White gold is created by nickel zinc and gold. It can also be made with silver. White gold doesnt tarnish. Only if its more silver then gold. If your having it dipped its getting dipped in rhodium. Rhodium protects the finish. If its tarnishing you could be having an allergic reaction to it.
Edit... I just thought of this too, if its a cheaper ring it could be just a yellow gold dipped in Rhodium, hence why it needs redipped every 6 months.