Large parts: how to wash a fluorescent penetrant

January 2012

We have had an exchange of emails with a reader about a "very simple" question; obviously, a good question!

Question:

"When washing parts, some time ago, after using a water-washable (WW) Level 3 fluorescent penetrant, I wondered about the washing time.

Imagine I work on very large parts: first, I spray water on the entire surface, to ease its washability. When I spray water for the second time (to wash off the excess of penetrant), is there a risk that the surface-active agents (surfactants) contained in the penetrant make it possible to remove penetrant from discontinuities, leading to an overwash situation? When these surface agents (surfactants) are wet, I think they diffuse in the penetrant, a bit as an emulsifier does. What is your opinion?"

Answer:
For WW penetrants, the washing time (and many other parameters: the mechanical action of water, due to pressure, to flux, to temperature, to the spray pattern … we have already written papers on this topic (1) (2)) is a MAJOR FACTOR.

Nevertheless, between two penetrants classified as similar in sensitivity, manufactured by two different manufacturers, sometimes even from the same manufacturer, the washability parameter may significantly vary. Some WW penetrants have a gelification reaction with water, which "protects" the penetrant when inside a discontinuity, while making it quite easily water-washable from the surface. Other penetrants are very difficult to wash, to prevent a removal from any discontinuity; but this leads to huge water consumptions … and to a fluorescent background. We have also already published a paper that explains it (3).

A post-emulsifiable (PE) penetrant, IN PRINCIPLE, cannot be removed from discontinuities only by water. Nevertheless, one of us has a first-hand experience: a PE penetrant was almost completely washed off by water, when using the parameters set on the user’s penetrant process line. It was almost impossible to have any indication on the Type 1, ISO 3452-3 reference test blocks. You may imagine the result after application of the emulsifier (diluted at 12.5% in water, when the maximum rate at which this emulsifier was qualified was 10%!) and washing off with water!

If your penetrant is of the “gelification-type” (ask your supplier), a first spray of water, followed "some time later" by the washing off of the excess of penetrant is not a concern and is not counter-productive.
The "some time later" may be in the order of ten to fifteen minutes. Our experience with this kind of penetrant is excellent. Along the years, we have made a lot of lab-tests to check the washability parameters, to compare penetrants, to compare formulae. We have "modified"penetrants to see what happens if we add as few as 1% more surface agents in the penetrant, or if we modify the hydrophilic/lipophilic balance (HLB) of the surfactant system, for instance, going from a 15%/15% content of two surfactants (one being lipophilic, the other one, hydrophilic), to a 14.8%/ 15.2% balance. Indeed, a manufacturer, going from the same "basic formula," may "adjust" its penetrants washability within a wide range, from very difficult to wash to too easy to wash, just by a small adjustment of the HLB and of the content of surfactants.

Our reader sent us an e-mail, in which he thanked us and wrote: "it is not that simple to wash off very large parts within a very short time."
Yes, we agree … and the process performed by our reader is the right one, when used in conjunction with a "gelification-type" penetrant. The important point is to wash the entire surface the same way without any overwash.

As you see, a very "basic" question, but an important one, especially when an auditor may ask it during an audit! Better to know what to answer … and better to be able to cite a very reliable source of information!

References

(1) Patrick DUBOSC and Pierre CHEMIN Penetrant testing: water pressure in the rinsing/washing booths: a myth?, June 2010. On our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info/site/en/news/pt-texts/173-ressuage-pression-de-leau-dans-les-cabines-de-rincagelavage-un-mythe-

(2) Patrick DUBOSC and Pierre CHEMIN, Editorial – July 2010: 10th European Conference on Non Destructive Testing Moscow – June 2010, Search and find papers on our Website, Penetrant Testing: water pressure in the rinsing/washing booths: a myth? On our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info/site/en/edito/edito-2010/86-juillet-2010-ressuage-pression-de-leau-dans-les-cabines-de-rincagelavage-un-mythe-

(3) Pierre CHEMIN and Pierre CHEMIN, Waste-water treatment and penetrant testing, June 2011. On our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info/site/en/news/pt-texts/185-traitement-des-eaux-usees-et-ressuage

Normative reference

• ISO 3452-3:1998 Non-destructive testing - Penetrant testing - Part 3: Reference test blocks, International Organization for Standardization, Geneva, Switzerland, 1998.

Tribofinishing before MT inspection

Another of our readers asked us whether part deburring by tribofinishing (also called vibratory finishing) before MT could significantly affect the detection sensitivity of MT inspection.

Here are our comments:
"It is right that tribofinishing before MT is a topic which is not often written about.
In surface preparation before PT, it is not recommended, as said in our paper (*).

For MT, generally, rough or very rough surfaces may be submitted to a mechanical cleaning such as grit blasting, sand blasting, grinding, etc. and even machining. The mechanical cleaning does not need to be followed by a chemical etching.
As a consequence, it seems there is a priori no problem to perform MT after tribofinishing.

However, it is difficult to answer your question in a so strong way.
Indeed, only knowing the parts and complementary tests would allow for telling which magnetization technique and which current form would be the most suitable.

On the one hand, the tribofinishing may turn an open-to-surface crack into a sub-surface defect and, in that case, a rectified or DC current is generally the best choice.
On the other hand, MT allows for detecting very tiny and closed defects (such as forging lap). Then, an AC magnetization is often the best choice, thanks to greater mobility of the magnetic particles."

We would be happy to receive pieces of information for those of our readers who have some expertise in this field.

Thank you very much by advance.

Reference

(*) Pierre CHEMIN and Patrick DUBOSC, Some of the limits to industrial uses of PT, December 2010. On our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info/site/en/news/pt-texts/58-quelques-limites-du-champ-dapplications-industrielles-du-ressuage