I have a section of galvanized pipe that has corroded at the threads that connects to a T from the toilet and vent to a elbow that connects to a waste stack.
The question I have is can I cut this section of gal pipe from the T to the elbow and relace it?and if yes how and what do I use? The section of pipe is about 4 to 6 inches long. Help Mark F

You didn't say but this sounds like 1 1/2" galvanized. Correct?
Are the female threads still good on the tee and ell?
If they are purchase 2 1 1/2" PVC male threaded adapters and small cans of PVC Cement and Primer plus a roll of Teflon tape. You will also need a 8" piece of PVC which you will cut to size.
Before we go any farther I have to know how rigid is the space between the two fittings. Can you spring them apart enough to make a solid connection or are things so tight that we'll have to put some kind on union in between? Let me know and we'll proceed. Regards, Tom

You didn't say but this sounds like 1 1/2" galvanized. Correct?
Are the female threads still good on the tee and ell?
If they are purchase 2 1 1/2" PVC male threaded adapters and small cans of PVC Cement and Primer plus a roll of Teflon tape. You will also nee a 8" piece of PVC which you will cut to size.
Before we go any farther I have to know how rigid is the space between the two fittings. Can you spring them apart enough to make a solid connection or are things so tight that we'll have to put some kind on union in between? Let me know and we'll proceed. Regards, Tom

I don't know if the threads are good and I didn't measure the size of pipe but I'm sure its bigger,its part of the waste line that connects from the toilet to the waste stack.

You're going to hafta help me out a little here. I have to know what we're dealing with before we can continue. The rotten pipe will have to be removed and the threads cleaned up. If you're not comfortable doing this perhaps you should call in outside help. Regards, Tom

Another idea: You can also cut the old gal. pipe at the T, than take blade from hack saw and cut the tread inside the T in several places. Take skinny flat screwdriver, put it under the old threads and pray it out. Then, reinsert new gal. nipple and connect with existing pipe...