×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

What would cause aluminum to peel when painted?

What would cause aluminum to peel when painted?

What would cause aluminum to peel when painted?

(OP)
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?

RE: What would cause aluminum to peel when painted?

Corrosion products?

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

RE: What would cause aluminum to peel when painted?

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.

RE: What would cause aluminum to peel when painted?

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.

RE: What would cause aluminum to peel when painted?

(OP)
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.

RE: What would cause aluminum to peel when painted?

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.

RE: What would cause aluminum to peel when painted?

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

RE: What would cause aluminum to peel when painted?

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

RE: What would cause aluminum to peel when painted?

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

RE: What would cause aluminum to peel when painted?

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.

RE: What would cause aluminum to peel when painted?

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources