Yes, authentic bags can do that, some are brass plated and when the plating wears off they are just a plain base metal that can corrode, as the pull ring on your bag has done.
I had a similar situation with a beat-up bag that I rehabbed. It wasn't corroded but all the plating had worn away and after I shined up the hardware as much as possible I still hated the look of that dull, gray metal pull ring that didn't match the rest of the goldtone hardware on the bag.
I finally decided to remove and replace the ring, although this meant altering the bag from the original condition. I borrowed a wire cutter from my husband's tool bench and cut the ring off, then I threaded a similar sized, brass split ring onto the zipper pull and the pull tab.
I was lucky and had an old keyring around the house that had just the right size ring for my bag, but you can buy split rings pretty inexpensively at hardware or hobby stores or online.
At the link below are before and after pics of the bag so that you can see the change. Good luck!
http://forum.purseblog.com/coach/resurrection-not-rehab-this-bag-is-back-dead-658297.html