Penetrant testing

Non-aqueous wet developer: how to apply a thin, even layer from a spray can?

April 2011

Updated in June 2011

While PT has been widely used the world over for many decades, we always wonder why so few people really know how to apply the non-aqueous developer (NAWD) the right way.
As it is such a critical step, and of the utmost importance for a reliable PT inspection, this paper is intended to help every PT operator get the best results.

We recommend all training centres to teach their trainees about this technique of application.

Standards, specifications and procedures require that a thin, even layer of developer be applied on surfaces under inspection, whatever the developer form.

The non-aqueous wet developer (NAWD), mandatory when using a colour contrast penetrant, may also be used with a fluorescent penetrant. It may be called “the universal developer”, i.e. the one that may be used whatever the penetrant type.

A non-aqueous wet developer is always based on a volatile solvent containing very small white particles, with some additives in very low concentration to help for getting a layer without an “orange-skin aspect” (which would be then contrary to an even layer), other additives to ensure satisfactory developer layer adherence without making it sticky and to ease the final wipe-off of the developer after inspection, etc.

The “volatile solvent” shall be volatile within the temperature range for which the developer is designed. For instance, a solvent volatile enough between 10°C (50°F) and 50°C (122°F) is not suitable for an inspection in a -10°C (14°F) area. For low temperatures or high temperatures ranges, specific developers are used. Refer to our papers (1) (2).
A non-aqueous wet developer must be sprayed on the surface under inspection. No other technique of applying this developer form is allowed, though, as “veterans”, we have already seen it applied with a brush!

An NAWD in bulk may be applied using dry and oil-free compressed air and some guns used for paint; nevertheless, the most convenient way, by far, to spray an NAWD is to use a spray can.

The propellent must be a liquefied gas (3) (4) (5).

The technical point is then: how to get this mandatory thin, even layer of developer from such an item?

Keep in mind that:

• The NAWD after shaking is a suspension of particles in a liquid.
• The liquid is a solution of liquefied gas in the solvent.
• Pressure is produced by the gaseous phase of the propellent.

Then, the way of doing becomes almost obvious:

- 1 - Thoroughly shake the can for at least 30 seconds before use.

- 2 - Target an area OUTSIDE of the area that must be sprayed. During the first second of spray, the product coming out of the spray can is not homogeneous. In fact, it is the liquid contained in the dip tube of the spray can which goes out first. If this is sprayed on the surface, it will give a poor result.

- 3 - Then target the area of interest, keeping a distance in the 20/30 cm (8 to12”) range between the nozzle and the surface.

- 4 - Firmly push down the push-button. Even if a thin layer is to be applied, the spraying will be right ONLY IF the flow from the can is not disturbed by any obstacle, such as a partially opened valve.

- 5 - Keeping the distance, a smooth, regular movement of the hand will allow for the right layer. In fact, it is better not to apply enough developer by the first spray. A second spray may be needed to adjust thickness. As we say in French: “it is always possible to add a bit more salt to a soup; but if there is too much salt in it, it is impossible to remove the excess”.

- 6 - When a thin uniform layer is achieved, one should still be able to see a metallic background of the part through the thin film of NAWD.  If the part is "painted white", you have applied too much NAWD and the part needs to be re-processed or small cracks may be missed by masking with too much developer.  It seems that this opinion is not widely shared, especially in the USA.
Indeed, some people think that it is applicable to fluorescent penetrant inspection and not to colour contrast penetrant inspection. Some developers contain titanium dioxide (TiO2) specifically to hide the surface. Obviously, titanium dioxide is excellent to get a white layer. Nevertheless, in France, in nuclear industry, if one cannot "perceive" the surface through the layer ... the layer is too thick!
In this context, French Electricity Board (EDF) makes it clear that even when using colour contrast penetrants (they use only them), the metallic surface shall be "guessed" through the developer layer.
One of our future papers is about titanium dioxide.

Spraying for a long time in a row may have adverse consequences:
• The can will cool down, due to the evaporation of some liquefied gas to compensate for the increased gaseous volume in the can as the total quantity of product is dwindling in it.
• The particles could settle down, the sprayed developer being then unable to give the right layer.

It is then a good idea to spray for, say, one minute maximum, then to shake again the spray can, and follow steps 1 to 5 as above.

The quality of the layer is vastly dependent on the quality of the developer, on the quality of the spray can, but also on the know-how of spraying by the operator. This last point is got only by experience: no standard, no specification, no procedure, no software may be of any help!

Reference

(1) Pierre CHEMIN and Patrick DUBOSC, Penetrant Testing industrial uses, November 2010, on our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info

(2) 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

(3) Patrick DUBOSC and Pierre CHEMIN, Propellent for PT: MT spray cans, March and April 2009, on our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info

(4) Pierre CHEMIN and Patrick DUBOSC, Colour contrast Penetrant Testing: old times young generations will never know! Oldies but goodies, February 2010, on our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info

(5) Pierre CHEMIN and Patrick DUBOSC, A deceitful good idea! Oldies but goodies, April 2011, on our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info

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.