Answer

The overpowering metal determines zakat on mixed articles. Hence, if the gold is more than 50% of the total article, zakat will be given according to the gold nisaab. (Fatawa Mahmoodiah p.71 v.3)

White gold is an alloy of gold and nickel (Lexicon Encyclopaedia p.226 v.4). Therefore, if the gold is more that the nickel, zakat will have to be given, otherwise, no. (Fatawa Hindiyyah p.335 v.5)

Only Allah Knows Best