How was this done?

thats exactly what we in the exhaust business use :D aluminized steel tubing, lasts longer than the normal run of the mill steel, usually starts to rust along the outside of the weld tho.

I've had people watch me weld flanges on that stuff in amazement before... almost everyone thinks it's aluminum. had someone come up and touch a magnet to it too, after I was done welding of course