Attached is a picture I took of a steel pipe I hauled out of the basement.  It had been leaking, and the previous owner used black electrical tape, and plumbers putty to try and stop the leak. 

So this steel pipe was fitted to a brass pipe.  Apparently when they built some of these houses in the 1920's, they used some brass pipe (so says our home inspector).  When you place two dis-similar metals together, you get this galvanic corrosion, which caused the leak.  And wouldn't you know it, the pipe was basically sealed shut with corrosion and gunk.