It has been a while since I have taken a chemistry class. But I do remember that aluminum is one of the most reactive substances known to man. In fact it is never found alone in nature. So if it is so reactive, why doesn’t it rust?
Thank you all in advance to all who reply 
It does. Almost immediately, in fact. And that rapid oxidation effectively seals the surface against further oxidation. What you see when you look at an uncoated piece of aluminum is the metal behind a transparent layer of aluminum oxide. Eventually, particularly if exposed to the elements, that layer will thicken over time and make the metal appear dull; for this reason, many aluminum items are lacquered or otherwise coated to keep them bright and shiny.
Because unlike iron, aluminum bonds with it’s oxide. So pieces of aluminum become instantly coated with a thin transparent layer of aluminum oxide, which protects it from further oxidation.
That said, in real world practical terms aluminum does rust away if exposed to the elements or to high heat for prolonged periods.
It does rust. However, the oxidized surface layer prevents further corrosion. In other metals, corrosion continues beyond the surface layer and eventually eats through the entire panel/girder/bar/whatever.
This formation of a protective oxidized layer is called passivation, but I can’t tell you why it happens to aluminium and not, say, iron. My WAG would be that it’s some property of materials with x number of free electrons in the outer shell of each atom.
ETA: On preview, what they said, plus the part about passivation, which **Q.E.D. ** will probably be able to explain once he sees this.
Oh, and aluminum oxide is sapphire, so the thin layer of “aluminum rust” is extremely strong.
Did I just pay a bunch extra for a wind gauge with sapphire bearings and all that means is they let a piece of aluminum oxidise?
You can passivate iron as well, which results in a black iron oxide that tightly adheres to the surface of the base metal. You’ve seen this if you’ve ever seen a cast iron pan. This type of oxide (as opposed to the rusty red type of oxide that flakes off) is created under high temperature/pressure conditions, IIRC.
No, the black coating on a cast iron pan is carbonized oil and grease. Black oxide passivation is also called bluing, and used to be a common gunmetal finish. Newer methods of ferrous metal passivation use an electrochemical conversion of an oxide coating with phosphoric or some other acid.
I can’t add much to what Q.E.D. and Lumpy have already said, other than to note that the difference between iron rust and the passivated oxidation layer that is formed by aluminum (and also stainless steel) is that rust is sufficiently permeable that it allows water or acids to penetrate it and create electrolytic cells beneath it. Under some conditions–particularly marine conditions in which aluminum is exposed to highly ionized sea water, or where it is subject to constant abrasion–aluminum will corrode through and through. It is also subject to a phenomenon known as stress corrosion cracking (SCC) where a part under constant tension will create microfractures that allow tiny corrosion cells which are extended by said tension. SCC is a serious problem with aluminum and some types of stainless steel structures, especially threaded fasterners.
Sapphires are an aluminum oxide, but aligned in a lattice structure. It wouldn’t be correct to refer to the aluminum oxide coating on an aluminum part as being sapphire, as it has a structure based on the underlying grain of the material. In any case, it isn’t hard enough to resist significant abrasion.
Stranger
Having at one time worked for a local floatplane airline flying old twin engine beechcraft and Beavers into the boonies of the coast of British Columbia, that does not always apply. They had 5 planes and about every seven years they would have to strip one down, inspect for corrosion (don’t ever say rust) replace panels or grind out corrosion to acceptable thicknesses. The owner often joked how he was really in the precious metals business.
There are about half a dozen types of corrosion for aluminum, general, pitting, exfoliation, crevice, filiform and fretting.
Are you sure about that being the same as “bluing?”
I’m too many years from my navy days to remember the exact terms used, but I do recall that when they made a new steam plant, there was a very specific series of steps that they had to follow in order to create good black rust instead of bad red rust. This involved maintaining certain temperatures and pressures while the protective coat was being formed.
Nobody ever compared this to bluing.
Ah yes, here we go:
Buecker, Brad. Power Plant Water Chemistry: A Practical Guide. Tulsa, Ok: PennWell, 1997. Pages 6-7
Pure aluminum will passivate. Many common aluminum based alloys, though do NOT passivate. To protect these, they are often manufactured with a pure aluminum coating, or anodized, alodined, or otherwise coated to prevent corrosion.
My recollection from my Navy days is the same as minor7flat5, i.e. that the desirable passivating iron oxide layer is a dense, black layer that looks not at all like blueing. I may be wrong about the coating on a cast iron pan, though. It could well be carbonized oil, but I thought that many cast iron pots and pans were manufactured with a passivating iron oxide layer already in place before any “seasoning” was done by the end user.
Just to add …
The protective Aluminum Oxide layer on aluminium is very sensitive to Hydroxide - just try spraying an aluminium baking tray with oven cleaner :smack:
Si
By the way:
[ul][li]Chocolate + peanut butter = good. [/li]Mercury + aluminum = not so much[/ul]