The nature of the passive film on iron and ferrous alloys

A review is given of experiments that have contributed to a better understanding of the nature of the passive film on iron and ferrous alloys. The important questions relating to the nature of passive films on iron that are considered are: (1) the thickness—are the films two- or three-dimensional? (2) the composition—do the films contain hydrogen and cations introduced by alloying elements or inhibitors? (3) the structure—are the films crystalline spinel structures or non-crystalline glassy structures? and (4) the electronic properties—are the films insulators or semiconductors?