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WHITE POWDER COATING 5052-H32 ALUM

Hi,
My name is Julie I am a Material Manager for a metal stamping house. My question is that we run some parts out of 5052-h32 alum. and we have these parts powder coat painted a sky white color. We started having problems within the last 6 months I would say that orange marks are bleeding through the paint. We have tried 3 different painters have had the part tested at Twin City Testing to see if there was contamination in the paint there wasn't. we have stripped parts that were painted that had the orange marks on them and actually sanded the bare metal a little to make sure there was no contamination then cleaned part and repainted (even with a different manufacturer of paint of the same color) and we are still getting the orange marks pretty much in the same spot. What would this be caused from and could it be in the material? Has anyone else run across this problem? This is totally baffling us.

Julie Switala
materials manager - Brooklyn Park, MN, USA
2007

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Hi,
This is an awkward one to answer - if you could e-mail a photo to me it may help.
My first thought is that it is not the powder but a defect caused by the substrate - possibly an ink dye used by the mf i.e. for a logo or batch marking. This has permeated into the grain of the alum. Try getting hold of some MEK / methyl ethyl ketone or acetone [on eBay or Amazon]. Lightly soak some cotton wool and swab the surface with one of these solvents. If there is any dyes there they should show up on the cotton swab.

Terry Hickling
Birmingham, United Kingdom
2007

I agree with the last response. The most probably is that there are impurities in your base material. Try to neutralize the prep material surface (finishing with a solvent). Strip one of your parts before do it. If you have a small sample of your base material you could "try" to find (in a fast way)some impurities. Good luck

Jose Castellanos
- Minneapolis, MN, USA
2007

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You might try saturating your metal with Phenolphathalein which is the indicator for titrating chemicals. If you have any residual alkalines or base, it will show up purple or pinkish depending upon the strength of the base. It might tell you your substrate is not cleaned properly. Again, Phenolphathalein is a different color in acid and base. In acid it is colorless, in base it is a dark reddish purple.

Bob Utech
Benson, Minnesota
2007

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Julie, the marks are caused by grease pencil that the manufacturer uses to mark the aluminum during their processing. Sometimes the al. is hot when the pencil is applied, thats why the marker keeps coming to the top even after you sand and use solvents. You can't see it until you apply the paint and then cure it. It generally only shows in the white powders, repaint after repaint until it's not worth keeping. I always scrap the piece after the first run.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
2007

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One more note, al. manufacturers also use temp sticks that are different colors, the different colors melt at different temperatures and are definitely applied when the metal is hot.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
2007