Visited HMS Warrior at Portsmouth recently ... ship was not in dry dock, but was still floating in the sea (built 1860, so that's over 140 years in salt water!)
On enquiring, was told that wrought iron "does not rust". On further investigation have found that whilst it does indeed rust, it does so very slowly.
Since wrought iron is simply cast iron that has been reheated, folded and beaten, why should this change the chemical reactivity in relation to rusting? ... any ideas?