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Degreasing prior to penetrant testing

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Written by Administrator
Saturday, 22 October 2011 13:20

November 2011

1- Introduction

Degreasing prior to PT is an ESSENTIAL and DECISIVE step to ensure a reliable inspection.

In fact, pollutants such as oils and fats, as any other material may clog discontinuities and prevent penetrant to do its job properly. As good as penetrant materials or inspectors are, nothing can be done against.

Very important information is the fact that in no way silicones may find their way on parts which may be PTested later. Indeed, it is awfully difficult, in fact almost impossible to wipe out silicones from surfaces (painters’ fear) which make surface hydrophobic and clog discontinuities.

2- Oily and greasy pollutants

Previous to determine the most suitable means let us try to list the oily and greasy pollutants which may be found on parts.

2.1- Pollutants due to manufacturing

Manufacturing processes (turning, milling, grinding, etc) require using whole oils or soluble oils.

Descaling processes require using water-based non-organic materials.

Tubes’ bending needs using very high viscosity oils.

Sticking and polishing leave residues, which shall be removed.

Welding processes very often come with anti-spatter materials, which nowadays are generally silicone-free … but oily and viscous.

Ultrasonic testing (UT) for crack or corrosion detection or for thickness measurements requires using couplants - that is a main reason to perform any UT AFTER PT.

Etc …

2.2- Pollutants found after an interstage storage or short-, medium- or long-term storage

White-spirit-based water-repellents may be used during manufacturing to dry parts by water-displacement.

Short-term corrosion preventatives are generally made of lanoline dissolved in white spirit. This coating may be unexpectedly difficult to get rid of if parts have been stored for too long or in plain sun light. Hydrocarbons then are not enough to dissolve these chemicals, which will need using chlorinated solvents or acetone.

Water displacing short-term corrosion preventatives are made of a blend of sulphonated hydrocarbons and waxes of different kinds dissolved in white spirit. They are removed with hydrocarbons solvents.

Very long-term corrosion preventatives are made of a complex mixture of partially neutralized oxygenated hydrocarbons, petroleum sulphonates amines in white spirit. They are a bit more difficult to wash away than the above chemicals.

2.3- In-service pollution


In-service produced pollutants may be grease, oils, lubricants, hydraulic liquids mixed with other pollutants, heavy-duty carbon residues, organic or non-organic pollutants, etc.

2.4- Pollution due to workshop

Quite often it is due to dust, grease, oil vapors, lubricants, loosening oils, hydraulic liquids, oil sludge, etc …

3- Degreasing means

First of all keep in mind to ban some inefficient means, which may even be detrimental to a performing PT:
• Sandblasting, shotblasting.
• Absorbent materials such as sawdust, etc.

3.1- Solvents

Halogenated and non-halogenated solvents should be separately considered.

- 3.1.1- Halogenated solvents

Some years ago, better to write at least two decades ago, parts were often degreased before PT with halogenated solvents, such as:

- Vapor degreasing:
• 1,1,1 trichloroethane ( T-111), which the Montreal protocol banned production and marketing of some time ago, along the chlorofluorocarbons (CFC).

• Trichoroethylene now classified as carcinogenic (risk phrase R45: May cause cancer).

- In spray cans:
1,1,1 trichloroethane, trichloroethylene, trichlorotrifluoroethane (CFC 113), a mix of T-111 and CFC 113, a blend of chlorinated solvents and hydrocarbons.

Other chlorinated solvents have been used on a small scale such as: perchloroethylene, dichloromethane (methylene chloride) including in vapor-phase units, etc.

All these chlorinated solvents had two benefits: non-flammable, highly volatile, they exactly fitted the needs of PT for degreasing parts before penetrant application, except for those made of titanium-based alloys and of austenitic steel.

Indeed, since the ‘70s using chlorinated solvents on titanium-based alloys and austenitic steels is forbidden. On aluminium alloys pitting corrosion may occur. Chlorinated solvents are prone to be photolysed (destructed by light); this gives birth to chlorine radicals. This "nascent" chlorine is chemically very reactive and captures an electron, becoming a chloride anion. Corrosion inhibitors were part of the formula: they trapped chloride anions, and were called "stabilizers". Nevertheless, as these often amine-based chemicals lost their efficiency as time passed, corrosion tests were mandatory at regular intervals on in-use solvents.

Hydrofluorocarbons (HFC) as substitute for chlorofluorocarbons (CFC) were thought of but to no avail in our PT world.

- 3.1.2- Non-halogenated solvents


Several kinds of these solvents may be listed:

• Alcohols: the most widely used by large in PT is isopropyl alcohol, sometimes used also as excess of penetrant remover.

• Ketones: acetone has been widely used some time ago on large stainless steel plates. In many companies using it is nowadays forbidden mainly because of fire risks due to its very low flash point. Methylethyleketone (MEK), classified as easily flammable and irritant is seldom used as a degreaser in PT.

