Oil-free penetrants are a very specific "species" of penetrants, comprising in fact
two quite different "subspecies". Almost every penetrant materials manufacturer has
them in its range. To the best of our knowledge these are not the core of their sales!
The two "subspecies" are:
Organic solvents- free penetrants. Note that some other penetrants of this subspecies
are almost organic solvents-free since their formula comprises liquid dyes (dyes
delivered diluted in organic solvents).
Water-based penetrants.
I-SOLVENTS-FREE ORGANIC PENETRANTS
Ca 1975(*) an American Company almost unknown by the NDT world designed and patented
so-called "biodegradable penetrants". Some competitors try to duplicate them, but
the patent was so well locked that they failed--for some time.
Note that the Type 1, Method A, sensitivity Levels 1 to 4 penetrants from this range
are listed in the SAE-AMS 2644 QPL.
The main difference with the "classic penetrants": organic solvents-free, their formula
comprises fluorescent dyes dissolved in a blend of non-ionic surfactants (surface-active
agents).
The different sensitivity levels are achieved using different concentrations of dyes
and different blend of surfactants.
One of the main aims was to ease waste water treatment. Even in these "old times"
Californian regulations were tight.
The trouble is that these penetrants are not, by far, as "biodegradable" as they
are claimed. In fact their BOD (biological oxygen demand) and COD (chemical oxygen
demand) are so massive that they could unbalance waste water treatment installations!
Yes they can be "destroyed" by bacteria faster than their hydrocarbons-based counterparts.
But this "advantage" requires far more oxygen in a shorter time, hence the imbalance!
Nevertheless, due to their formula, these penetrants are easy to wash and show an
outstanding overwashing resistance.
But a drawback for users of tanks of penetrants is their high viscosity: dragout
by processed parts and equipment (baskets, hooks, etc.) may be impressive. Consumption
is higher; there is more penetrant to wash from the surface...and more penetrant
to retain in the waste water treatment installation! Add this to a higher materials
price, due to the cost of the raw materials....and it is quite easy to understand
why users have been reluctant to switch to these penetrants!
II-WATER BASED PENETRANTS
The first water-based penetrant was designed in the '60s: NASA was looking for a
means to check liquid oxygen circuits for leaks on rockets, among them the Saturn
5--the largest, tallest, heaviest rocket ever manufactured.
Some users ask the penetrants manufacturers/suppliers to supply them with certificates
stating their materials are compatible with liquid oxygen. This cannot be done as
the suppliers have no influence on the way their products are used. Liquid oxygen
is very dangerous when coming in contact with any organic material (a mere fingerprint!).
Even water-based penetrants have some organic (i.e. carbon-based) materials in their
formulae : dyes, surface-active agents.
If "classic penetrants", those oil-based, are used for inspection of parts designed
for out-of-Earth space, a slow degassing of hydrocarbons may occur, polluting telescopes,
for instance. After a penetrant inspection with water-based penetrant, it is easier
(not easy, easier) to have water evaporate before assembling inspected parts.
Water-based penetrants may be cheaper than their oil-based equivalents. Some penetrants
are supplied as concentrates which may be diluted up to 50/50 in water. Further less
oil means less activated carbon needed for waste water treatment: the same filter
may have a longer life with water-based penetrants.
Nevertheless it is not easy to forecast any cut in activated carbon consumption.
Several parameters interact: carbon form (powder, granules, etc), what it is made
from (cocoanuts, coal, etc), how it is activated (chemically-activated or steam-activated),
the ability to retain this or that hydrocarbon, other chemicals which may affect
the way hydrocarbons are "attracted" by the activated-carbon, flux speed and time
of treatment, and probably some effect due to the surfactants. Keep in mind that
less hydrocarbons equals more surfactants! Therefore a consumption drop of 20% only
may be seen, and luckier users may get 50%, even 60%! Only real tests performed with
the real waste water could tell. And quite often it's even more complex: imagine
how many waste water treatment installations are connected to several penetrant lines.
For instance, one water-washable (WW) Level 2, one post-emulsifiable (PE) Level 3
and one PE Level 4. You may understand that along the days, the weeks, the mix in
the waste water tank will greatly vary. Some days, 80% of the parts will go through
the WW Level 2, while the following day, maybe 80% of the parts will be PE processed!
Water-based penetrants are mainly used for:
Leak testing.
Light alloy foundries.
Ceramic parts (hip replacement prosthesis, other prosthesis, valves for high-performance
engines, etc).
Parts made of thermoplastic and thermosetting plastics (after long-term compatibility
tests have been satisfactorily completed).
Some composite parts (same compatibility concerns).
Porosity of organic coatings: polytetrafluoroethylene (PTFE) for instance.
Porosity of metallic coatings: electroless nickel (EN) plating, for instance.
