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March-May 2015 : Impurities in PT and MT materials: where do they really come from?

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Written by Dubosc
Thursday, 19 February 2015 11:40

March 2015

Penetrant Testing and Magnetic Particle Testing materials are made of industrial-grade ingredients. There would be no technical advantage to using analytical-grade substances, the costs of which would be exorbitant and unjustified. Therefore, some impurities are to be found in the raw materials and in the finished products. However, are you sure that this is the sole source of impurities?

What are the impurities of concern in PT and MT materials? Mainly those, which could impair the inspection, by reacting with some of the chemicals in the formulae. Further, and this is the main concern, the impurities that could lead to corrosion of the parts under test.

In fact, regarding the impurities that could react with some of the ingredients in the formulae, it is up to the designer/manufacturer of the products to choose the right sources of raw materials, those that can reliably provide a batch-to-batch consistent quality.

Thus, by far, the main concern is due to impurities that could be a natural component of some raw materials, but that could corrode some materials. Halogens and sulphur are the major culprits. Add, sometimes sodium.(1)

All the renowned PT and MT materials manufacturers comply with all the international standards and companies specifications requirements, as for the maximum allowed content of impurities such as fluorine, chlorine, sulphur, and, for some applications, bromine. They comply, as far as it is...possible!!! We have already seen requirements of...25 ppm of halogens MAXIMUM (this is almost impossible to get) while the relevant method of analysis did not allow for the measuring so small traces of impurities, as it was is clearly written in the scope chapter of the analysis method! In the mid-'90s, the ASTM methods, as specified in the ASME Code, for the chlorine and sulphur dosage, were commonly required: the minimum chlorine figure that the ASTM D808-87 analysis method was able to detect was 0.1 %, i.e. 1,000 ppm, when we had been already required to supply certificates of analysis stating less than 25 ppm after this method!

Note that this procedure assumes that compounds containing halogens other than chlorine will not be present.

Another point to consider is whether the entire analytical process complies with the process requirements of the applicable analysis procedure. The specific case we have in mind: a manufacturer of PT materials supplied a non-aqueous wet developer with certificates certifying a content of fluorine + chlorine less than 100 ppm, after the XXX specification of a prime, which required less than 200 ppm. Well, for users of the developer, everything was going well.

The only, tiny, so tiny, so minuscule, problem, was that this developer was T-111-based (its non aqueous, volatile base was 1,1,1-trichloroethane; yes, this chemical was still then allowed, because it was only on Jan 1, 1989 that the Montreal Protocol was implemented, which phased out production and use of several substances believed to be the cause for the stratospheric ozone layer depletion, among them, T-111. Obviously, the developer could not contain only 100 ppm of chlorine. In fact, a quick, easy calculation showed that the developer contained...65 % (650,000 ppm!) of chlorine. How could the manufacturer get 100 ppm only?

In fact, he had not followed the sampling procedure: he let the solvent evaporate for an hour at its boiling temperature, then, performed the analysis as per the method established by the prime. Indeed, the 100 ppm figure was that obtained on the dry extract...not the entire product, as required in the sampling process of this same method.

There is another source of “pollution” of the PT and MT materials. This source is never mentioned, though it could lead to local contents of chlorine far above the maximum allowed value by the applicable specifications.

When weather is hot, or when somebody works hard, in tough conditions, there is a natural reaction of the body to cool down: sweat.

What does sweat contain in large quantity? Sodium chloride...salt, as it is usually called.

Drops of sweat may fall down on the surface under inspection, leading to local...corrosive concentration of chloride ion. The same reaction may come from sweat on the fingers, when taking a part to move it, or from sweat on the hand palm, when somebody uses the part as a support to stand up. The concentration of chloride may be high, and the ions may stay on the part for a long time if, for instance, the hand palm has been applied after inspection. If corrosion occurs, the chance is that the designated “culprit” will be one of the PT materials used for inspection, and not the sweat produced by the inspector.

This topic has already been talked about many times many years ago by one of the author, and the Penetrant Professor, June 2013 issue(2), from Met-L-Chek, gives a self-explaining conclusion to our paper:

“Elemental requirements keep getting tighter and tighter but the inspection materials have a 1000 times less of these elements than the inspectors finger prints. Where are the dirty hand controls?”

Further, what to think about operators and inspectors who take their snack without washing hands afterwards?

 

References

(1)Pierre CHEMIN and Patrick DUBOSC, The specifications which changed the penetrant materials, August 2008 (document completed and updated in April 2012).

(2)William E. MOOZ: Pesky Penetrant  Contaminants, The June 2013 issue Penetrant Professor from MET-L-CHEK®.

 

Normative references

ASTM D808-87 Standard Test Method for chlorine in new and used petroleum products (bomb method). ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 1987.
Supersedes by the ASTM D808-11 Standard Test Method for chlorine in new and used petroleum products (high pressure decomposition device method). ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2011.

Last Updated ( Thursday, 19 February 2015 11:52 )