Here is what I think you may be referring to:
8.15.3.2 Protection of Piping Against Corrosion.
8.15.3.2.1* Where corrosive conditions are known to exist due to moisture or fumes from corrosive chemicals or both, special types of fittings, pipes, and hangers that resist corrosion shall be used, or a protective coating shall be applied to all unprotected exposed surfaces of the sprinkler system.
It is the conditions due to moisture that gets a lot of people. Some will claim that relative humidity will be an issue and require galvanizing for exterior piping. Others will say that if there is a covering so it does not get directly rained on that you don't need galvanizing.
Good luck