I know TIP reccomends black iron. Their reason for not using galvanized is that a flake of galvanizing can come off. I've been in the chemical industry since 1964, visited or worked in more than 2 dozen plants (I lost count long ago)and of all those that used threaded pipe for plant air & instrument air I can't remember seeing anything but galvanized. I've also never seen a case of galvanizing flaking off inside the pipe in air service.
