# Why is aluminum oxide resistant to corrosion?

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.

• 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/… – Buck Thorn Oct 3 '20 at 9:25
• Also Al is not unique in this regard. – Alchimista Oct 3 '20 at 10:07
• @Alchimista - true, but only 3 elements have a self-limiting oxidation layer so it is close to unique... – Jon Custer Oct 3 '20 at 19:56
• @JonCuster ... chromium, titanium, and nothing else? – Karl Oct 3 '20 at 19:59
• @Karl - aluminum, silicon, and beryllium (or boron - I really don't deal with those). Neither Cr or Ti self-limit. – Jon Custer Oct 3 '20 at 20:03