Exhaust pipe material?

Anyone know the exact exhaust pipe material on the B5.5 TDI? The guy b-4 me had a mech "troubleshoot" a plugged exhaust. He put cuts in front of the muffler and resonator. I had an exhaust shop weld them but CS does not last on SS. Looks like a 400 series. IF I have the spec, I can get the exact rod material to repair this properly.

Thanks!