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How can I keep my lapped aluminum shiny?

I had some 6061 aluminum disks lapped to 1.0mm thick ± 10um TTV. They wre dull gray and then turned black in color. I took them to a metal finisher who put them in Brite-dip. They cam out looking like aluminum metal, but within a day had turned white. What can I do to keep them from turning color?

Dave Benzing
product designer - San Jose, California, USA
August 10, 2011

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August 2011

Hi, Dave.

Aluminum is a very active metal, so bare aluminum will quickly darken. There are three alternatives that I can think of:

- You can clear coat the aluminum; but many clear coats have significant thickness, probably exceeding or at least compromising your TTV. But you could probably consider single-part clear coats like Everbrite [a finishing.com supporting advertiser].
- You can anodize the aluminum, but this adds significant thickness -- probably a minimum of about 5x your TTV.
- You can have a clear chromate conversion coating applied to the aluminum; this would probably let you hold your TTV. I would suggest spec'ing MIL-DTL-5541 [on DLA] trivalent chromate on a sample part. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Thanks Ted, but I need the surface to remain pure Aluminum. We have had some improvement here by improving the surface finish. The parts that turned black were lapped to about a 1-2 micron Ra. By using a cerium oxide optical lapping slurry we reduced the Ra by 10X to 1000-2000 angstroms and the surface is now only a medium gray and much shinier in appearance. Does anyone know what we can use in the lapping tool to polish it to a bright, shiny mirror finish? It must be done on the lapping tool in order to maintain our flatness and TTV specs. Thanks!

Dave Benzing
- San Jose, California, USA
August 12, 2011