What would cause aluminum to peel when painted?

[deayer]

We sent out some 6063-T5 aluminum parts to be painted; the vendor called stating that aluminum (not paint) was separating and peeling back after 12 minutes @ 450 degrees. Does anyone know what would cause this?

 

[CoryPad]

Corrosion products?

Subsurface irregularities (laps, folds, seams, fins, etc.)

 

[ornerynorsk]

This is an extrusion, right? I have seen this once before when the extruder did not cull out the initial footage from each bloom that contains a lot of contaminants. Each "push" through the die will have loads of mica, graphite, and other crap in the first 5 to 8 feet or so, which makes for some nasty metallurgy.

 

[SMF1964]

You picture also looks like the tearing you get when you extrude too fast?

Is the painting process a powder coat or a liquid that is then heat cured? If it's a liquid, maybe you're getting liquid into these tears and the heating process causes them to split further?

I would have a sample sectioned to look at these peals/laps/folds/etc under a metallographic microscope (any metallurgical laboratory could do this) to identify what exactly is present there.

 

[deayer]

Ornerynorsk: Correct, this is an extrusion. I do not believe that this is the beginning of a run. We had sent several 20' sections to our vendor to be painted and the majority of the pieces peeled, except 5-6 pcs.

SMF1964: The paint is a liquid and then baked at 450 degrees. Unfortunately the material only lasted 12 minutes before it started to separate/peel. I requested a sample of the material for further analysis. I should receive it tomorrow. I thought that maybe there was oil or some other kind of impurity in the billet that may have caused this, not sure if there is anyway to tell. I've asked our QA to inspect the rest of the material from the lot for surface imperfections. I have never heard of aluminum extrusions separating and peeling before.

Thank you for your responses.

 

[Ron]

You're right to look at surface contaminants. Another thing to consider is that the coating facility did not properly prepare or pre-treat the aluminum. Usually a conversion coating is required, such as a phosphate wash or other pre-treatment.

 

[swall]

You don't have a paint problem; you have an aluminum problem. I'm with SMF1964--looks like speed cracks in the extrusion.

 

[limey1]

Apart from cracks caused by extruding too fast, could you be overheating the billet/container?

 

[DesignerGuy16]

Normally you'd see that type of speed "chatter" at the edges where you're pushing less material through. I designed aluminum extrusion dies for a time, and it doesn't feel like that's the issue here.

I'd be more inclined to say material imperfections, or corrosion from the paint are your leading cause. Is the peeling localized in a certain area? How is the extrusion oriented during the paint baking? Is the paint pooling in the areas it's peeling the base material away? Just curious.

James Spisich
Design Engineer, CSWP

 

[swall]

I see big tears in the aluminum substrate. Not a paint issue.Too short a time to be a corrosion issue from what I took from the original post.

 

[saberblue]

I have seen this type of problem caused by the die lube -- silicon anti foam or reacted fats that were not cleaned off before painting etc.


A.R. "Andy" Nelson
Engineering Consultant
anelson@arnengineering.com