Ed,

Even the cheapest aluminium doesn't rust. Alu is a self-protecting metal, i.e. the oxyde that forms on the surface of the metal acts as a protecting coat for the sane metal underneath. You can even dip a piece of alu in sea water for a month, it won't rust. It takes aggressive chemicals not usually found in nature, acids and bases, to attack a piece of aluminium to the core. Also, aluminium oxyde forms almost immediately, within minutes of aluminium being expose to oxygen, it's dull white and it doesn't change volume. Only iron changes volume (expands) when it turns to rust.

In short, if the paint slowly bubbles up because of corrosion, you have a steel frame. to make sure, just chip the bubble of pain with a nail and see if it's reddish brown underneath. If you're afraid of doing that, check with a magnet: it'll only stick to a steel frame.