The Passive State in Our Reactive Metals-Based Civilization

The history of the development of the Point Defect Model (PDM) for the growth of passive films that form on reactive metal and alloy surfaces when in contact with corrosive environments is presented. Three generations of the PDM have been developed, with each addressing issues that have arisen from experiment or theory. The present review updates four previous reviews with more recent information, particularly with respect to the application of the PDM in accounting for phenomena observed in the growth of passive films under controlled electrochemical conditions. These topics include the adaptation of PDM to account for the potentiodynamic polarization of passive metals; for describing the evolution of passive systems under open circuit conditions; in the use of Mott-Schottky analysis to ascertain the electronic character of a passive film and hence to identify the dominant point defect; development of the theory of potentiostatic transients; and a description of the structures of passive films. Finally, the application of the PDM in modeling practical corrosion problems, such as the disposal of high-level nuclear waste, over ultra-long exposure times (10⁶ years). This latter application represents one of the greatest challenges ever posed to a sector of the scientific community.