Djacks02:
First and foremost, lets address what hot-dipped galvanized pipe is and how it is applied - especially for sprinkler systems.
Most, if not all, downstream sprinkler applications are screwed fitting joints. Galvanization, by common sense, is thoroughly removed when the pipe is threaded! There is no way to protect this area again with additional galvanization. It is forever doomed to a short, corroded life in the application it finds itself in (if the static water it contains is corrosive in the first place). Many young engineers don't analyze what they are applying or spend time studying the skills and application at hand. This leads to false dreams or anticipations about what results will be. Practical, experienced engineers will testify that galvanized pipe is for PLUMBERS - not for serious pipefitters in a process plant application. Plumbers LOVE galvanized pipe; the more they install, the more they will come back to replace. It's called job security.
In other words, galvanized pipe inherently has the weakness of having it's threaded section totally exposed to corrosion and thereby is weakened by this. Black iron or cast iron pipe many times is more effective - depending on the fluid and the application. I would not apply galvanized piping to serious Fire Sprinkler applications where I was not going to routinely inspect and monitor pipe integrity - and this costs money and time.
Think about it. If you are trying to protect a steel pipe by galvanizing it, isn't this a statement that you know you have inherently weak steel pipe that you must protect? And if you have an "Achille's Heel" (a breech in the galvanized coating) because you have forcefully removed the protective coating due to the installation need, what can you expect? I don't trust such a scenario.
I hope this experience helps.
Art Montemayor