Originally Posted by
rseeker
The process is essentially the same: The phosphoric acid is reacting with the iron corrosion products to form a dense, insoluble iron phosphate.
In a zinc phosphate containing coating, the zinc phosphate is moderately soluble. In the presence of moisture and iron oxides, the phosphate combines with the iron, the zinc combines with the oxide.
Yes, you do not want to use phosphoric acid on aluminum. As for brazing, it's also something to avoid. For that matter, any dissimilar metals in the presence of a strong electrolyte, acid, or base. Aluminum is also peculiar in that it is attacked by both acids and bases.
Using a commercial product that contains phosphoric acid is generally the best and or easiest way to go, just a little more expensive. Just remember to RTFM.