Plumbing in the Home/tub spout
I AM REPLACING MY TUB SPOUT. WHEN I TOOK THE OLD ONE OFF IT BROKE OFF. THE COPPER TUBE IS RUSTED AND STUCK IN THE GALVANIZED ELBOW.HOW DO I GET IT OUT.
BUMMER! Unfortunately, I've been there myself. Copper and galvanized steel are not compatible. This was a poor installation. The dissimilar metals create what is called a galvanic reaction that in this case accelerates corrosion.
Since you are talking about "copper" pipe being threaded into a galvanized elbow, I am assuming that the current spout that you had was a "slip fit" compression style spout. I would consider changing to a threaded style spout that requires a nipple of the correct length to attach it to the galvanized elbow. Note: this nipple should be BRASS.
Now, onto the question of getting out the stub. Depending on your access, there are a couple of options. In order to attach copper to a galvanized fitting, there has to have been a threaded adapter sweat onto the copper nipple that sticks out of the wall.
The first thing I would try would be to soak the corroded joint with something like CLR or a spray on product like PB Blaster to try to break the corrosion loose. If the broken stub is back inside the wall where you can't get any tools on it, you will have to use an extractor. An extractor is essentially a reverse thread, spiral flute that you drive into the fitting and then turn counterclockwise to remove the broken off nipple. Smaller ones, meant for half inch pipe are not that expensive. This process is going to take a bit of work but if it works you are home free.
This is what I use: http://www.plumbingsupply.com/extractors.html
Make sure you measure the inside diameter and get one that fits.
If you can't extract the broken stub this way, you will probably need to open the wall at valve height from the backside for access using normal tools. It's a lot easier and less expensive to repair drywall or plaster that it is to replace tile.
If you don't have access to the backside of the valve, you will have to go in from the front. There are "repair plates" that you can place over a carefully cut hole in the tile once the repair is accomplished.
This whole process can be challenging depending on how much difference there is between where the tub spout stubs out and where the actual valves are located. This is something that would need to be worked out on-site. Obviously, I can't do that. It might be time to bite the bullet and call in a pro.