Originally Posted by dpkelly
Usually it's not the pipe that rusts first but the hangers that are welded on to the pipe. This has happened all the time. Eventually the hanger rust spreads to the pipe causing a hole to start in the pipe. The welds are probably not made of the same grade of steel as the pipe.
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Where the welds are, the materials actually change their molecular composition from the heat of being welded and the fact that the filler rods have to be a different material for Stainless Steel. Different materials than the pipe will cause a cathodic effect (like a battery), and cause one of them to corrode.
There are several different kinds of stainless steel. Some are way more resistant to corrosion than others; but after you heat it up, it gets weaker. That process is called annealing. You have to anneal stainless steel pipes in order to bend them. That's why a stainless exhaust is discolored where the welds and bends are. It can be polished out, but no matter how shiny you make it, some of the strength of the pipe is gone forever.
I have a 1 year old Apexi N1 and a hanger recently broke off, taking a 1" x 2" chunk of pipe with it. My solution... weld it back on there. (Edit: It's not stainless, though... if it were, it would take like 5x as long to go.)