If something is rustable, it means it can get rust on it.
Rustable things are usually made of metal and can get old and not work well.
Examples:
Metal is rustable if it gets wet.
My bike is rustable if I leave it outside.
The old car is very rustable.
My spoon is not rustable because it is made of plastic.
If you leave your toys outside, they will become rustable.
The old ship is very rustable and might break soon.