I find this article fascinating. It seems that a steel containment structure has rusted halfway through, and yet they say it's good for another 20 years. One side says it doesn't meet code anymore, another says that it's good.
I'm all for relaxing code on existing structures, because the code usually has a high margin built in. I had big fights with 'code fundamentalists', mainly because something is originally built to code, and it won't meet code anymore once it ages. But the code has built that in!
Still, extensive rust seems a bit much! There's no experience database on this, and I didn't see any extensive analysis. As well, who knows what other damage there is? The original designers did not foresee extensive rust, caused by sloppy maintenance. All in all, it sounds rather stupid. (and not very earthquake-safe!)