It's called Heater Return Line...
Be sure to get a new o ring seal for the heater return line also.
I think its more likely that you have a leaking water pump shaft seal .... unless you can see the leak coming from the line. These usually do not corrode that much...but maby? Can you wiggle the water pump pulley? Normal is no movement or very very little movement even when you tug on the pulley with est. 20+ lbs of force or more.
It would be a shame to replace the pipe and then find that the leak was from the water pump !!
Parts can be ordered from Fastlane--Call Phil at the 1-888-333-4642 number on the top of the page. The line can be ordered for 25 or 30 dollars most likely.