[Bibio]

its not so much galling as galvanic corrosion. all metals react different when they come in contact with each other. any form of aluminium will "anodise" and if one surface is not "fixed" it will slowly eat into the other surface which will sort of weld the two together.

this can also happen on bare aluminium when it comes in contact with other substances such as water and salt and a classic example of this is the build up of oxidisation behind calliper brake piston seals.

funnily what we call anodising is no more than a pre oxidisation of an aluminium surface which is then sometimes dyed and then fixed, but its still a surface of corrosion. if the skin of the fixed surface is broken then further oxidisation will take place. this in turn will seize the surfaces (threads) together, just like steel and rust but at a much higher rate of time.

to stop the corrosion (anodising) a barrier surface has to be used and this is usually in the form of a lubricant with the most common being copper grease.

btw, stainless steel and aluminium have a pretty violent reaction with each other so you must always use a barrier lubricant.