts of penetrant must be detectable optically, and a developer, in the form of a dusted-on white powder (eg as described in US-A-3,748,469 or US-A-3,561,262) , sprayed-on powder suspension (as described in US-A-3,803,051 or US-A-3,928,046) , or a solution containing dissolved salts from which the solvent is evaporated is commonly used to enhance the penetrant indications.

The visual impact of the penetrant material is increased by the incorporation of visible colour or colour contrast dyes, often red, of high tinting power, or by the use of suitable dyes which can be excited to fluoresce strongly by black light (UV light) . These fluorescent materials are typically much more sensitive because of the much greater contrast ratio achievable compared with the simpler visible dye system. For most sensitive fluorescent penetrants, the dye system employed must be highly absorptive for UV light, and must be able to convert the absorbed radiation efficiently into visible light. Usually a combination of fluorescent dyes is used consisting of a sensitiser dye, which absorbs efficiently UV light and converts it to visible light by emitting at a longer wavelength, and a colour dye which absorbs the emitted light and emits it by fluorescence at a higher wavelength, which is easier to see.

For a given fluorescent dye system, higher absorptivity may be achieved by increasing the dye content of the penetrant. This however, above a certain dye concentration, results in increasing self-quenching which acts to reduce the efficiency of conversion of UV into visible light. This has the effect that small defects, although visible, are less obvious to the naked eye, so that the probability of detection is reduced.

Another way of increasing the efficiency of the conversion of UV to visible radiation is by modifying the solvent system of the penetrant. The addition of phosphate esters for this purpose is well known. These esters have the added advantage of increasing the dye solubility in the base oil. Thus many of the currently available commercial fluorescent penetrants contain such esters, for example, tricresyl phosphate, in relatively large quantities, eg 5-50%. These additives suffer from the disadvantages of being sometimes considered to be toxic, and they are of relatively high density and cost.

Other additives having this booster effect are ethoxylated alcohols as described in GB-A-1,57 ,892 and ethoxylated alkyl phenols, and alkyl diethanolamides. These materials also tend to confer water washability, which is undesirable in the case of post-e ulsifiable systems. Additives which do not confer water washability include esters such as phthalates, eg octyl, butyl, benzyl or tridecyl phthalates. Two or more booster solvents will often be found to have a synergistic effect. For example, a mixture of 25% lauric diethanolamide with 75% (w/w) Reofos 95 (a proprietary phosphate ester) is more effective as a solvent booster than either component when used at a total concentration of 40% by w/w in a typical penetrant formulation.

Solvents which increase the efficiency of the UV to visible light conversion dye can also be incorporated into the developer. Alburger, in US-A-3,928,046, describes the use of lower alkyl pyrrolidones in such compositions.

The use of N-lower alkyl 2-pyrrolidones as a base solvent for penetrants is known. For example, US-A-3,965,350 claims a liquid penetrant vehicle consisting essentially of an N-alkyl 2-pyrrolidone, where the alkyl group contains from one to four carbon atoms. Advantages stated for such a formulation include increased dye solubility and water miscibility, the latter assisting water washability. This feature is of course undesirable in post-emulsifiable penetrants, and additionally may act to precipitate dye from solution when water washing is used, thus aggravating background fluorescence problems. In US-A-3,965,350 a surfactant may be added specifically to suppress this precipitation effect. In some of the examples the incorporation of a ketone is mentioned and it is believed that acts to suppress the precipitation described above as well as enhancing the wetting properties of the penetrant.

The patent further points out that the vinyl pyrrolidone produces a nauseous smell on contact with water, and is therefore unsuitable for general use. In fact the other lower homologues also give rise to an unpleasant smell when incorporated in penetrant formulations such as those disclosed in the patent. The pyrrolidones are known to have a boosting or activating effect on fluorescent dye solutions. The patent cites N-methyl 2-pyrrolidone as the preferred material, and illustrates examples in which the N-methyl 2-pyrrolidone is replaced in Examples XIV, XV and XVI by N-ethyl 2-pyrrolidone, N-ethy1-2-pyrrolidone and N-propyl-2-pyrrolidone respectively. In each case "similar results" were obtained.

According to the present invention a new process for detecting flaws in a metal surface comprises applying a penetrant composition comprising a dye to the surface, removing the penetrant composition from the surface by rinsing with a rinsing liquid and then observing the penetrant retained in the surface flaws in the presence of an N-alkyl-2-pyrrolidone characterised in that the N-alkyl-2-pyrrolidone is an N-C₅.₁₈alkyl-2-pyrrolidone.

