Paint in one form or the other or plating.

Alodine is a conversion coating that produces an oxidized surface to promote paint adhesion and provide corrosion resistance (on its own).

A quart bottle is cheap, and it's easy to apply.  The easiest way to find a bottle is at an automotive paint store or an aircraft supply house.

I've never used an aerosol version, but it ought to work the same as slopping the liquid on with a mop.  Alodine will cause a yellow cast on the stainless steel of your kitchen sink.

Here's one source:  http://aircraftproducts.wicksaircraft.com/category/metal-preparation

Different forms (plate, forging, extrusion, or sheet, and their thicknesses) and heat treats (-T6, -T7351, -T7451, -T76, for just a handful of the possibilities), the loading direction with respect to the grain, and cross section size of 7075 alloy demonstrate different degrees of susceptibility to corrosion.  I would not ever rate 7075 as naturally corrosion resistance, it's prone to exfoliation corrosion and stress corrosion.  Stress corrosion occurs in joints under preload load.

A part fabricated from 7075 and heat treated to -T76 condition will exhibit greater resistance to exfoliation than one heat treated to -T6.  A part heat treated to -T73 condition will exhibit even better resistance than the other two.

The argument will be made that hard coat anodize on aluminum is used for wear resistance in addition to improved corrosion resistance.  A lower receiver is subjected to wear in the magazine well, and practically nowhere else on the inside, and most shooters modify the inside of the well so magazines will drop free, so the anodize is removed anyway.  Good fits at the pins and a good wipe down to remove water after the gun gets wet removes the moisture leg of the corrosion stool, and a little oil at the pins removes the galvanic current leg.

For grins I'll put up some mechanical allowables for a few heat treats of 6061 and 7075 materials for comparing ultimate tensile strength Ftu, tensile yield Fty, and compressive yield Fcy.  I think these are the only images I have loaded here, I'll more if they are available.