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Specular polishing of aluminum

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Dear Colleagues,

I have been working for some years in the forming of aluminum parts (either 1070 or 1085). Only in a few cases our clients asked for a good specular finishing. So, we polished the parts mechanically and manually.

As a specular finishing is an increasing demand, I am forced to adopt some other processes. I have heard of chemical or electrolytic polishing processes, though I am not familiar with them. Could anybody give me any idea of ...

1) which one is more suitable for an industrial process?,

2)what are the advantages/disadvantages of each one?

3)can they produce homogeneous good specular surfaces?,

4)Is there any special preparation for this parts?

5)Are the processes highly controllable and reproducible?

Inigo Braceras
- Hernani, Spain.

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How specular is 'specular'?

Aluminum lamp reflectors for outdoor fixtures, and Doctors and Dentists lamps, and indirect lighting are produced by the millions via chemical polishing, more often referred to as "Bright Dipping".This is done in a nasty mixture of near-boiling nitric & phosphoric acid which fumes terribly and requires special 'lab hood' or 'garage door' ventilation systems. There are proprietary solutions as well as generic ones, but I believe that all employ nasty solutions.

Telescope mirrors, and very special lighting devices, where the utmost of reflectivity is required from very pure aluminum, are electropolished. This is also an industrial process, but is presumably more expensive.

In either case, the aluminum must be subsequently anodized if it is not to be protected from corrosion in some other way.

Ted Mooney, P.E. finishing.com Brick, New Jersey

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I am very familiar with electrolytic brightening for the production of bright trims in the luxury automotive industry. The "electro-oxidation" replaces the old boiling nitric acid process (Phosbrite process). The new method requires some extraction (aerosols) but operates at a lower temp than the old method (75 deg C vs 105 deg C).

I suggest you find the very excellent bible of aluminium The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick, R. Pinner, P.G. Sheasby or contact a specialist company such as Henkel or Nabu. Let me know how it goes.

John Tuohy
- Ireland