In the new process, where the penetrant composition is a water-washable composition, the rinsing liquid may consist of water, eg tap water. Where the composition is a post-emulsifiable composition the rinsing liquid may contain a surfactant or a remover can be applied to the surface, optionally after a water pre-rinse, followed by a rinse with water, usually tap water, as described above. Where a pre-rinse is used the recovered liquid may be separated to recover the penetrant components, for re-use or disposal. Sometimes the rinsing liquid may be non-aqueous.

In the new process the N-C₅.₁₈alkyl-2-pyrrolidone may be incorporated in the developer, in the manner described by Alburger in US-A-3,928,046. It is preferable, however for the compound to be incorporated into the penetrant composition.

In the process, after the final rinsing step, there is usually a drying step and then generally a development step eg of the usual type. Development may consist of any of those methods described above. The cracks are observed after development in the normal way, by illuminating with light of an appropriate wavelength. For fluorescent dye systems the radiation which is used is UV ("black light").

A new dye penetrant composition according to the invention, which is homogeneous and comprises a dye in solution in a solvent, which comprises N-alkyl-2-pyrrolidone, is characterised in that the N-alkyl-2-pyrrolidone is N-C₅._1galkyl-2-pyrrolidone.

The N-alkyl-2-pyrrolidone should be a liquid at ambient temperatures. Usually the N-C₅.₁₈alkyl-2- pyrrolidone has at least 6 and no more than 12 carbon atoms and best results are achieved with lower numbers of carbon atoms, for instance between 6 and 8, inclusive. The alkyl chain can be straight, branched or cyclic. One particularly preferred example is N-cyclohexyl pyrrolidone and another suitable compound is N-n-octyl pyrrolidone. The penetrant composition of the invention may be a post-emulsifiable composition, as described above. Such a composition is substantially immiscible in plain water but forms an emulsion or miscible solution with water containing a surfactant. Alternatively the penetrant composition may be water-washable, that is the composition contains surfactant so that on contact with water it can be washed away as an emulsion or miscible solution. Suitable surfactants for use in a water-washable composition are for instance alkylphenol ethoxylates or alcohol ethyoxylates.

The composition may contain co-solvents as part of the solvent. Such co-solvents include for instance hydrocarbon solvents, such as kerosene or other hydrocarbons, or aliphatic alcohols, for instance C, ,_β alkyl alcohols, preferably C₈.₁₆ alcohols for instance a mixture of ₉. primary alcohol such as that available under the trade name Dobanol 91 from Shell Chemicals. One advantage of the use of pyrrolidones compared to phosphate ester-based compositions is that aromatic co-solvents can be omitted and high dye content compositions formed.

The dye that is used in the composition can be any of those conventionally used in dye penetrants, a colour contrast dye or a fluorescent dye. Mixtures of dyes can be used, for instance, in the colour contrast system to modify the colour. Preferably it is a fluorescent dye or dye mixture. Any of the dyes disclosed in GB-A-1,574,892, GB-A-1,239,791 or US-A-3 , 965, 350 can be used.

Fluorescent dye compositions may, as is conventional, comprise a system containing two or more dyes which act in conjunction with each other. Such a system may comprise a sensitiser dye and a colour dye which act by the so-called cascade effect, described above. It is believed that incorporation of the

N-CC₅.₁₈alkyl-2-pyrrolidone has a particularly beneficial effect on such systems as it may enhance the efficiency of fluorescence of the system.

In the compositions the dye or dye system is generally present in an amount in the range 0.1 to 20%, preferably in the range 0.2 to 8% by weight. In a dye system comprising colour dye and sensitiser dye the dyes are preferably present in the ratio of 1:1 to 1:5, preferably 1:2 to 1:4.

The N-C₅.₁₈alkyl-2-pyrrolidone is generally present in an amount of at least 1% by weight, preferably at least 2% by weight and more preferably at least 5% by weight. The amount of cyclohexyl pyrrolidone generally does not need to be more than 50% by weight, and is preferably less than 25% by weight.

Co-solvents, which act as diluents, may be present in amounts of up to around 95% by weight, and are generally present in an amount of at least 20% by weight. Preferably the amount of co-solvent is in the range 30-90% by weight.

If any surfactant is present in the composition it is usually present in an amount in the range 0.1 to 25% depending on the degree of washability required.

