Corrosion electrochemistry passivity

This book consists of nine chapters. The second chapter provides an overview of the important thermodynamic and kinetic parameters of relevance to corrosion electrochemistry. This foundation is used in the third chapter to focus on what might be viewed as an aberration from normal dissolution kinetics, passivity. This aberration, or peculiar condition as Faraday called it, is critical to the use of stainless steels, aluminum alloys, and all of the so-called corrosion resistant alloys (CRAs). The spatially discrete failure of passivity leads to localized corrosion, one of the most insidious and expensive forms of environmental attack. Chapter 4 explores the use of the electrical nature of corrosion reactions to model the interface as an electrical circuit, allowing measurement methods originating in electrical engineering to be applied to nondestructive corrosion evaluation and... 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

Helfand, M. and Clayton, C. R., "Variable Angle XPS Studies of Passive Ni-P-Cr Metallic Glasses, Corrosion, Electrochemistry and Catalysis of Metastable and Intermetallic, C. R. Clayton and K. Hashimoto, Eds., The Electrochemical Society, Pennington, NJ, 1993. 

A great many papers have been written about the important role that solution anions play in corrosion and passivation, especially of iron. An excellent ehapter was published some years ago by Hensler in Encyclopedia of Electrochemistry of the Elements [68]. Examples of other, more recent publications are artieles by Sato [69] and Kuznetsov and Valuev [70]. The coneept of solution anions interacting with the electrode surfaee and forming surfaee-ligand eomplexes, as well as influencing the potential distribution at the surfaee, is being developed. It is becoming apparent that in order to understand the meehanism of passivation and its breakdown, it is necessary to understand both the eleetrode and the electrolyte solution and the interaction between these two eomponents of the eorrosion process. 

The thermodynamic properties and the application of electrode kinetics have been described. As corrosion and passivity are in principle determined by electrochemistry at metal surfaces, a good understanding of the equilibria and the kinetics of electrode surfaces is a necessary requirement for any further study with more sophisticated methods. This involves complicated transients studies as well as electrochemical methods like the RRD electrode with or without hydrod5mamical modulation. Here a systematic research is still needed for a better understanding of pure metals and especially alloys. Results for simplified conditions give answers for the often more complicated situation of corroding systems in a real environment. 

V. Maurice, P. Marcus, Structure, corrosion and passivation of metal surfaces, in Modern Aspects of Electrochemistry, Vol. 46, S.-I. Pyun, editor. Springer, New York (2009), pp. 1-58. 

As outlined above, electron transfer through the passive film can also be cmcial for passivation and thus for the corrosion behaviour of a metal. Therefore, interest has grown in studies of the electronic properties of passive films. Many passive films are of a semiconductive nature [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102 and 1031 and therefore can be investigated with teclmiques borrowed from semiconductor electrochemistry—most typically photoelectrochemistry and capacitance measurements of the Mott-Schottky type [104]. Generally it is found that many passive films cannot be described as ideal but rather as amorjDhous or highly defective semiconductors which often exlribit doping levels close to degeneracy [105]. 

The lag between the time that nitinol, was first produced and the time it was used commercially in medical devices was due in part to the fear that nickel would leach from the metal and not be tolerable as a human implant. As it turns out, with a correct understanding of the surface electrochemistry and subsequent processing, a passivating surface layer can be induced by an anodizing process to form on the nitinol surface. It is comprised of titanium oxide approximately 20 mn thick. This layer actually acts as a barrier to prevent the electrochemical corrosion of the nitinol itself. Without an appreciation for the electrochemistry at its surface, nitinol would not be an FDA-approved biocompatible metal and an entire generation of medical devices would not have evolved. This is really a tribute to the understanding of surface electrochemistry within the context of implanted medical devices. 

Alloys represent special problems for -pH diagrams. Overlaying of the diagrams for the individual elements has shown some, albeit limited, promise (16). The problem may indicate that the effects of alloying are manifested more in the kinetics than in the thermodynamics of corrosion and electrochemistry. For example, passivity of pure iron in acid is not predicted from -pH diagrams it is a kinetic effect. 

Refs. [i] Strehblow HH (2003) Passivity of metals. In Alkire RC, Kolb DM (eds) Advances in electrochemical science and engineering. Wiley-VCH, Weinheim, pp 271-374 [ii] Vetter KJ, Gorn F (1973) Electrochim Acta 18 321 [Hi] Strehblow HH (2002) Mechanisms of pitting corrosion. In Marcus P (ed) Corrosion mechanisms in theory and practice. Marcel Dekker, New York, pp 243-285 [iv] Strehblow HH (2003) Pitting corrosion. In Bard AJ, Stratmann M, Frankel GS (eds) Corrosion and oxide films. Encyclopedia of electrochemistry, vol. 4. Wiley, Weinheim, 337... 

Ref. [i] Schultze JW, Hassel AW (2006) Passivity of metals, alloys, and semiconductors. In Bard A], Stratmann M, Frankel GS (eds) Corrosion and oxide films. Encyclopedia of electrochemistry, vol. 4. Wiley-VCH, Weinheim, pp 216-270... 

In spite of some lack of detail, the main aspects of the subject have been covered, it is hoped impartially and adequately. There has been some tendency in recent electrochemical texts to pay scant attention to the phenomena at active electrodes, such as overvoltage, passivity, corrosion, deposition of metals, and so on. These topics, hich aie of importance in applied electrochemistry, are treated here at sucli length as seems reasonable. In addition, in view of the growing interest in electrophoresis, and its general acceptance as a branch of electrochcun-istry, a chapter on electrokinetic phenomena has been included. 

High temperature and high pressure processing of materials often involves the use of supercritical fluids. Corrosion studies are quite essential for evaluation of the equipment in supercritical fluid operations. Previous electrochemical measurements for alloys in supercritical fluids are rare (1-1). The reported measurements (3) show that passivation of iron alloys is different at supercritical conditions compared to ambient conditions. The study of the electrochemistry of iron alloys can lead to control of corrosion of equipment utilizing the alloys. Thermodynamic analysis provides the information about stable species, i.e. corrosion products under given temperatures and pressures. 

Fig. 14 Electrochemistry of propagating crevice corrosion. Two cases are shown (a) the material in the crevice corrodes actively and (b) the material in the crevice undergoes an active-passive transition.

C.M. Rangel, T.M. Sdva, M. da Cunha Belo, Semiconductor electrochemistry approach to passivity and stress corrosion cracking susceptibility of stainless steels, Electrochim. Acta 50 (2005) 5076-5082. 

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