I've noticed that just about every dead exhaust I've seen is rusted through at the joints, hardly ever in a length of pipe or a box.
Since the sheet/tube metal seems to be galvanised I guess it's the non-galvanised nature of the welds that's causing it to be more rust prone there.
So next time I get a new exhaust should I get some zinc paint/spray and go over the joints to make it live longer?
Or am I wrong about the causes?
I'd also think the most corrosive elements are inside the exhaust acting on the side of the weld I can't get to - nothing I can do about that without getting a stainless exhaust, I expect :-(