Passivation of Metal Electrodes

In both cases of Eqns. 11-13 and 11-14, in which the corrosion reaction is controlled by the transfer of interfacial charge or by the diffusion of oxidants, the polarization resistance, Rf, can be used to estimate the metallic corrosion rate (corrosion current, 

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Oxide and salt layers on metal electrodes are of great practical value. Electrodes with thick phase layers are used in batteries, and varions types of thin layers will pro-dnce passivation of metals. 

Passivation of an electrode with respect to a certain electrochemical reaction is the term used for the strong hindrance experienced under certain conditions by a reaction which under other conditions (in the electrode s active state) will occur without hindrance at this electrode. Passivation of metals imphes the hindrance frequently observed with respect to anodic metal dissolution. 

A special problem can be the passivation of the electrode surface by insulating layers, for example, formation of oxides on metals at a too high anodic potential or precipitation of polymers in aprotic solvents from olefinic or aromatic compounds by anodic oxidation. As a result, the effective surface and the activity of the... 

It is weD known that metallic iron corrodes violently in dilute nitric acid solutions, but metallic iron is passivated in concentrated nitric add solutions as shown in Fig. 11-14(a). This passivation of metallic iron results from a strong oxidizing action of concentrated nitric add that changes the iron electrode irom the active state to the passive state. 

In the passive state, metal electrodes normally hold extremely small potential-independent dissolution current as shown in Figure 22.7 for metallic iron in acid solution. For some metals such as nickel, however, the passive state changes beyond a certain potential into the transpassive state, where the dissolution current, instead of being potential-independent, increases nearly exponentially with... 

Data on the chemistry and structure of thin oxide layers (passive films) produced by anodic polarization of metallic electrodes are necessary to understand and predict the properties of these films, in particular their corrosion resistance. There are now many available data on the chemical composition of passive films formed on metals and alloys. Surfece chemical analysis techniques have been, and still are, very useful to obtain such data. In sharp contrast, there is a lack of data on the structure of passive films. This is in part due to the difficulty of any structural analysis of very thin films on... 

The metal can enter into interactions with environment components and form compounds that are stable to corrosion during anodic dissolution. Passivity is the thermodynamic state of reaction in metals at which their corrosion retards abruptly. The transfer of a metal into the passive state is called passivation, and the layer formed on the metal electrode surface is called the passivating layer. Oxide and salt passivation of metals are distinguished by the composition of passivating layers. 

EIS examines the response of the corroding system to ac excitations at frequencies from 100 MHz to 100 KHz. The current or voltage is measured when a small amplitude voltage or current is applied to the working electrode. The method was initially used by DoHn and Ershler [50] and developed later by Randles and Somerton [51] and Grahame [52]. In corrosion research, EIS was extensively used to study the passivation of metals [53-57] to determine corrosion rates and to study the inhibitor performance, sacrificial and barrier coating performance, and the disbondment of polymer-coated metals [9,23,58—66]. Macdonald and Me Kubre pubHshed an excellent review on the use of EIS [67]. 

The earliest electrochemical studies of biological systems conducted with simple metal electrodes were not especially meaningful, yielding only irreversible behavior due to poor electron transfer between the metal center and the electrode, and passivation of the electrode caused by aggregation or denaturation of the biological species. Thus, early studies made use of indirect methods involving mediators to aid in electron transfer between the electrode and redox species. Kuwana and Hawkridge utilized potassium ferricyanide in coulometric titration of heme proteins as illustrated in Equations (28) and (29). ... 

The establishment of active-passive cells is facilitated by pH changes induced by the anodic and cathodic partial reactions. To understand this behavior, we recall first that passivation of metals is favored by a high pH (Chapter 6). The Evans diagram of Figure 7.17 shows the anodic partial current of a passivating metal exposed to an aerated electrolyte. Curve (b), which exhibits a lower passivation current than curve (a), corresponds to a solution of a higher pH. Oxygen is assumed to be reduced at the same rate on both metals. Clearly, at open circuit the electrode (a) is in the active state, while the electrode (b) is in the passive state. Electrical contact between the two therefore leads to the establishment of a corrosion cell in which metal (b) is the cathode. 

The motivation behind the Symposium on Electron Spectroscopy and STM-AFM Analysis of the Solid-Liquid Electrochemical Interface was to assemble in one place some major players in electrochemical surface science. The obvious rationale was that such a gathering would help distill and focus future work to issues deemed most critical to further progress in the area. The processes that were discussed at the symposium included electrodeposition and electrocrystallization, passivation of metals and alloys, anodic dissolution of metals and semiconductors, oxidation of small molecules, assembly of semiconducting layers, hydrogen adsorption, and charge transfer at surface-modified electrodes. 

The passivity of metals like iron, chromium, nickel, and their alloys is a typical example. Their dissolution rate in the passive state in acidic solutions like 0.5 M sulfuric acid may be seriously reduced by almost six orders of magnitude due to a poreless passivating oxide film continuously covering the metal surface. Any metal dissolution has to pass this layer. The transfer rate for metal cations from this oxide surface to the electrolyte is extremely slow. Therefore, this film is stabilized by its extremely slow dissolution kinetics and not by its thermodynamics. Under these conditions, it is far from its dissolution equilibrium. Apparently, it is the interaction of both the thermodynamic and kinetic factors that decides whether a metal is subject to corrosion or protected against it. Therefore, corrosion is based on thermodynamics and electrode kinetics. A short introduction to both disciplines is given in the following sections. Their application to corrosion reactions is part of the aim of this chapter. For more detailed information, textbooks on physical chemistry are recommended (Atkins, 1999 Wedler, 1997). 

The standard electrode potential is a thermodynamic parameter and expresses only whether a given reaction is possible or not, and nothing else. It says nothing about the kinetics of such a reaction, that is to say about the speed at which this reaction will proceed. It may happen that the rate of reaction is zero and that the reaction will thus not proceed at all. Passivation of metals is a good example of this phenomenon. 

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