Thank you for your reply. I understand that the plastic lined nipple is best for permanently attaching a brass ball valve on it, because the brass is a noble metal and will corrode the unprotected galvanized pipe and tank. Hence, the need for the plastic lined nipple.
But I did not think that a nipple lined pipe with a galvanized cap would be less reactive than a galvanized, unlined pipe with a galvanized cap.
That is interesting.