My 'daily carry' stopper is a PSI black titanium. I've been using it for about four years, with no indication of any problem.
CSUSA used to have a page in their catalog that compared the various platings used on pens. The gave the titanium platings the highest rating of any of the platings they compared. Obviously, pen duty is different from stopper duty, but I suspect that the information is still pertinent.
Of course, if you want absolute permanence, use stainless steel.
Update - this morning, I replaced the black titanium stopper that has been my EDC for the past five years with a Ruth Niles stainless stopper, retaining the turned top.
The problem was that the plating was failing - the original black shiny surface had become dull and the plating was starting to 'bubble'. And when I unscrewed the metal base from the turning, I found that the threaded stud showed signs of rust. I never wash the stopper, but I do rinse it off and dry it after use, but apparently that wasn't good enough.
I kept the original turning because it was special - white oak from a tree harvested from our property, my first captive ring turning, and my first experience finishing a piece with pure Tung Oil.
Its interesting to note that the Tung Oil finish on the turning lasted longer than the finish on the plated metal stopper.