Moisture on iron will make it rust. Rust is not the end of the world but you will have to do a bit of work to have the rust removed, however, a rusty cast iron cookware can almost always be restored to a new condition.
What to do if you have found rust on your cast iron cookware?
Regardless of the extent of rust, there are options to throwing away a cast iron cookware. Moisture can result in what is called "profile rusting" which can be seen and felt on the surface of the cast iron cookware. Luckily, profile rusting is easily removed at home in an afternoon. For severe rust that covers most of the cookware, take the cookware to a machine shop to have it sandblasted and restored to raw cast iron, then season immediately.
How To Restore a Rusty Cast Iron Cookware
Use fine steel wool to remove rust from affected areas. Scour the cookware until the area returns to raw cast iron. Then follow the steps for seasoning the skillet in our manual.