Pretty amazing that this happened. In the old days of UP&L, this would never had gotten by Dan and Sheldon, nor would have any of the contractors have tried.

My best guess is that this is a transition weld between carbon and a stainless alloy in the superheater section.

It probably not likely to fail in the near future, nor catastrophically, but would certainly have a much shorter life span, especially during the rigors of start-up and stopping.