• Organic esters: ethyl acetate is rarely used as a degreaser in PT. Butyl acetate may be seen in Germany.

• Oxygenated solvents such as glycol ethers from the ethylenic family are not used due to their mutagenic risks. Those from the propylenic family are not such classified but they are not used as degreasers in PT.

• Light aromatic hydrocarbons such as benzene, toluene, xylene, etc are not used as they are very dangerous for health.

• White spirit, a petroleum distillate, is not the right solvent to use before PT due to its quite high distillation range (150°C-200°C/302°F-392°F)

• Some other petroleum distillates made of aliphatic hydrocarbons are quite often offered as the right alternative by suppliers with a low understanding of the specific requirements of degreasing before PT and who do not balk at selling some of them as "nonflammable" substitutes to flammable degreasers. But these degreasers have a boiling point above 145°C (ca 295°F), they do not evaporate in a reasonable frame of time, leaving residues in the discontinuities! These "degreasers" may be used to clean mechanical parts, but NEVER as a degreaser prior to PT!

• Low boiling aliphatic hydrocarbons: heptane only is of some interest. No other aliphatic or isoparaffinic hydrocarbon is suitable for degreasing prior to PT: they are not volatile enough and may remain trapped in the discontinuities.

• Terpenic hydrocarbons generally come from living organisms: they are found in conifers (pine tree, for example) and in citrus fruits (orange, citrus for example). One of them, limonene and its derivatives, has already been used in formulae designed to replace halogenated solvents.

- 3.1.3- Solvents used nowadays for PT applications


To get rid of large oily and greasy layers one need to use a solvent, which goes well deep in the layer and mixes with the pollutant. Then this solvent must not be too volatile. Almost all the PT materials manufacturers have at least one solvent of this kind in their program. It may be:

•  A blend of catalytically hydro-desaromatized and desulphurized hydrocarbons: aromatic content is less than 0.003% (30 ppm) and benzene content is less than 1 ppm (typical figure), hence non carcinogenetic.

• An hydrotreated heavy naphtha solvent with less than 0.1% benzene, hence non-carcinogenetic.

• A light aliphatic naphtha solvent with less than 0.005% benzene and less than 0.01% toluene, hence non carcinogenetic.

As an example a French aeronautical equipment manufacturer has approved, agreed by its parent company in the USA, use of such a solvent in a tank fitted with a lid- just to lower evaporation and pollution of the workshop air. Better to comply with the MSDS requirements, as always. Further, due to its low flash point and its explosive limits (between 0.6% and 7% V/V) flashproof equipment shall be used.

This degreasing step may be a "cascading process". Soiled parts are immersed in a first tank of solvent (for a “rough” cleaning), then are immersed in a second tank (for “finish” cleaning). When the solvent in Nbr 1 tank is too heavy in pollutants it is disposed of in a suitable licensed facility for reclaiming. Solvent in tank Nbr 2 then is transferred to tank Nbr 1 after a thorough cleaning of the inside of the tank, and tank Nbr 2 is filled with brand-new solvent. And so on.

Ultrasonic cavitation may be used to improve the quality of the cleaning.

Nevertheless after using a low volatility solvent it could be a good idea to use a volatile solvent to remove all the traces of the solvent and pollutants from discontinuities.

Heptane in spray cans is marketed by some PT materials manufacturers to remove light pollutants.

But thinking of the wide range of pollutants and knowin"‘‘triple play" solvent - to go along the new generation’s vocabulary; the right wording is a ternary solvent! - may be the best answer.

Very often on the same surface, there are different pollutants: light or heavy oils and water (due to condensation for instance). Hydrocarbons, which are non-polar solvents, dissolve oils while a polar solvent will dissolve water.

Among various formulae the most often used are:
•  Petroleum distillates/n-heptane/methylcyclohexane.
• N-heptane/acetone/isopropyl alcohol.

This latter is probably the most efficient.

3.2- Corrosion inhibited alkaline cleaners

Sometimes referred to as "alkaline detergents", especially in French ("détergents alcalins"), they are another alternative to the halogenated solvents with one major advantage: they are not volatile organic chemicals. They do their job by saponification and detergence of oils and greases.

Alkaline degreasers are applied either by immersion of parts or by spraying.

After washing parts are rinsed with water; rinse water shall then go through a waste water treatment installation.

- 3.2.1- Immersion process

Alkaline cleaners, delivered as powders or liquid concentrates, are generally dissolved at a rate of 10-20% in hot water (60-75°C/140-167°F). Parts need a water rinse after degreasing.

Some low-temperature cleaners are marketed but they are less efficient. It is more a kind of “à-la-mode” commercial case about lowering energy consumption.

An alkaline cleaner must comprise corrosion inhibitors but almost NO SILICATES as these latter chemicals may form a layer on the surface when cold parts are immersed in hot water. This hard layer then prevents penetrant from entering discontinuities. Further, silicates clog open discontinuities, which are, then, not detected during a PT! It is an "established" classic! Add to this that, once made, the layer of silicates is almost impossible to dissolve again!