According to a further aspect of the invention a liquid developer composition for use in developing a penetrant composition, comprises a pigment and an N-alkyl-2-pyrrolidone, characterised in that the

N-alky1-2-pyrrolidone is N-C₅.₁₈alkyl-2-pyrrolidone.

In the context of this specification a pigment covers finely divided solids which are insoluble in the developer liquid and also soluble compounds which form fine divided solids on removal of the solvent.

The pigment used in the developer may be an insoluble material which is suspended in the developer composition, such as talc, calcium carbonate, magnesium oxide, magnesium sulphate, silica, clays or titanium dioxide. Alternatively the pigment may be dissolved in the developer and be such that a precipitate of fine crystals is formed on the surface on drying, for instance sodium benzoate or sodium nitrite. Usually the developer is aqueous-based, or it may be solvent-based. An aqueous based composition can contain co-solvents such as glycols and surfactants to aid wetting. Since the pyrrolidone is non-volatile it remains on the surface after the drying of the developer so that it can interact with the dye in the penetrant composition to provide the beneficial effects.

In such developer compositions the N-C₅.₁₈alkyl-2- pyrrolidone is usually present in an amount in the range 1 to 50% based on the solids content, preferably in the range 1 to 20%. The use of cyclohexyl pyrrolidone has many advantages over any of the lower alkyl pyrrolidones described in US-A-3,965,350 and GB-A-1,239,791 (US-A-3,838,160) and US-A-3,928,046. In particular the pyrrolidone has a higher boiling point, does not produce a nauseous smell on contact with water, is less toxic, and, more importantly, for fluorescent systems, provides compositions having a higher fluorescent brightness.

The following example illustrates the invention: Tests Used Comparative testing was carried out as follows. Black plastic bottle caps of 3.5 cm diameter were inverted and used to contain the test fluorescent penetrants to a depth of 0.4 cm. Two samples were set side by side and exposed to UV (black) light of intensity 200 μw/cm . The resulting fluorescence was measured using a CCD camera equipped with suitable optical filters designed to reject radiation of wavelengths outside the visual green-yellow range. This was coupled to a digital frame store. The captured image was analysed with both specimens in the same frame. The software was able to compare repeatedly the brightness of two areas of the screen adjusted to be in the middle of the specimens, sampling 100 times. Each sample area consisted of nine pixels, the brightness values of which were averaged. The averages of the two zones were compared to produce a brightness ratio. The brightness ratio from the 100 consecutive samplings were recorded and a final average obtained. In this way the effect of noise of optical and electrical origin was minimised.

Examples 1-8

Comparative measurements were made using the above equipment on the following basic fluorescent penetrant composition: Sensitiser dye l.OOg Colour dye 0.33

High flash kerosene 18.67 Dobanol 91 6.66

Additive 6.66 The sensitiser dye used was known as "Calcofluor White RWP Cone", or Fluorescent Brightener 61, supplied by the American Cyano id Company of Bound Brook, New Jersey 08805, USA. The colour dye used was known as Hudson Yellow D250, or Cl Solvent Yellow 43, supplied by the Dayglo Colour Corporation of 4732, St Clair Avenue, Cleveland, Ohio 44103, USA.

"Dobanol 91" is a blend of C₉_^ primary alcohols supplied by Shell Chemicals. The high flash kerosene used was known as "Exxsol D100" supplied by Esso Chemical Ltd, Arundel Towers, Portland Terrace, Southampton, UK, S09 2GW.

In the reference composition, the additive was Dobanol 91 (Example 1) . In examples 2 to 8 the additive used was N-methyl 2-pyrrolidone (Example 2), N-ethyl 2-pyrrolidone (Example 3), N-vinyl 2-pyrrolidone (Example 4) , N-hydroxyethyl 2-pyrrolidone (Example 5) , N-cyclohexyl 2-pyrrolidone (Example 6) , N-n-octyl-2-pyrrolidone (Example 7) and N-dodecyl-2-pyrrolidone (Example 8). All solutions were clear ie homogeneous except for the hydroxyethyl homologue (Example 5) , which was not tested.

