will it rust

Any cast iron will rust eventually. We suggest NOT using a stainless steel brush to clean it. Use a brass brush or a nylon scrubby brush. You can even wad up some aluminum foil to rub off cooked on food. When the grill cools, spray the grids with a light coating of cooking oil.
Just remember, no metal exposed to high heat and moisture - which occurs every time you cook - will break down eventually.