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Lack of adherence on aluminum

2004

I work for an aircraft maintenance facility and we are certifying our painters, the problem arose with our test panels. We apply wash-primer, epoxy primer and polyurethane coating on 2024-T3 aluminum and after the curing time we perform thickness and adherence test. When doing the cross tape test we have trouble with the adhesion but the weird thing is the lack of adherence is on the wash-primer (when the test is done on the part of the panel with the whole paint system) but if we do the same test on the primer (wash-primer plus primer) surface we get a pass with flying colors... our thought is the topcoat is somehow reacting with the primer which in turn does something to diminish the adhesion of the wash-primer... any thoughts? We are going to vary some parameters, namely the cure time and surface preparation between the coatings to try to figure out what's going on, but at the moment all ideas are welcome.

Thank you!

Pedro Martinho
aircraft maintenance - Lisboa, Portugal

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2004

Dear Sirs,

I have not come across this problem but would like to start the ball rolling with some observations.

Wash (etch) primer: Firstly you should be investigating an alternative as this type of product is now considered a hazard to the H & S of the applicator due to the chromate content. If you have been informed that it contains no chromate then it is not a true wash primer. With that out of the way, two pack (the best)wash primer should be mixed and left for 15 mins prior to use to enable a chemical reaction to commence. At the end of the day throw away any unused material.

Always try to apply a thin continuous wet film.

Check that the substrate is clean. We had poor adhesion on one occasion and discovered that this was due to plasticiser left behind from the strippable coating. This is sometimes applied by the aluminium supplier to protect the alloy surface from damage. Clean with MEK / methyl ethyl ketone.

A few years ago I read that R & D work on aircraft painting had discovered that instead of the zinc chromate anti corrosive pigment, better adhesion was achieved by substituting, part or all, with strontium chromate - but again an H & S issue.

The way you describe the problem it would appear that the intercoat adhesion between the coating and the substrate is stronger than the intercoat adhesion of the wash primer/aluminium. So make sure you are using a two pack wash primer, leaving a 15 min gap between mixing and application. Check what experience your paint supplier has on strontium - check the substrate is clean. Better still evaluate waterborne - it will be safer for the environment and the applicator.

Terry Hickling
Birmingham, United Kingdom

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2004

We use a two component wash primer, zinc chromate and the induction time is respected. In aerospace there is nothing with the same corrosion resistance so we are forced to use zinc dust ⇦ this on eBay or Amazon [affil links] ers for the time being.

Next week I'll have some answers from the tests we've been doing. The failure is in the wash-primer itself because we get wash primer on the substrate and on the tape... so substrate/wash-primer, wash-primer/primer and primer/topcoat adhesion seems to be ok but it fails on the thin wash primer coat (6 to 10 micrometers), and it has happened to almost all our test samples so it's not a one off occurrence.

Thank you for your answer in any case.

Pedro de Oliveira Martinho
Aircraft maintenance - Lisboa, Portugal

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2004

Dear Pedro,

The original (still the best)wash primers consisted of 10% resin (PVB) 10% Zinc chromate 2% Talc; the rest made up with Meths 74OP & small amount of Toluol. The reaction that takes place when the catalyst (phosphoric acid dispersed in meths) is very complex. A good reaction requires the small amount of water that is in the Methanol also the phosphoric acid reacts both with the substrate (etches it) and the chromate part turning it into phosphates.

Try
a) normal ratio of wash/catalyst
b) 25% less catalyst
c) 25% more catalyst

Many modern wash primers substitute butanol for the meths (flash point reasons) but they are not so good as these lack the required water content.

Keep me informed.

Terry Hickling
Birmingham, United Kingdom

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We have the results from some test panels, this time there was no failure in the wash primer, we are now thinking it was a problem with the application technique, apparently our painters were worried with thickness measurements so they applied a small coat and then, after measuring it, a mist coat to get up to the right thickness, this happened with the primer but it might have happened with the wash primer as well which could explain the failure... in any case the test panels are now ok so we'll only pursue the subject further when the workload lessens (right now things are a bit busy).

Thank you for your reply. When we resume testing I'll give your idea a try, just to get some info.

Pedro Martinho
- Lisboa, Portugal
2004

Q. What is the composition of wash primer used on exposed metal in paint shop. Please specify all kinds of it

Sudheer kumar
- Lucknow, Uttar Pradesh, India
July 4, 2012