Alodine is acid - chromic acid.

Now that I didn't know...or had forgotten

And the acid etch applied before alodining to remove any
corrosion products is phosphoric acid.
Aluminum is never found in the free state as many other metals
sometimes might be. And it doesn't smelt easily, requiring huge amounts
of electricity to electrolyze it out of the ore. So it's a relatively
recent metal, being rather scarce (and super-expensive) 100 years ago,
in spite of the fact that it's one of the most abundant elements in the
earth's crust.
I suppose we don't find it in its free state in nature because
it reacts so easily with just about anything else and returns to its
former oxides or sulphates or chlorides or whatever. And I imagine
that's why it hasn't seen wide service in cars, what with all the road
salt used in many areas. Even inside an airplane wing we find surface
corrosion on uncoated 2024, just from condensation. Any good floatplane
will have its interior zinc-chromated.

Dan