[Cajundaddy]

Yep old galvanized sucks. My method is to fully support and hold firm the joint fitting, get a big wrench on the nipple I am removing, and add a 4' helper for leverage. An old salt once showed me that the first turn should be clockwise just enough to break loose the threads and then you can reverse it counter clockwise and remove it. This method has worked well for me on corroded gas lines for 30 years.

If the nipple is completely corroded and thin to the point it collapses under a wrench, the best plan is probably to replace all of that thin, hollowed out galvanized stuff so you don't have an in-the-wall flooding problem in the future. Good luck!