Is the original pipe black pipe?

I have a 110 year old multi family house that has asbestos covered FHW supply and return lines feeding radiators on the first floor. (BTW: the asbestos is already encapsulated). The previous owner uncovered the 1 1/4" pipe, to a fitting, then connected copper reducer fittings down to 3/4" in order to install a Slantfin 4 ft. baseboard radiator in the bathroom. Due to major renovations that are needed in the bathroom, I will be replacing the baseboard heater, since it was damaged during the renovations and the copper connections now leak at the copper fittings.

I am looking to replace the copper with PEX.

1. Is the original 1 1/4" piping black pipe? 2. If so, will I be able to use a black pipe 1 1/4" to 3/4" reducer directly to a Pex brass connector without an issue of corrosion or electrolysis? 3. What would I use on the threads? Is tape OK?

Thanks
Dave

 

The original most likely WAS iron pipe, and most likely a steam system.

It's fine to hook up brass / bronze / copper to iron in hydronic heating systems.

Teflon tape is fine, so is pipe-dope.