Our Service Chemical Surface Treatment of Aluminium Reflectors (continued) The process of chemical polishing explained in the March issue of our newsletter is followed by the anodising as an electrolytic process producing an oxide layer on the surface. An oxide layer can only be created on a metal used as the anode. Using direct current atomic oxygen is produced at the anode during the electrolysis. The oxygen reacts with the aluminium forming aluminium oxide. In contrast to the natural oxidation this oxide layer is more than 100 times thicker and tightly fixed with the metal surface. The layer thicknesses needed for reflecting surfaces of 4 to 10 micrometer are mainly achieved by means of direct current in sulphuric acid electrolyte (15 to 20% sulphuric acid, 80 to 85% water, bath temperature: 18 to 22 degrees Celsius). Elektro-chemically created oxide layers are transparent. The incident light is reflected at the contact surface oxide/metal. The degree of reflection depends on the quality of the aluminium used and the transparency of the oxide layer which deteriorates with increasing layer thickness. The reflectivity deterioates with a decreasing purity of the aluminium used and with an increasing layer thickness.

The chemical surface treatment is concluded by the sealing process. After the anodising the oxide layer is still micro porous. The optimum wear and corrosion resistance is achieved by sealing that means closing the pores. The sealing process is normally effected in distilled, boiling water exceeding 96 degrees Celsius. The aluminium oxide is hydrated and converted to boehmite. This process is linked to an increase in volume and results in a sealing of the pores which makes the layer impermeable for liquids and far more resistant against external influences.

The result is an highly specular reflector with a reflectivity of more than 86%.

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