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If a thin layer of aluminum oxide is formed on the surface of aluminum to prevent further oxidization, why are optical components using extra "overcoat" materials to prevent dulling of the metal over time?

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Sapphire is an aluminum oxide which is transparent when in crystalline form. However when aluminum metal corrodes the aluminum oxide takes up more volume that the aluminum metal from which it was formed. So the aluminum oxide can't form the same crystalline structure as aluminum metal. Thus instead of one nice nice uniform crystalline sheet of aluminum oxide the oxide forms small crystals. Such an oxide coating reduces the reflectivity of the mirror. (Thinking of it like dropping table salt on a mirror.)

The overcoating materials keep oxygen away from the aluminum metal so that the oxide coating doesn't form.

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  • $\begingroup$ I think you should read about allotropes of Al2O3 $\endgroup$ – Mithoron Nov 3 '15 at 23:51
  • $\begingroup$ "Such an oxide coating reduces the reflectivity of the mirror." How much and any source? Theory is fine but the reduction might be so low it wouldn't even matter. I googled tests claiming 92% reflectance in the visible spectrum. $\endgroup$ – Leo Ervin Nov 4 '15 at 0:49
  • $\begingroup$ Well I tried giving a half-answer that I thought would be satisfactory. If there is 92% reflectivity, where does the other 8% go? Some is randomly scattered and some is absorbed. If you think of taking a picture with the telescope, then some of the scattered light from the surface of the mirror would fog the film. So it isn't just reflectivity. // Adding a coating is an expense. If a coating wasn't necessary for long term stability then it wouldn't be done. $\endgroup$ – MaxW Nov 4 '15 at 19:57
  • $\begingroup$ Adding silicone monoxide coating increases the reflectivity, because it's less detrimental than a layer of aluminum oxide to the reflectance of aluminum. I think it's around 96%. Yes, it also protects from corrosion, but in what conditions and how long? For outdoor telescopes exposed to humidity, extra $ wasted on silicone coating might make sense. For indoor projects, who knows, maybe the natural aluminum oxide layer is sufficient? That's why I want more info. $\endgroup$ – Leo Ervin Nov 6 '15 at 18:23
  • $\begingroup$ There is humidity and oxygen indoors too. In winter the humidity is usually higher indoors for comfort. // I'm not prepared to debate the economics of silicone monoxide coatings. $\endgroup$ – MaxW Nov 6 '15 at 18:28

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