Your thoughts are correct David.
But it isn't galling, it will be corrosion that is the problem. With copper pipes and a nice humid atmosphere, it will start to become like a very low voltage battery (galvanic action). As you used to do with battery posts on your car, a thin smear of petroleum jelly on the slip joint should stop it causing any problems. But expect it to break out in little patches of white dust almost anywhere. After it has dried out after a run, a little polish with Mr Sheen or any other waxy type stuff should keep the shine and stop the rot appearing.
Now to explain galling in laymans terms.
When you see a nice shiny lump of ali, it looks like it is perfectly smooth. In fact, even under a fairly low powered microscope, it will look like a mountain range on the surface.
So in a piston/cylinder configuration, when the two parts rub together, with normal lubrication, gradually the mountain peaks get worn down, and the parts slide together more easily, and the more it beds in.
If you put it in there without a good lubrication film, the peaks rub together, and the friction between the two surfaces will cause them to fuse together, similar to a very low temperature welding. That is galling.
It isn't just aluminium that does it, almost all metals will gall together under certain conditions.
In fact, I use that to my advantage. You will find on my little engines, I almost exclusively use stainless cap headed fastenings. When tightened up in a metal threaded hole, or the point of a grub screw, stainless has a 'grab factor' built in (very minor galling), and it saves me having to use Loctite. In fact sometimes, on small cap screws, it is too efficient, and I round the hex out when trying to break the grip of the screw.
Even though you didn't ask for it, I hope this has explained it.
John