Aluminum protects itself. When it rusts (aka oxidizes,) which it does
very very quickly, it forms aluminum oxide (duh.) A coating of aluminum
oxide is about the hardest, most impervious thing you could put on a
piece of metal. The "process" does not "continue on its own." As soon as
a microscopically thin layer forms, your aluminum is protected from
further degradation.

I'm no expert, however my understanding is...

6061 and 2024 are not pure aluminum. In order to obtain the desired strength
properties, other metals are added to the aluminum. I believe 2024 has
something of the order of a couple percent of copper. The surface will not
be pure aluminum oxide, since there are "impurities" within the metal. You
are now open to pitting corrosion as well as granular corrosion.

The alodine forms a passivated layer, that uniformly protects the whole
surface. Being a delicate surface itself (as a previous poster already
mentioned), the passivated layer needs to be protected against mechanical
abrasion.

6061 is quite immune to most of the above corrosion. It is still a good idea
to protect anywhere you will have metal to metal contact though, since you
can have moisture trapped between the parts which accelerates the corrosion.

I use 2024, so I do the scuff, etch, alodine, 2 part epoxy prime routine.