If it's a film that appears on the back of the card, more than likely most if not the entire card, the it's probably nothing to worry about. Alot of the time when the cards are run through a bath to remove the excess solder from all the solder joints and the board itself, a thin film layer will appear, looking like the card is oxidized. It however is not oxidized, its just a left over layer of the bathing solution. This is common on most electronics.
Can you put up a good hi res pic of the card? Might help to determine if it is actually oxidation or just leftover bathing solution.
i'm not sure,but what i knew.the graphic can't run smoothly when i play Grid in ultra setting...my graphic card is XFX ati HD 4850...