Passivity–the key to our metals-based civilization

Abstract

Humankind has been able to develop a metals-based civilization primarily because the reactive metals (Fe, Ni, Cr, Al, Ti, Zr, . . .) exhibit extraordinary kinetic stabilities in oxidizing environments. From the time of Schonbein and Faraday (1830s), the reason for this stability has been attributed to the existence of a thin reaction product ®lm on the metal (or alloy) surface. This ®lm effectively isolates the metal from the corrosive environment. However, attempts to elucidate the mechanisms of the formation of passive oxide ®lms, which generally comprise bilayer structures consisting of a defective oxide that grows directly into the metal and an outer, precipitated hydroxide (or oxyhydroxide on even oxide) layer, have yielded only a rudimentary understanding of the chemistry and physics of the growth and breakdown processes. In this paper, selected aspects of passivity and passivity breakdown are reviewed, with emphasis on the physical models that have been proposed to account for the experimental observations. One such model, the Point Defect Model, is shown to account for most, if not all, experimental observations, and to provide a robust basis for predicting the occurrence of passivity breakdown in any given system. By combining the Point Defect Model with deterministic models for pit growth and crack growth, it is now possible to predict the evolution of localized corrosion damage in a wide range of systems.

Investigation of corrosion defects in titanium by positron annihilation

“…Our research has confi rmed this effect [4,8]. The so-called point defect model (PDM) of corrosion [9] describes the huge role of lattice defects in the process of transferring cations outside and anions into the metal. Linhardt and coworkers [10] demonstrated that as long as the metal is passive, the magnetic fi eld has no impact on the kinetics of corrosion pit formation in AISI 304 austenitic steel, but once the passive layer is broken or disappears, the magnetic fi eld infl uences the rate at which pits are formed.…”

Perspective—Localized Corrosion: Passive Film Breakdown vs Pit Growth Stability

“…Related processes are described by the Point Defect Model, [12][13][14][15] which suggests that transport of cation vacancies through the oxide lattice and their condensation at the metal/film interface form a vacancy that leads to breakdown. Voids can also form by the Kirkendall effect as exhibited by oxides formed during room temperature oxidation of Fe nanoparticles.…”

Characterization of the passive films on electrodeposited Fe-Ni-Cr alloys in borate solution at pH 8.4

“…However, it becomes essentially resistive (semicircle in the Z plot) if the dissolution/growing of the passive film and cathodic reaction occur through the layer and at the oxide-solution interface. It has also been reported that the processes of passive dissolution at the anodic potential are governed by the electrical and mass transport properties of the passive layer [24][25][26][27]. However, at the mixed potential, the corrosion processes at the oxide/solution interface also need to be further investigated.…”

“…Therefore the anodic reaction is governed by electron and transport properties of the passive layer. Taking into ac- count the point defect model described in the literature by Macdonald et al [24,25] and modified by Bojinov et al [26,27] for the description of the passive dissolution of the anodically formed passive films of Fe-Cr and Ni-Cr alloys, the transport of interstitials or vacancies ¶ involves the outward movement of the cation defects (mainly Fe…”