Passivity, as Seen Through the Eyes of Electrochemical Impedance Spectroscopy

Digby D. Macdonald, Department of Nuclear Engineering, University of California at Berkeley, Berkeley, CA, USA

Abstract

Passivity, in which a metal or alloy assumes kinetic stability in the face of high thermodynamic reactivity has been termed “The Reason for Our Reactive Metals-Based Civilization” in which metals such as Fe, Ni, Cr, Al, Zr, Ti, and Mg can be used to fabricate high precision machines that display useful service lives (typically 40 to 80 years). While the mechanism of the growth and breakdown of passive films is still a matter of some controversy, at least as far as the details are concerned, one model, the Point Defect Model (PDM) has successfully provided a broad framework within which the phenomenon may be considered. Passive films are known to form as bi-layer structures comprising a point defective barrier layer that grows into the substrate metal via the generation of anion vacancies at the metal/barrier layer (m/bl) interface and a porous, outer layer (ol) that forms via the hydrolysis of cations that are transmitted through the bl at the barrier layer/solution (bl/s) interface. For the transition metals, the bl is typically 1 – 8 nm thick whereas no limit is placed upon the thickness of the ol, and the bl, at least, forms as a metastable phase. In this presentation, I will discuss the formation of point defective oxide, sulfide, and hydride barrier layers on various reactive metals in aqueous solutions containing anions of those species using Electrochemical Impedance Spectroscopy (EIS) as the experimental probe. Optimization of the impedance version of the PDM, ImPDM, on wide band EIS data will be discussed in detail to illustrate the kind of information that can be gleaned from studies of this type.

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