Etc.
Some water-based penetrants are designed as colour contrast penetrants. As a colour
contrast penetrant must have a far higher dye content than a fluorescent one, the
dye must be easily dissolved in water. Depending on raw materials, dye concentration
and other parameters, water-based colour contrast penetrants may appear as pinky-red
to carmine red under white light, and give a more or less pink fluorescence under
UV-A radiation.
They are in fact dual-purpose (Type III as per ISO 3452 standard) penetrants. Keep
in mind that NO DUAL PURPOSE PENETRANT IS APPROVED AS A FLUORESCENT PENETRANT as
per the SAE-AMS 2644 QPL.
Rhodamine is THE dye, and if there is more than 1% in the penetrant, it shall be
labelled as:
Xn = Harmful.
R40: Limited evidence of a carcinogenic effect.
Up to now when speaking of water-based penetrants it is assumed they are water-washable.
Do PE water-based penetrants exist?
Your answer: "I've never heard of".
"Probably because none exists".
You're right.
Would it be a good idea for a manufacturer to design such a new penetrant? Though
the "average user" may answer "yes", we see this as a "think twice" case.
Some technical explanations may help.
The current, well-tried PE system is based on a principle of non-miscibility between
the PE penetrant and water. The hydrophilic emulsifier moves the goalposts a bit.
Even highly diluted in water the emulsifier may more or less quickly mix with the
penetrant.
If we put the system topdown, that means the PE water-based penetrant should come
in contact with an emulsifier containing...no water at all! Well that means the emulsifier
would be hydrocarbons- and surfactants-based...and used as delivered. Reminds you
of something?? Sure?? The lipophilic emulsifier, of course!!
This emulsifier would probably be used in a tank, parts immersed. By the way, as
the remaining penetrant on the parts is water-based...the emulsifier would take more,
and more, penetrant...but also water. Well, we have to wonder about the technical
advantages, and as far as costs are involved...no way in sight!
III-WATER BASED PENETRANTS SENSITIVITY
Some water-based fluorescent penetrants are SAE-AMS 2644 QP listed. Level 1/2, 1
and 2 approved penetrants may then be used in accordance with this specification.
At the time of writing this paper (Jan '09) no Level 3 is listed. As a general matter
WW Level 4 penetrants are rarely used, and it is likely that no penetrants manufacturer
will spend time and money to try to design a water-based one.
Nevertheless in the first years of the 2000 decade one manufacturer pointed out two
targets:
Removing from formula every potentially harmful substances such as suspected carcinogens,
neurotoxins as well as endocrine disruptors.
Designing a complete range of water-based fluorescent penetrants, from Level 1/2
up to Level 4.
The first target was successful.
The second one was far too difficult to reach. Only Level 1/2 and Level 1 penetrants
were listed.
Furthermore people who thought that a complete range of water-based fluorescent penetrants
would be a tremendous leap forward just had not the feedback from users!
Many users of WW Level 2 fluorescent penetrants, at least in the aircraft industry,
use electrostatic spraying to apply the penetrant. Replacing oil-basis by water makes
it mandatory... to buy a brand-new equipment, as technical and safety requirements
differences are so huge that the same equipment cannot be used for both. Forget it!
IV-CONCLUSION
Oil-free penetrants show some advantages, and some drawbacks, when compared to oil-based
penetrants. Water-washable, water-based penetrants, in our opinion, are not close
to replacing "classic penetrants" on a large scale, at least in the aircraft/aerospace
industries.
Quality people in these industries prefer to rely upon tried and tested products.
Water in penetrants means higher risks of corrosion on some materials; penetrants
supplied as concentrates shall be diluted in water by users. Water evaporates from
tanks far quicker than hydrocarbons, hence risks of quick modifications of concentrations
even in a one-day span of time, and non-controlled change in sensitivity, washability,
overwashing resistance.
We have seen in France that approval for water-based fluorescent penetrants use is
always on a case-by-case basis.
On the other hand these penetrants have got an impressive part of the car industry
for inspection of aluminium castings, either in Germany or in France, and probably
in other European countries.
(*) Please refer to the tabular form of Penetrant Testing History by Pierre CHEMIN
and Patrick DUBOSC.
(**) Jean VAERMAN, “Fluorescent Penetrant Inspection Process. Automatic Method for
Sensitivity Process”, 11th World Conference on NDT Las Vegas, November 1985.
(**) Jean VAERMAN, “Fluorescent Penetrant Inspection, Quantified Evolution of the
Sensitivity Versus Process Deviations”, Proceedings of the 4th European Conference
on Non- Destructive Testing, September 1987, Pergamon Press, Maxwell House, Fairview
Park, Elmsford, New York, Volume 4, pp. 2814-2823.
