sodium fluoride

Aluminium needs to have its oxide film disturbed and prevented from re-forming. When electric welding the mechanical stirring effect of the arc is enough to break it up, and the shield gas (MIG or TIG) stops it forming again.

Flux is used for gas welding, or for brazing. These are quite agressive fluxes compared to those for brazing steel and they're based on fluorides (watch the toxicity hazard of hot fluorides). A flux is needed because gas welding doesn't have the stirring effect of the arc, and there's no shield gas even if the atmosphere around the flame is reducing (i.e. oxygen starved)

thank you that helps clarify things - is it true that sodium fluoride paste can be used to 'clean' an aluminium surface of oxides prior to arc-welding? and would this be a common practice? jpm

The fluoride isn't especially active. But when you heat it, it forms an active mixture, including hydrofluoric acid which is the toxic part. Used cold it's ineffective.

There are lots of aluminium pickles that are used before some processes (particularly anodising), but I'm not aware of any before arc welding. However I'm not an aluminium welder, so there may well be.