By increasing the thickness of the natural oxide film on evaporated aluminum mirrors, a much better surface protection can be achieved. Heat treatment in air is not practical for producing protective coatings on aluminum because the temperature required is more than 400°C and oxide films formed at this temperature are rough. Oxide films with precisely controlled thickness can be produced on evaporated aluminum mirrors by anodic oxidation in electrolytes such as ammonium tartrate. The thickness of the oxide layer formed in a given time increases linearly with the applied voltage and is 13.0 A-units/volt for 2 minutes anodizing time. The current efficiency of the anodic process in ammonium tartrate is in the neighborhood of 80 percent. The thickness of the oxide coatings is 1.38 times thicker than that of the aluminum layer replaced. The anodic oxide films formed in ammonium tartrate are amorphous and free of pores. They exhibit no noticeable absorption in the ultraviolet, visible, and infra-red. Their refractive index between λ = 3000A and λ = 6000A varies from 1.67 to 1.62. Anodized aluminum mirrors show excellent abrasion resistance. To obtain highest reflectivity in the visible the anodic oxidation must be performed with 120 volts for 2 minutes. The precisely controlled uniform anodic coatings on opaque aluminum mirrors are suitable for the preparation and investigation of reflection type interference filters. The thickness of a pure aluminum film can be determined by the voltage required for complete oxidation.
© 1949 Optical Society of AmericaFull Article | PDF Article
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