Old Baseboard grey pipe/alu fins?

My house was built in the 1950's. The upstairs (an addition) has hot-water baseboard heat that just runs the perimeter walls. Baseboard covers run wall to wall to cover the pipe/pipe with fins . When they got to walls or even the bathtub they went straight through. The pipe is not copper or for sure not copper like today. The fins are aluminum and are folded to make fins but are really all connected. It's loud on cold nights (in upstate NY)... loud ticks run up and down these fins (even with the air removed from the pipes). I'm replacing a section of this... where it went through the tub and the section with fins that follows. Problem is... after I got it all together my joint leaks. Any Idea what this metal is? It is grey pipe and has soldered (looks like) sleeve type connections like copper. It's hard so I don't think its lead. I sweated the connection with non-leaded solder and it leaks but only a little. Before I drain the system and take this apart again I want to make sure I'm using stuff that should work. Anyone know what this pipe is and the best way to connect it to copper?
Thanks, Jim

You should check the pipe with a magnet to determine if it is iron or steel. I suspect that it is copper that may have been tin plated to prevent corrosion of the aluminum fins. If it is tin plated, then you can solder it like anything else.

If the plating is aluminum or something like that, you would probably have to clean it down to copper to tin and solder it.

If the pipe is copper, you should be able to remove the plating down to copper with emery cloth such as is used for cleaning prior to soldering.