The term is oxidiation. Metals are not stable as pure metals and want a more stable chemical form. Extremes are Sodium which cannot exist in air and have to be kept under oil to Gold and Platinum which are very difficult to Oxidise. Iron Fe can exist as Ferrous (insoluble) and Ferric (soluble) the acid primers convert to the Fe203 rust into Fer3?PO4 Ferrous phosphate, insoluble in water. In reality pure wate doesn't promote oxidation it's the salts of chlorine, Sulphite, Sulphate all of which are soluble to some extent. Passivation works ish but a long term solution is only to grind out and passivate the bare metal and then coat with impervious resins the better cross linking (epoxy 2 pack) the better. Sorry it's long.
Re Aluminium panels, No it's not phosphoric acid it's a whole different system. Those old enough will remember Comet wings falling of due to corrossion in the wings. The primer is usually Strontium Chromate (horrible stuff) and is I believe the only primer chemical approved for Aircraft. The yellowy green colour. Was involved with a BMW 2002tii in the Martini colours restoration. The usual stuff as far as primers doesn't work as it doesn't on galvinised. (Is galvinised (zinc fumed) recent 'cause I'm out of date.) You need specialist primer to my knowledge for galvinised. Try Trimite in Uxbridge they do it. or PPG and buy the complete system as individual components may not be compatible. If it's a Mercedes then make sure they know it's an aluminium panel to be painted and buy the lot. You shouldn't get "rust" on Aluminim normally it inter coat adhesion. (top coat falls off!).
