Yes, metal license plate frames can rust. Rust is caused by the interaction of iron and oxygen in the presence of water or moisture. When these three things are combined, they form a compound called iron oxide, which is what we see as rust.
Rust can happen to any type of metal, including steel and aluminum. However, it's more likely to occur on steel license plate frames because steel is more reactive than aluminum. License plate frames made from stainless steel are less likely to rust because stainless steel contains chromium, which protects the metal from oxidation.
If your license plate frame does start to rust, you can remove the rust with a wire brush or sandpaper. You can also prevent rust by regularly cleaning your license plate frame and keeping it dry. If you live in an area with high humidity or salt air, you may want to consider using a clear coat sealant on your license plate frame to protect it from corrosion.