The heat right out of the manifold is likely to be too high for galvanizing to be safe as bought, plus galvanized ducting is generally thinner than black stovepipe.
You can render the galvanized duct safe for the mass by heating it in a well-ventilated area outside until the galvanizing goes from shiny to dull gray; then the zinc has turned to a more stable form that will be safe unless it is exposed to welding temperatures or the like.
It still would be thinner, and might not be as durable as you want. You will have to make your own decision there.
So run the piping through a fire until it turns from shiny to dull grey?
Here is my dilemma. How far out should I go with stove pipe before I attach to the galvanized pipe. The piping was given to me and I can't afford to replace it all with stove pipe.
Correct me if I am wrong, I have seen many a picture with the mass heater part being galvanized piping.
Never paid attention to it actually coming out of the manifold.
About five feet of black stovepipe is generally recommended. If your mass is very good cob and has sufficient layers or thickness to eliminate the possibility of leaks, you can probably use whatever you want and expect that even if the duct does deteriorate, the cob will hold up.
I will not be using cob for the mass heater. I am creating a cement block box and filling it with pea gravel. Periodically I will be uncovering the mass heater piping to look for leaks I will be using cob for the drum manifold to seal it around the firebrick and to create the 45 degree lip on top of the heat riser. Then the jay tube will be bricked in with perlite as the insulation.