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It is common knowledge that aluminum is relatively resistant to corrosion due to its tough oxide layer. The ultrathin (1 nm even) layer is impervious to water and oxygen. What I can't figure out is, why? What makes $\ce{Al2O3}$ so strong compared to $\ce{Na2O}$ or $\ce{MgO}$? My theory is that the aluminum oxide molecules are tightly bound together due to the high charge density of aluminum ions, and this forms a physical barrier where water and oxygen molecules can't come through. This will explain why this only applies to aluminum, as its small size and 3+ charge should make its charge density amongst the highest known. I can't find anything corroborating this claim, though.

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    $\begingroup$ While waiting for an answer you may want to search for "passivation" and either spelling of aluminium on this site, as there are many relevant anwers, for instance: chemistry.stackexchange.com/questions/125691/… $\endgroup$ – Buck Thorn Oct 3 '20 at 9:25
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    $\begingroup$ Also Al is not unique in this regard. $\endgroup$ – Alchimista Oct 3 '20 at 10:07
  • $\begingroup$ @Alchimista - true, but only 3 elements have a self-limiting oxidation layer so it is close to unique... $\endgroup$ – Jon Custer Oct 3 '20 at 19:56
  • $\begingroup$ @JonCuster ... chromium, titanium, and nothing else? $\endgroup$ – Karl Oct 3 '20 at 19:59
  • $\begingroup$ @Karl - aluminum, silicon, and beryllium (or boron - I really don't deal with those). Neither Cr or Ti self-limit. $\endgroup$ – Jon Custer Oct 3 '20 at 20:03

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