Uh duct work is galvanized. But so are many of the cheap chimneys. Maybe that explains things?...
When I was working I used to make them out of 8 " 16 ga. spiral duct .
I still have a small one I made last year out of 4 " . Holds about 8 pieces . I use it to light the snake , or slow burn pile .
...
Maybe that explains thing?