Alodine/Chem Film Defect 2

[Peter the Engineer]

Hello,

I recently received multiple parts with the same kind of defect after being treated with chem film and anodizing. I took a picture of it (please see below).

I am wondering what might have caused this "bleeding". This had also happened in the past, but the company doing the plating (chem film + anodizing) was never very clear on why this happened. Could anyone please offer a clue? The bleeding does not seem to cause any dimensional variation, but I am really bothered by the appearance of the part. I tried looking for explanations online but could not find anything.

Thank you.

 

[WKTaylor]

The oxide film was not removed... and/or the surface area was not cleaned to 'bare aluminum'.

An old mechanic explained this. "When applying chromate conversion coating [CCC, alodine] onto aluminum, if it beads-up and runs-off and doesn't form an adherent colored film... the surface simply isn't clean enough for the CCC to bond with the aluminum molecules.

Regards, Wil Taylor
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[IRstuff]

Oxide on aluminum grows pretty fast, too.

Any aluminum oxide that's been formed in high temperatures with oxygen can even resist chemical etches; we once has a part that required reactive ion etching to clear out contact vias on a chip

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[Peter the Engineer]

Thanks Everyone!

Etching the oxides could be a tricky balancing act between making sure all the oxides are gone and making sure dimensional change is kept at minimum.