Rusted thru pipe stub in exhaust

[forsailbyowner]

Ive been watching this ferrous pipe stub that is between the aluminum exhaust manifold and the aluminum mixing elbow. Last time I tapped it with a hammer I found its shot. I was able to take out the remains with a chisel and now Im wondering, is it ferrous for a reason? Maybe like a sacrifice so it goes away before the much more expensive manifold or elbow?
there is another cast iron piece that bolts to the manifold and accepts the pipe thread. Im wondering if I would be making a mistake replacing the ferrous pipe with a bronze piece. It would be mating with an aluminum mixing elbow and cast iron on the other end, thanks in advance.