If you store your grill outside, it can be exposed to moisture which can cause corrosion. Additionally, suppose you don't clean your grill regularly. In that case, ashes and debris can build up and cause the metal to break down. To prevent rusting, make sure that you clean your grill regularly and store it in a dry place. If the bottom of your grill is already rusty, you can try scrubbing the area with a wire brush to remove any loose rust.