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Penetrant Testing and painting: that's the question!

Written by Administrator
Saturday, 05 May 2012 11:38

In the '80s, in the evening, a phone call from a man who needs help!

He explains his incredible problem:
"I've used a colour contrast penetrant to check welds on an LPG reservoir; I've sand-blasted the area, then I applied the white paint coating as ordered. Almost immediately I've noticed pinkish-red rings appearing close to the undercuts of the weld beads. I waited for the paint drying, then, I applied a new layer of paint. The rings appeared again.
I've now applied 7 paint layers, and the rings are still visible! What can I do? My customer does not accept this visual effect!"

- "Well, Sir:
• Either you strip the paint layers, at least in the areas of interest, and you make sure all the penetrant traces disappear before painting again.
• Or you ask your customer to allow for a dark colour paint".
(This latter option would not be accepted: all the LPG reservoirs shall be painted in white, to lower temperature increase under sun).

This man was working in a large vessel manufacturer at a time when these manufacturers painted vessels by themselves.

Nowadays, due to strict rules, paint application is a job for specialists: primes specifications, safety rules when using products, corrosion-prevention guarantee, etc., they all make such a situation quite unlikely. Everything is recorded, products references, batch numbers, application equipment, meters calibration due dates/certificates, surface preparation, paint layer parameters: thickness, adhesion, pliability, gloss, coating's porosity, etc.
So coming back to our guy, what should he have done then?

• Immediately after Penetrant Testing inspection, he should have thoroughly cleaned the entire surface covered with penetrant, either using a solvent and rags or better using spray cans of non-aqueous wet developer (NAWD), in fact exactly the one used for developing penetrant indications. He should have sprayed quite a thick coating wetting the surface so that the solvent dries within 5/10 minutes, or more, instead of the short time specified for inspection. Doing so, the developer would have drawn out the traces of penetrant from the discontinuities which were not detected, or which were accepted as such and not repaired. In fact, he should have done that several times, exactly the same way as used for cleaning test panels.

• Another way was maybe to think a bit prior to the Penetrant Testing. If the vessel was made of carbon steel, if the prime's specification allowed for it, a Magnetic Particle Testing (MT) would have been less time consuming, easier, at least as efficient -- and would not have had rings appear after painting!

However, MT requires some equipment, yes. PT seemed easier.

Then, many people thought sand-blasting is enough to remove all the traces of penetrant - in fact, that sand-blasting is very efficient to clean surfaces! (Maybe we could meet people who still think so nowadays!). Sand-blasting removes surface contaminants (corrosion, etc.) and above all gives the surface the roughness needed to help for paint adhesion.

Another point to have in mind: paint and penetrants often are not compatible: better not to have penetrant in contact with paint, as this could have a very negative effect on the quality of the layer!
We have had cases where, in the same workshop, PT inspections were carried out using spray cans, while only twenty meters away, parts were painted. Aerosols (tiny droplets) of penetrant, which settled down on the part waiting for painting induced problems with paint adhesion. Of course, to find the solution to the problem (the quality of the paint, the professional expertise of the operator, the cleanliness of equipment were involved), a  curious man "wandering" in the workshops was needed to point out that, due to the ventilation, it was extremely likely that penetrant aerosols could be found in the area in which parts were stored for painting. Then, only one question was necessary, “Do you have any problem of adherence of paint on your parts?" Quite easy, then, for the penetrant materials supplier … to solve a head-scratching problem of painting quality.

Penetrants and paints are not friends!

In the '60s a UK Company had a motto: "For every surface treatment problem, there is a product xxxxxx" (trademark we do not display).
Engineers and Commercial people in this Company had made a "translation": "With every surface treatment product xxxxxx (trademark we do not display), there is a problem".

This anecdote is there only to remind everyone that problems met in workshops may be due to the suppliers/manufacturers as well as to the users.
Our idea in these documents is NOT to target anyone, but on the contrary to bring to your knowledge some interesting cases which may prevent you to duplicate the same mistakes while performing Penetrant Testing (PT) or Magnetic Testing (MT).

All the ministories you will read are TRUE. We think they will be helpful:
• First as examples of specific technical - or non-technical - requirements or peculiar problems.
• Second to let you see that the problems do not always come where you think they should come from.
• Third so that users feel free to ask for help from people (the experts) who may know more than they do.

Last Updated ( Saturday, 05 May 2012 11:56 )