September 2009

Water tolerance vs Water content: Water-washable penetrants

Many specifications require that, when used in tanks, fluorescent water-washable (WW) penetrants shall be tested, for instance once a month, for water content.

Along the years we have noticed that some laboratories gave the result not as the "water content" but as the "water tolerance".

The difference is very important: water content is the quantity of water in the penetrant, given as a percentage. A WW penetrant may contain 2% of water. Almost all the specifications allow up to 5% maximum. Nevertheless, better to have the lowest achievable figure!

Water tolerance is the quantity of water which renders a WW penetrant cloudy; this means there is then some kind of "separation" of the raw materials. The penetrant is then unable to meet its sensitivity requirements. This noticeable modification is one of the parameters that every operator shall look for when making the visual inspection of the penetrant in the tank: level, aspect, at the beginning of the shift.

A lot of WW penetrants have a water tolerance in the 9 to 15% range. But measuring the water tolerance of an in-use WW penetrant is not equivalent to measuring the water content. Stating that the sample under test has a tolerance of, say, 5% when the water tolerance of the same unused penetrant is 9% does not imply that the water content is 4%. There are other parameters which may lead to a wrong assertion.

So a water content measurement shall be carried out.

Is it useful to remind you that the ubiquitous ASME D95 method, very often the only one accepted by the American primes, IS NOT AT ALL THE ONE TO BE USED FOR MEASURING WATER CONTENT IN WW PENETRANTS?

Simply read the "SCOPE" of D95. This method is designed for "bituminous" petroleum products, i.e. very viscous ones, which do NOT contain surfactants. A penetrant is not a "bituminous" product and contains a huge quantity of surfactants. When using the distillation method as described n the D95 method, it could be that some surfactants come with the distilled water (steam distillation). Therefore a more important volume will be measured, the "water content" figure will be higher than the real one.

Further a method based on the Karl-Fischer reaction is more precise. This is ISO 12937 standard ''Petroleum products-Determination of water- Coulometric Karl-Fischer titration method'' issued January 2001. Derived colorimetric Karl-Fisher methods may also be used.

A laboratory can make 5 measurements on the same sample and average the results, while a distillation as per the D95 method takes more time, and generally is carried out only once per sample!

So:
- Specify in your own specifications that only a Karl-Fischer method shall be used.
- Argue with your primes to have this requirement written.
- Check the laboratory which does the test really measures the water content.

In June 2007 (*) Patrick DUBOSC has already drawn the attention of the NDT people to the importance of using a Karl-Fischer method and in no way the ASTM D95.

In spite of this paper some rumors (**) have recently been heard of: some auditors demand that the water content of WW penetrants and lipophilic emulsifiers be tested using ONLY the ASTM D95 method; these auditors tell, if our information is right, that the Karl-Fischer method has been designed many years ago and that it would not fit the needs for this analysis!

THIS IS COMPLETELY WRONG: BOTH WRITERS of this DPCNewsletter and of your preferred site http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info ARE CHEMISTS AND THEY STAY STUBBORN:
- The ASTM D95 scope in itself is unequivocal: this method cannot be used with penetrants or lipophilic emulsifiers.
- The Karl-Fischer method and the colorimetric derivatives have been used for decades for numerous analyses. It is a reliable, fast, reproducible method.

We do not know the auditors who "broadcast" these fake news.

We are really sorry to see that many aerospace primes involved in NADCAP are still allowing either the ASTM D95 or the Karl-Fischer method, when some of them FORBID the Karl-Fischer. Once again reading the ASTM D95 scope is sufficiently self-sustaining.

That's why we recommend that the ASTM D95 be deleted from the ASTM E-1417 as an acceptable method of measuring the water content of penetrants.

In the ISO 3452-2 bibliography only Karl-Fischer-based methods are listed.

(*) Patrick DUBOSC & Andy BAKEWELL: Water content of water-washable penetrants- NADCAP NDT-Non-Destructive Testing Newsletter- June 2007.

(**) Rumors-The August 2009 Penetrant Professor from Met-L-Chek.

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.