Will the frame rust if I use the chair at the beach?

It depends on the frame, and the product page for each chair says which one it has. The aluminum-framed chairs are the ones to choose for salt air and wet sand: aluminum does not rust, which is why the lace-up and high-seat beach chairs are built on it. A steel frame is stronger per pound and costs less, but it is the one that will mark if the chair goes back in the car wet and stays folded in a hot trunk for a week. The habit that keeps either of them alive is the same: rinse the frame with fresh water after a day on salt water or in a pool, open the chair up, and let it dry before it is folded away. Keep a folded chair indoors over the winter rather than in an unheated shed, where condensation collects on metal every thaw.