One of the major drawbacks of alkaline cleaners is parts drying after rinsing to remove any trace of water.

We may emphasize the ROLLS-ROYCE RPS 702 specification which requires an oven drying for 60 minutes at 120°C (ca 250°F) so as to get a surface temperature of 100°C (212°F) minimum. Making water evaporate from discontinuities is not an easy task, mainly due to the capillary pressure; water in a crack makes it difficult for a penetrant to enter, especially for water washable penetrants.

As a matter of course, such a drying must have no adverse effect on the parts.

- 3.2.2- Spraying process


This process combines the mechanical action of the spray with the chemical action of the cleaner which must low in foam.

› 3.2.2.1- Washing booth or tunnel

The overall cleaning process quality depends on a part on the quality of the washing step and on the quality of the rinsing step.

The part or the basket of parts is set on a rotating table.

Nozzles set up in suitable positions spray the water-based solution, rated at 10% for instance at a temperature in the range 50-80°C/122-176°F.

If some areas of the parts may retain some liquid, a good idea is to have an air-scissor- like stream to blow the drops.

Parts are then water-rinsed.

The final step is drying them in such a way that any trace of water is evaporated, this requiring an oven temperature ca 120 °C (250 °F) for 60 minutes.

› 3.2.2.2- High-pressure/hot water-jet washing machines


Why not use the corrosion-inhibited alkaline cleaner as a diluted additive in these machines? If the concentration at the spray lance nozzle is ca 0.5% there is no need to rinse the part … and parts dry by themselves quite quick fast.

This process may be used only on parts, which may withstand the mechanical action without any distortion.

4- Degreasing quality check

Several tests may be used to check the efficiency of the degreasing step:

• Visual inspection.

• The "white-rag test" for a quick check of standard degreasings.

• The water-break test: pour water on the tilted or vertical part. If the water film does not come as a continuous film and displays a break, this is an indication that the surface is not enough cleaned. This test is described in ASTM F 22-02 titled "Test Standard Method for Hydrophobic Surface Films by the Water-Break Test" approved in 2007.

An other test specifically designed to check that parts have been properly degreased before a PT has been marketed ca 1997 (*). It is the KC-QPON (KLEEN-CHECK® COUPON).

Supplied as a kit it comprises:
• 6 starburst panels (Qpons).
• 3 flasks of oils of different viscosities (one low viscosity, one medium, one high viscosity): Kleen-Check Soil oil nbr 1, nbr 2, nbr 3 respectively, each one made fluorescent by adding a small amount of a suitable fluorescent dye.
• 1 Material Safety Data Sheet (MSDS).

 

The 6 panels are made of stainless steel covered with a layer of hard-chromium on one face. Manufacturing is in fact very close to the PSM-5™’s (TAM panel). Then depending on the viscosity of the oils which are likely to be on the parts to be cleaned one selects one of the three Kleen-Check® Soil oils to match as far as possible the pollutant’s viscosity.

Then one panel is "polluted" with the right Kleen-Check® Soil oil and put with real parts in the degreasing machine. When the degreasing step is completed, the panel is checked in the UV-A inspection booth. If the discontinuity fluoresces, that means the pollutant has not been washed out of. One may assume that the open-to-the-surface discontinuities, if any, on the parts would be in the same situation.

Degreasing parameters need to be adjusted…and these parts need to be reprocessed.

5- Conclusion

By experience, we know replacing halogenated solvents as degreasers prior to PT has not been that easy. Non-halogenated solvents nevertheless are an acceptable answer.

Alkaline cleaners have several advantages: non-toxic, not even harmful. Further, they do not emit volatile organic compounds.
However, they do have several drawbacks. After an efficient water rinsing parts shall be thoroughly dried then cooled down to a temperature suitable to penetrant application. Waste water treatment is mandatory. An alkaline degreaser line needs many square meters (or feet!), requires a lot of energy, of chemicals, is a source of impressive volumes of waste, requires a check of the process quality much more time-consuming than vapor degreasing’s. That is a main reason why some manufacturers try to duplicate the vapor degreasing process with a similar process using hydrocarbons, or some exotic solvents. However, depending on soils to be removed results are sometimes dubious.

Another problem comes from areas where the liquid is retained: how the parts are set in the line is of the utmost importance, and sometimes they even have to rotate so that the user is sure every bit of surface has been degreased and rinsed and that no, or very few product remains in grooves, recessed surfaces, dead-end holes, blind passages, etc.

References


(*) Pierre CHEMIN and Patrick DUBOSC: "Penetrant testing history" on our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info/site/en/news/pt-texts/179-historique-du-ressuage

• ASTM F 22-02, "Test Standard Method for Hydrophobic Surface Films by the Water-Break Test", ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2007.

• Rolls-Royce, RPS 702, Non-destructive Testing, Fluorescent Penetrant Inspection, Derby, United Kingdom, 1999.

Last Updated ( Saturday, 22 October 2011 14:02 )