On Sun, 5 May 2013 17:05:31 -0700 (PDT), Sonny
wrote:

Well, it may have had a pint/quart of water, but the rusting factor would depend on the amount of oxygen reacting with the iron to produce a quantity of rust. In a closed container, the rust may be much less than if contents were exposed to more open air.

An air tank has plenty of oxygen in it. High pressure == more oxygen.

Not just open it, but can you remove the whole petcock assembly, without breaking it? What size hole is in the tank, there, 1/2", 3/4"? If you can remove the assembly, rinse out the tank, well, then rinse with denatured alcohol. Allow to dry, as best it can, before reinstalling the petcock.

It's just going to get "wet" again, at the first use. It's sorta
unavoidable, unless you live in AZ. ;-)

If I had a questionably badly rusted tank, I might entertain the idea of rinsing/sloshing the bottom of the tank with muratic acid, to see what all might be washed out. Rinse with water, afterwards. I would think the very bottom is rusted the most, if significantly.

That sounds like a really bad idea. The rust isn't going to hurt
anything more than it already has. HCL does corrode steel, which is
*not* useful. If the worry is polluting the air stream, filter it. If
that doesn't work, filter it some more. ;-)