Prepping Anodized Aluminum for good adhesion

A. Hi Ben!
I'm thinking about 2 things.
One is, residue from the stripper/thinner. Sometimes a second thinner or solvent is necessary to fully remove traces of the first. A vapor degreasing step after the initial cleaning may help.
The other one is, you mentioned that the surface had been pretreated. Do you know if it was conversion coated? That is very common for aluminum parts destined for paint, and creates a good surface for painting operations.

Q?/A? Hello Ben,

I am a Chemical Engineer in the surface finishing industry. I am currently troubleshooting paint adhesion failures our customer is having on their parts and it is similar to what you are experiencing. Did you have paint peeling off the part after receiving them from storage or did it only occur after trying to repair them? Was the part pretreated in any way, such as chemical conversion coated or anodized? What alloy of aluminum are the parts? Some alloys are prone to corrosion and if moisture was trapped within the material it may be corroding under the paint.

A. Clyde,

Generally speaking, chromic acid anodizing provides you with the best adhesion to organic coatings; other processed includes phosphoric acid anodizing and boric-sulfuric mixed acid anodizing. However, the adhesion is not dependent only on the anodizing process but also on both pretreatment and post-treatment. Post-treatment and handling procedures after anodizing are especially important to ensure good adhesion. Oil, solid, and water contamination of the anodic coating surface will definitely cause the paint peeling-off problem. Also, painting procedure and quality need to be concerned about.

Good luck.
Ling

A. I agree with Ling above but you also may want to consider trying an epoxy based primer and paint rather than a lacquer over an anodized surface as anodizing has an excellent adhesion quality; but as with lacquer, surface preparation is critical to making it stick and it seems to me that you're defeating your purpose to try and put a lacquer on it.

There must be an alternative to a clear lacquer finish.

I think this should be your next course of action.

A. My experience is that the largest single thing one can do to improve organic coating adhesion on anodized aluminum details, is to not seal it. Procure it from your supplier specified as non-sealed, handle after anodizing with clean cotton gloves and package with kraft paper. After receipt of the unsealed, paper wrapped, aluminum details, again handle them with clean cotton gloves and then prep and apply your organic coatings. Excellent adhesion will be the result.

A. We have had good experience sealing surfaces with a rapid melt & resolidification process. Sealing after anodizing may significantly reduce your susceptibility to contamination prior to organic coating. At the same time, we also remove organics and other contamination from the surface. This has led to superior adhesion for hard coatings on gears. Combination of sealing and cleaning may provide you with a robust and superior surface for subsequent coating.

A. You didn't say which phosphate you use? Iron or zinc?

I wonder if phosphating is your problem? Chromate would be a better pretreatment for aluminium.

A. Martin, phosphate doesn't effect aluminum, fluorides do. The chemical you're using should be a multi-metal formulation with fluorides to handle aluminum.
Also, phosphate is not a cleaner, if you want to use it as a cleaner you need a formulation that has a detergent. I don't know the formulation of the phosphate that you are using but find out from your vender. It needs to have the necesary detergent and fluoride additives that will allow you to properly pretreat this metal. As Geoff has already pointed out, if you only paint aluminum than chromate conversion is the way to go.
Also, your power washer is the bottom of the barrel system for doing what you are doing, so expect some variating results from your application.
Also, 150 °F is too hot for phosphate, try 110-120 °Fmax
Also, 55 min is too long to cure at 400 degrees, try 20-30 min. Good luck.

A. Hi, Sushil. Aluminum sheet should be chromate conversion coated (non-etch caustic cleaning, light etch, desmut, chromate conversion coat) before painting.

Regards,

SEM Adhesion Promoter

on eBay or

Amazon

(affil links)

A. The usual adhesion defect can be attributed to the protective strippable coating that nearly all sheet aluminium has to protect it during transit. When removed it sometimes leaves some plasticisers behind to inhibit adhesion of the following coat. It may require a solvent like MEK / methyl ethyl ketone a constituent of cell thinner. If this is not the problem then Ted is correct and you will require a adhesion promoter to be applied even after cleaning. A method used for at least 50 years would be to apply a single coat of two pack etch (wash) primer. The acid will bite into the aluminium and when dry provide an excellent base for any following coating system.

A. Hi Alok. We appended your inquiry to an existing thread which addresses the subject. Kimberly makes a good point, but I am not familiar with the rapid melt & resolidification process she describes. So, for now, I'd have to go with Ward's advice to not seal the parts.

Regards,