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Chemical Surface Treatment of Aluminium Reflectors
 The
chemical surface treatment represents the final step
of the manufacture of highly specular aluminium reflectors.
By means of this treatment, the so called „finishing“,
the reflector gets its mirror-brightness with a reflectivity
of up to 86%.
The treatment process mainly consists of four phases,
that means:
- Cleaning/Degreasing (Pretreatment, Pre-Clean),
- Chemical Polishing,
- Anodising,
- Sealing.
Prior to the polishing and anodising, the surface
of the mechanically formed (“spun”) raw
reflector has to be thoroughly cleaned of grease and
oil. The cleaner is alkaline. Cleaning
primarily involves the dissolving off the surface
layers of aluminium by a small amount of etching and
leaving the thin oxide coating normally present in
a uniform condition.
The cleaning process is followed by the chemical
polishing as a controlled dissolution of
the basis metal. In order to achieve a mirror-bright
surface with a maximum reflection factor, pure aluminium
Al 99,5% up to Al 99,9% as well as super pure aluminium
with a percentage purity of up to 99,99% has to be
used. The best-known chemical polishing processes
are the Erftwerk process and the
Alupol process. The main chemical constituents of
the Erftwerk process are ammonium bifluoride, nitric
acid and hydrofluoric acid. Depending on the operative
conditions, the amount of material removed is between
10 to 15 micrometer at temperatures of 80 to 90 degree
Celsius. The typical treatment time is between 15
to 30 seconds. The cleaned raw reflectors, fastened
to special frames, are immersed in a chemical bath
(water-heated metal or plastic tank, protected by
heat-resisting and acid-proof vulcanite) and moved
by hand to avoid so called “tracks of gas bubbles”
on the surface.
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