I didn't know that they could/would seize up. But, it stands to reason that it could happen, especially if salt is involved.

I had a metal car tire valve cap seize on to the valve stem last year. I literally couldn't get if off with pliers. The entire valve was turning. Had to take it to the shop. The tech said that salt caused it to seize up. And this is why all valve caps should be plastic to avoid this very situation.