Cast iron is a mixture of graphite (carbon) flakes in a matrix of steel (iron with carbon in solution). The graphite, which has the shape of corn flakes, does not contribute much to strength. If anything, it makes the cast iron somewhat porous or sponge like. The graphite does makes it easy to machine and has a dampening effect on the cast iron. However, it also makes for a lot of surface area, which allows plenty of air (oxygen) to get to the iron and form rust.