Decorative hard anodizing? 1

[Sheep]

I've been in the aerospace field for many years and have worked extensively with both decorative anodized finishes and hard anodized working finishes BUT I am now working in a commercial environment and have a problem that I don't have an answer too in regard to anodizing. Is it possible to obtain a a decorative (in this case, translucent medium blue) finish with the hard anodize process? It would be the ideal answer answer to my current quest but I have yet to find a local process house that can give me a good answer to this question. Anyone? Thanks!

 

[MikeHalloran]

The color comes from a dye added to a protective sealer infused into the microcracked aluminum oxide surface that anodizing produces. Hard anodize is thicker than decorative anodize, but you still need to seal it, and you can still tint the sealer. Don't expect to get the exact same shade.

For more info, start here:

http://www.finishing.com/2600-2799/2621.shtml

Mike Halloran
Pembroke Pines, FL, USA

 

[kenvlach]

Hard anodize has some inherent coloration due to alloying elements in the aluminum. So it's doped alumina, a la ruby or sapphire. Also, some intermetallics and silicon form particulates (inclusions) in the alumina, so can get speckled effect. There is also some change with thickness. The natural coloration adds to any dyeing, so it is somewhat difficult get lighter colors on certain alloys. Using a sulfuric+oxalic acid (rather than simply sulfuric) gives a yellowish hue, as does dichromate sealing.
The Aluminum Anodizers Council's 'Anodizing Reference Guide' gives the natural colors expected for various alloys.

http://www.anodizing.org/reference_guide.html

Kaiser Aluminum developed a number of 'Kalcolor' alloys which developed natural colors (amber/gold, bronze, black)when anodized. Alco had a similar Duranodic process which depended more on organic acid additives to the anodizing electrolyte to develop color. Don't know whether still available.

http://www.alu-info.dk/Html/alulib/modul/A00305.htm

Most frequently, hard anodizing is dyed black or a darker blue or red. Some colors used in anodizing (the same dyes are used for both hard & conventional anodize):

http://www.usspecialty.com/