Keith s elliot : This one is simple actually, this pipe will only last longer than other types of Iron/steel. the problem is a condition called High Temperature Hydrogen
Attack, and hydrogen embattlement, there are good articles on these effects in Wikipedia!
You can try looking in the oldest hardware store in your part of the country for a thin gaged very lite weight grade of Blued or Japanned stove piping suitable for
ductwork, as these pips (if found) come in large sizes you can find a 6'' piece to be the inside wall of your Heat Riser, and a 10'' piece to make up the outside
wall of the Heat Riser, then filled with
Cob and perlite. There have ben a few reports of The expansion of the inside metal pipe applying enough pressure to cause
cracks in your penlight /cob Heat Riser and some people are making up the Heat Riser with son tubes the kind you fill with concrete-
( never use
concrete any where near the
Rocket Mass Heaters RMHs burner base, it can not take the heat ether! )
These tubes are available at Lowes, Menards, Home Depot, the big box home improvement stores ! Hope this helps and is timely !
used as a form into which you pour a mixture of
Cob and perlite,