(Corrosion Division H.H. Uhlig Award) Perspectives on Passivity and Its Breakdown
Authors: Natishan Paul M. 
Published: February 2020

Aluminum, stainless steels, and many other metals and alloys are protected by a passive film that protects the surface from corrosion. However, in environments that contain aggressive anions such as chloride, Cl‾, the passive film becomes unstable and degrades locally causing oxide film breakdown and pitting corrosion. This presentation will discuss two aspects related to passivity and its breakdown that have been a focus of our research; the surface charge character of the oxide film (1-2) and chloride adsorption and incorporation into the oxide film (3-6). Most of the discussion will be related to passivity and its breakdown of aluminum, Al, in Cl‾ solutions.
The first step in the initiation of metastable or stable pitting corrosion of Al is the adsorption of Cl‾ on the surface of the passive oxide. Chloride adsorption by the oxide film has been primarily discussed in terms of surface attractive forces. The isoelectric point (IEP) or pH of zero charge (pHpzc) of an oxide defines the surface charge character as a function of pH. The pHpzc of Al is ~ 9 in aqueous solutions. Thus, at pH values less than 9, which is the case for most natural water systems, the surface charge of Al is positive and this leads to the attraction of negative ions such as Cl‾ to the surface. The relationship between the pitting potential value of oxide-covered Al and the pHpzc, of its oxide was studied by ion implanting aluminum (99.999% purity) with elements whose oxides were of known pHpzc (1-2). The results showed that all the implanted ions chosen because of the low pHpzc of their oxides, i.e. lower than that of Al, (Si, Cr, Zr, Nb, Ta, and Mo) produced binary surface alloys that had higher (more positive) pitting potential values than Al. Conversely, the implanted ions chosen because of the high pHpzc of their oxides relative to Al (Li and Zn) produced binary surface alloys that had pitting potential values that were lower than that of Al. The implantation of Al into Al had no effect on the pitting potential, indicating that the increased and decreased pitting potential values observed for the other implant ions were chemical in nature and not a result of the ion implantation process.
The second aspect of this presentation will discuss Cl‾ adsorption and incorporation into the oxide film. X-ray absorption spectroscopy (XAS), specifically X-ray absorption near edge spectra (XANES), and X-ray photoelectron spectroscopy (XPS) were used to investigate Cl‾ adsorption and incorporation into the oxide film on aluminum (3-6). Samples were polarized at selected potentials below the pitting potential in deaerated 0.1M NaCl solutions. XANES and XPS spectra both showed the presence of two distinct Cl‾ species associated with the oxide film on Al for samples that were polarized below the pitting potential. Chloride was found to be present as an adsorbed species at the surface, as well as an incorporated species within the passive oxide.

Acknowledgements

The author gratefully acknowledges the Naval Research Laboratory and the Office of Naval Research for financial support of this work. The author also gratefully acknowledges his colleagues especially the co-authors of the cited papers without whose contributions this work would not have been possible.

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