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Need cosmetic finish with type 3 anodizing: What aluminum alloy/temper?

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Q. My name is Brian Hoskins and I'm a mechanical engineer for a R&D company that produces advanced sensor technologies for homeland security, force protection and commercial applications. One of our products uses a multi-piece Al enclosure. We are having the common problem of spotting and streaking caused by what I read is an acceptable defect in the Al billet used. Due to technical issue we cannot get away from anodizing these parts, but we need them to look good. So the question is what Al alloy and temper will give the most consistent cosmetic finish using type 3 anodizing. I have heard rumors about spec'ing an ultra fine grain Al but no one has said how to do that. We are currently using 6061 T6. During my search 6063 has come up a lot, but no temper is ever mentioned. The posted picture has been anodized and striped back down.

water spots on aluminum
Brian Hoskins
Mechanical Engineer - Stillwater, Oklahoma
July 29, 2010

A. 6061 is not a bad choice at all. The stripping process (most likely a caustic etch) caused the defect as seen in your picture. If you have to strip your parts, I would suggest re-finishing them too, in order to prevent the above defect from showing. Consistency can be helped by a couple of things.

#1: Use Alcoa aluminum, in my 24 years of experience, they produce the most consistent product in the USA. Manufacturers like Kaiser and the like tend to have widely varying grain structures from lot to lot, which may have an effect on the end appearance.

#2. Your anodizer has to have his act together. Fluctuations in process parameters (current density, tank chemistries, tank temperatures) will have an effect on the color/consistency of the end product.

Marc Green
Marc Green
anodizer - Boise, Idaho

A. Just be sure the 6061 is NOT cold drawn. Any other spotting and streaking is something wrong in the process line.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

A. The picture makes me think of fingerprints that did not get cleaned off in the prep steps.
6061 is a common alloy that normally has few problems compared to 2XXX and 7XXX alloys. 1XXX normally has the least problems, but lacks desirable properties of the high alloy material.

James Watts
- Navarre, Florida

A. I'm gonna agree with Marc on this one. If you strip a part in etch then you enhance your problem by eating away at your material. The more surface you remove the more pattern you see.

Jennifer Curry
- FWB, Florida, USA

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