Think of chrome as a somewhat fragile coating for ferrous metal. As long as it is intact, it will protect the base metal from corrosion. But once there is a breach in the coating, corrosion of the ferrous metal will ensue, rapidly, under the right conditions. The problem is that chromium, not to mention the intermediate coatings between it and the parent ferrous material, is higher on the galvanic scale than iron, so once there is an electrical path between the iron and the other metals (because of a scratch or a pit, along with moisture or humidity, facilitating a path for ion transfer), the other metals will rob the iron of electrons, resulting in accelerated rusting of the iron.