The brightness ratios recorded by the comparitor were as follows: 16

Example BRIGHTNESS RATIO

1) 1.00

2) 1.28

3) 1.33

4) 1.17

5) 6) 1.42

7) 1.38

8) 1.34 These results clearly demonstrate the increased effectiveness of the higher alkyl-2-pyrrolidones in enhancing fluorescent brightness compared with the lower homologues that have been used in the prior art. Example 9 The composition of Example 6 was used to detect stress cracks in a hard nickel and chromium plated layer approximately 20 μ thick on a brass base. Following immersion in the penetrant fluid for five minutes, the test piece was allowed to drain free of excess penetrant. It was then immersed in a 5% w/w aqueous solution of Synperonic NX (a nonionic surfactant available from ICI) for one minute, washed under a running tap for 30 seconds, dried in an air circulating oven for ten minutes at 50C, and finally dusted with a proprietary developer powder (Ardrox 9D4A) . After allowing the indications to develop for ten minutes, they were compared with those of a paired (identical) specimen processed using a commercially available post-emulsifiable penetrant system approved to MIL-I-25135D Level 4. The crack pattern was equally visible with both penetrant systems, showing that the new process is technically satisfactory.

Example 10

A water washable penetrant was made up of the following ingredients (parts by weight) :

Alcohol ethoxylate

Surfactants Ethylene oxide oleate

Colour Dye Sensitiser Dye Butyl glycol

N-cyclohexyl pyrrolidone High flash kerosene

The sensitiser and colour dyes and high flash kerosene were as in Examples 1 to 8. The penetrant was compared to a conventional water-washable penetrant using a process in which the penetrants were applied, drained and then washed with pure water, dried and developed using the developer powder used in example 9. The crack pattern was equally visible with both penetrant systems.

Claims

1. A process for detecting flaws in a metal surface comprising applying a penetrant composition comprising a dye to the surface, removing the penetrant composition from the surface by rinsing with a liquid and then observing the penetrant retained in the surface flaws in the presence of an N-alkyl-2-pyrrolidone characterised in that the N-alkyl-2-pyrrolidone is an N-C₅.₁₈alkyl-2- pyrrolidone.

2. A process according to claim 1 in which the rinsing liquid is aqueous.

3. A process according to claim 2 in which the rinsing liquid is tap water.

4. A process according to any preceding claim in which the process involves the application of a liquid developer following the rinsing step.

5. A process according to claim 4 in which the N-C₅.₁₈ alkyl-2-pyrrolidone is incorporated into the developer.

6. A process according to any preceding claim in which the N-C₅.₁₈alkyl-2-pyrrolidone is incorporated into the penetrant composition.

7. A process according to any preceding claim in which the penetrant comprises a fluorescent dye.

8. A process according to any preceding claim in which the pyrrolidone is a C₆.₁₀ preferably C₆.₈alkyl-2- pyrrolidone.

9. A process according to claim 8 in which the pyrrolidone is N-cyclohexyl-2-pyrrolidone.

10. A homogeneous dye penetrant composition comprising a solution of a dye in a solvent which comprises a N- alkyl-2-pyrrolidone, characterised in that the N-alkyl- 2-pyrrolidone is N-C₅.₁₈alkyl-2-pyrrolidone.

11. A composition according to claim 10 which comprises a cosolvent selected from hydrocarbon solvents and aliphatic alcohols.

12. A composition according to claim 10 or 11 which is water-washable and contains a surfactant.

13. A composition according to any of claims 10 to 12 in which the dye is a fluorescent dye.

14. A composition according to claim 13 which contains a sensitiser dye and a colour dye.

15. A liquid developer composition for use in developing a penetrant composition for detection of surface flaws in metal comprises a pigment and a N- alkyl-2- pyrrolidone, characterised in that the N-alkyl- 2-pyrrolidone is N-C₅.₁₈alkyl pyrrolidone.

16. A developer composition according to claim 15 in which the pigment is insoluble in the composition.

17. A developer composition according to claim 16 in which the pigment is selected from talc, calcium carbonate, magnesium oxide, magnesium sulphate, silica, clays and titanium dioxide.

18. A developer composition according to claim 15 in which the pigment is soluble in the composition.

19. A developer composition according to claim 18 in which the pigment is selected from sodium benzoate and sodium nitrite.

20. A developer composition according to any of claims 15-19 which is aqueous based.

21. A composition according to any of claims 10-20 in which the N-C₅._1βalkyl-2-pyrrolidone is present in an amount in the range 1-50% by weight.

22. A composition according to any of claims 10-21 in which the pyrrolidone is a N-C₆.₁₂-alkyl-2-pyrrolidone.

23. A composition according to any of claims 10 to 22 in which the pyrrolidone is N-cyclohexy1-2-pyrrolidone.