304ss 16gauge for my piping. I use 304ss flanges but you gotta be careful to not warp them, 304ss warps easily so most use mild steel. I use 304ss for my charge side as well for winter protection from rust.
Intermediate pipe is 3" 304ss. I can attest to the warping issues found when welding the flanges on. Tack the piss out of them and clamp them to the turbine until they cool completely