Tire Always Going Flat

I recently had new tires (Very Good Qaulity Tires) put on my 2001 Nissan Maxima SE with V6 Engine (60,000 miles), but since, one tire in particular goes flat every couple of months. The tire store where I bought them indicates that the seal is not properly sealing because of rust on the rim and that I will need a new rim. I have never heard of this, usually hold my cars for over 10 years and never experienced something like this. Before I go spending money on a new rim, can someone advise if they have heard of this problem with Nissans or any car for that matter? Thank you.