Does cast iron rust?

Yes it does rust.

Ductile iron has better corrosion resistance than gray cast iron. Some alloy cast iron has better corrosion reisistance. However, any cast iron will rust without any surface treatment.

There are many types of surface treatments that can pretect iron castings from rust, such as primer painting, surface painting, electophoretic coating and zink galvanizing etc. As for the detailed information about surface coatings of metal castings, please refer to the links below "Surface Coating of Iron Castings ".