I've had aluminum dive tanks (not anodized) for about 15 years (yes, they won't hydro them anymore so I have to ditch them next year).

Used to dive in salt and fresh water (well, they've been sitting in the garage for the past two years) banging around in caverns/caves. Not much paint left on those things....

Thankfully, never had problems with cracking, because 100 Cubic feet of air at 3500 PSI might do more than just "leak" if a crack showed up...

I thought aluminum just oxidized along the surface, and that prevented further oxidation (unless scratched).

Call me old fashioned, but I still think those old US Steel 103 cu.ft. steel tanks were the best ever... didn't float when empty and you could put a lot of bottom mix in 'em. Yes, they do corrode, but I never saw a spot of rust on mine, even where they were banded (doubles). But that's for another thread...