I know about the protective layer of aluminium oxide from the reaction with oxygen but why does this stop it from rusting? Iron still rusts after forming a layer of iron oxide so why is aluminium oxide better protection? All that I can find is that iron oxide is crumbly which leaves other layers exposed which can then react but that doesn't explain why the iron oxide can then bond loosely with water.
Aluminium has a high charge density 3+ It is also very small with a high charge to size ratio. Now if you stick something very electronegative such as oxygen next to a highly charged cation the bonds are going to be very strong. More energy is needed to break the bonds. Which is why it is quite unreactive. Covalent bonds form.
Anti rust paints are made out of oxides. The oxide layer you mention is the protection for aluminum against further corrossion. you can see rusting aluminum to the limit if you induce it, for example, I once saw an old airplane whose wings and rivets created spots of heavy corrossion, due to the fact that both aluminum ribets and aluminum metal in wings had different formulas, of blends, which created a galvanic par that created holes where once were clean, neat ribetts, and the the plane became unusable, kaput!
You are confusing aluminum with aluminum oxide. The first is a metallic element and the second is a compound of aluminum and oxygen. Aluminum doesn't rust, as that term is reserved for ferrous metals, ie containing iron. Aluminum will oxidize but not rust.
Rust as we know it is a common term for ferric oxide, which is corrosion/oxidization of iron alloys. So technically, aluminum does not rust. However, aluminum does oxidize very rapidly when exposed to air. So aluminum always has a thin coating of aluminum rust (perhaps alumrust would be the term?). The good news is that aluminum oxides are very thin and remain well bonded to the surface forming a protective coating that helps protect from further, deeper corrosion. Ferric oxide on the other hand flakes easily from the surface and forms cracks that actually assist the corrision process. Caution though that under the right conditions, heavy corrosion of aluminum can occur resulting is similar flaking of the oxides and loss of the protective oxide layer. I've seen dusty piles of aluminum oxides when aluminum was used in a highly corrosive environment. Also, consider that there are steels that are highly resistant to corrosion. Think surgical stainless steel. All metals can and will form oxides.
When water molecules contact aluminum oxide, the aluminum and oxygen atoms on the surface move apart as much as 50%. As a result, when the outer layer of aluminum oxide gets wet, its structure changes just enough to become chemically inert and thus unable to react rapidly with additional water molecules or atmospheric oxygen. This change in molecular structure is why aluminum oxide resists corrosion. Hope this helps you.
