steam boiler radiator return pipe
I have a steam boiler radiator system. The return line of threaded black pipe in the basement is corroding. I've already spotted a leak. I replaced the part with a new piece and it was full of water. Because I would like to finish the basement, the return piping needs to be replaced. It's quite an investment. Currently, parts of the piping is laying on top of concrete and partially embedded in concrete. It only lasted for 25 years. The current pipe is 1-1/4 diameter.
Is there another material I can use to replace the return pipe. Something better than black pipe. Stainless steel, Cooper ? If so, what grade?
To last longer, I would like to bury (not embed) the pipe under the concrete slab with a waterproof wrapping and surrounded with a gravel bed to keep it dry. Is this possible ?
Also, is it normal to have water in these pipes ? I had to close the water valve to the boiler and drain it? It does have a Hartford Loop.
thanks for looking