Black spots on stainless exhaust headers

[Grumpy Goat]

I showed these photos to a materials engineer in our office and the first thing he said is that it looks like an acid attack of some sort underneath the chromed outer layer. I thought similarly, but I was thinking more like rust in the alloyed metal used in the headers. He said that he'll look into it a bit more but since I won't see him till next year who knows when we might have more from him.