Mike. Such Corrosion is not unusual on oder boats. It would be great if you could share some pictures of your chainplates. Might help other sailors with this issue.
Here is what one of mine looked like. I polished the pit out and then pacified the metal before reinstall.
I used Butyl on the reinstall. No leaks.
Mine looked worse, but it was still a toss of the coin as to whether to replace them. It should have been simple but they turned into a PITA to replace. I might have kept them a while longer if I'd known. I'll take some pictures this evening and post them.