Inner-liner is type 304-stainless steel.
. M & G Duravent 4PVP-24 4 Inch x 24 Inch Pelletvent Pro Pipe 304-alloy Stainless Inner Pipe Galvalume Outer PelletVent Pro pipe for Pellet and Corn burning-appliances features fully unitized twist-lock construction. Listed at 1-inch clearance to combustibles and can be used in both vertical and horizontal installations.