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Pre-cleaning for anodizing left white marks on etched 6063-T5 extrusions
Q. I sent 41 extrusions to my anodizer. All from a new lot of 6063 T5. 14 came back with white snow flake like marks all over them made in the cleaning process.
The Anodizer says it's a problem with the base metal. The other 27 were anodized black and look fine. Any ideas as to what caused this?
Joe Stantonbuyer - Waldoboro, Maine, USA
August 8, 2012
A. There is a long list of causes of white spots, too many to list here. Come back with more data.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
August 8, 2012
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Ed. note: Mr. Probert's reply predated the posting of the picture.
Q. Mr Probert,
Thanks for your quick response. I need to get to the bottom of this. I am currently throwing away a couple hundred dollars worth of materials and have no way of knowing if this will happen again. The returned parts remind me of the bubbles on the inside of a seltzer glass. Why 14 out of 41? The anodizer suggests a problem with the base metal. Do I face this for the remaining 156 pieces yet to anodize? Should I ask my anodizer for the composition of his tank? Do I need to go back to the extruder and ask them a question?
- Waldoboro, Maine, USA
August 12, 2012
A. Joe
If your processor is running 14 parts at a time, I would question the process. Consider making some witness coupons to be co-processed. Make some from the spotted parts as well as some of your drops/rems. Refinish both so they look the same raw. Serialize and have one of each run with your parts next time. While not conclusive as you do not know the quality of the drops, you might glean something from the results.
Also verify all of your extrusions are from the same lot.
- Colorado Springs, Colorado
First of two simultaneous responses -- August 13, 2012
A. We cannot tell exactly from the picture. Let us know if the spots wipe off and leave a clean plain anodic film or is there a pit underneath the white/bare spot. It may be that the heat treatment has separated the alloying ingredients. Recently when I saw this, we could strip, grind and re-anodize and spots came back in a different place. When the alloy is still good; "white/bare" spots can be caused by galvanic current in the dye tank (looks like you are not dyeing), floating soil, an anodizing solution with too high aluminum sulphate (old solution with dissolved aluminum that stays in the pores and cooks out when it hits the hot seal), chloride contamination, on and on, but this would be on all parts, not just 14.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
Second of two simultaneous responses -- August 13, 2012
A. Hi.
My opinion, check your process. Firstly check your degresing bath (may be too long duration for those parts) or check your acid dip bath (may be too long duration for those parts) or acidic-alkaline solution can be drop bus bar. But I don't think it's a base material problem because (also you say, too) why not all parts problem? 80% possibility it related to process. One more idea, you can strip it so if spots disappears than we can say related to process; if not, then related to base material.
Sincerely,
- Sakarya, Turkey
August 17, 2012
Thank you all for responding. I don't have any input to the process so I'll have to take the word of the people that do this work for me that it won't happen again.
- Augusta, Maine, USA
August 20, 2012
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