Mike Schofield : There is a little controversy about the Zinc coating on the Steel ductwork, This is my best understanding of the situation ! During initial firing
the galvanizing layer is heated to a temperature where part of the shiny coating changes in a mechanical way how it is bound to the Steel. There is some out
-gassing of Zinc volatiles at that time, the process is complete when a visual inspection shows that the zinc coating is now a dull or frosted gray that feels less
smooth of slick than the previous surface.
After the Zinc coating has been checked and is physically changed it is safe to use indoors ! If there is any question in your mind then the only 2 further choices
you have are -to only use that pipe out doors, or substitute Black stove pipe for it completely !
Below is a link that will allow you to scroll down through a series of Building and Working Rocket Mass Heaters
RMHs the one that shows the way the zinc coated
pipe is physically changed is the one that shows how to prepare a barrel for RMH
use Link below:
Scroll down till you find the one labeled at the top as Rocket flames . . . . RMH
For the Good of the Craft ! Big AL