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Alodine and Anodize paint adhesion problems

We are experiencing paint adhesion problems on Alodine 1200s and chromic acid anodize parts .032" 2024 T3 Clad. I am wondering if the henkel 6/16 deox process could be a factor? our current etch rate is .00015" per side/hour.

Steve Davis
Aerospace - Victoria B.C. Canada


How long are you immersing in the deoxidiser solution for? 0.00015" per surface per hour is at the bottom end of the etch rate but is not excessively low. Dependent on specification requirements I would suggest a deox time of 10-20 minutes. I know some specifications specify a much shorter time (up to 3 minutes, which has always seemed too short).

Have you checked that you don't have any problems with your painting process? All sorts of things can go wrong there, associated with paint mixing, application, curing etc.

Brian Terry
Aerospace - Yeovil, Somerset, UK

Our deox times are from 1 to 3 minutes. This is by customer spec. We are finding that the Alodine seems to look better without deox. We are doing some testing now to identify the best length for deox. The problem is that it takes about 7 days for the primer to cure before paint adhesion testing can be properly verified!

Steve Davis
- Sidney, BC, Canada

The problem with such a short etch time is that so little material is removed that the oxide layer is often not completely removed. At an etch rate of 0.00015"/surface/hr you are only removing 0.0000025-0.0000075" per surface. Leaving any oxide on the surface will give you a patchy coating at best. With specification requirements on time being so tight you might try upping the etch rate (either by the addition of Deox 16 if your spec allows it or hydrofluoric acid. Check how much you would raise the etch rate in the lab first, I suspect you have a maximum etch rate around 0.0004"/surface/hr and maybe aiming for the top end of the etch rate may help).

Another problem that we encountered a few years back was that the aluminium sheet manufacturers were coating the sheet with a material to reduce corrosion but this coating was not consequently removable by degreasing, alkali cleaning or deoxidising! Took us a long time to figure that one out. So it may be worth having a word with the sheet manufacturers, just in case.

Does the paint adhesion problem occur on both Alodine and Anodising or does it tend to be the Alodine?

Without knowing which primers you are using I can only make generalisations about the curing. If the primer is water reducible you will find that drying can be hampered by all sorts of conditions, not least atmospheric humidity. This is less so for the solvent based primers but atmospheric conditions do have an effect. Are you able to force cure at all? We have found that a level of force curing helps with adhesion. Some of the high adhesion primers stick to almost anything when force cured.

Brian Terry
Aerospace - Yeovil, Somerset, UK

Thanks Brian for your response. I think that we agree that this is a material issue as both Alodine and anodize are seeing patchy results. We have tried to deox for longer with little on no improvement. Was your solution to not purchase from that suppplier or did you find a processing solution?

Steve Davis
- Sidney, BC, Canada



We ended going back to the material supplier and asked them not to coat the material with the protective they were using, which they were happy to do.

If increasing the etch rate has not worked could you abrasively clean? I don't know if this is an option you are allowed to use but problem parts we have, on occasion, lightly abrasive blasted using a fine abrasive (180-220 mesh) or, if the problem is local we have used Scotchbrite. This has definitely helped and we have certainly improved the Alodine quality this way.#

One other thought, have you checked for contamination of the deoxidiser or the previous alkaline cleaner? Is there any possibility that there could be oil contamination on the surface of the baths? This would cause patchiness in both the Alodine and the Anodising.

Brian Terry
Aerospace - Yeovil, Somerset, UK

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