3 paths-

path1: assume that the plating on the key was poor. Has it         "worn"
or "peeled/flaked" off? If worn, that is not very long for a key to
wear like that. I would make sue tha tthe locks are lubricated with
dry graphite (NEVER use a wet lube in a lock, no matter what the
can/bottle says). If flaked, then I would worry about where all those
bits of chrome went.

path 2: There is a problem with one of the cars's locks- presumably
the door or ignision lock that is wearing off the chrome.

path 3: You keep the key on a keyring next to other keys that are
wearing off the chrome as they rub together in your
pocket/purse/whatever.

If this is one of the later digital/chipped keys, they are expensive
to replace.

            __  __
    Randy & \ \/ /alerie's
             \__/olvos
'90 245 Estate  -  '93 965 Estate
  "Shelby"     &      "Kate"