"I said to a friend to put a stainless patch in part of his exhaust. He said the stainless would corrode. I replied that it would not. He cited that it would and that is the reason Stainless rots if you do not put in a rubber packing between stainless pipe and the support shoe. we generally use 316L grade stainless. I could not see how this could occur as I thought that it would be the shoe that would corrode. I can see the problem of contamination of a weld by mild steel grinding dust but not by simple metal to metal contact."