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Clear anodize turning yellow after powder coating
July 2, 2015
Q. As an Inspector/QA in the machine shop industry for 20 years, I'm now coming across a new problem: We turn a cone-shaped part that is clear anodized, then the inside of the part is masked, then the outside of the part is powder coated. Just in the last few months, when the part has come back from the paint shop, the inside surface (which was masked from the powder coat process) has turned yellow, looking like a poor Chem-Film yellow process.
I have verified repeatedly the clear color before the paint process. Our anodize vendor has no idea how the anodize could turn color, saying that even being baked at close to 500 °F should not effect the anodize. The Paint vendor has heard "rumors" that the source of the purchased gas being used in his baking ovens may have an affect on anodized parts.
Has anyone else ever experienced, heard of, or has had the time/resources to research this problem?
Machine Shop Inspector - Oakdale, California USA
July 2, 2015
A. The (dis)color may come from the alloying ingredients of the aluminum alloy, or from impurities in the seal, or from aluminum sulfate left in the pores before sealing (which you can get out with a nitric dip before sealing), or if unsealed (need a seal test) from ingredients in the powder.
Robert H Probert Technical Services
Garner, North Carolina