Corrosion mixed-potential theory processes

An electrochemical model for the process of electroless metal deposition was suggested by Paunovic (10) and Saito (8) on the basis of the Wagner-Traud (1) mixed-potential theory of corrosion processes. According to the mixed-potential theory of electroless deposition, the overall reaction given by Eq. (8.2) can be decomposed into one reduction reaction, the cathodic partial reaction. 

Given sufficient quantitative information about the electrochemical processes occurring, mixed potential theory can be used to predict a corrosion rate. Unfortunately, in the vast majority of cases, there are few data that can be applied with any confidence. In general, experimental measurements must be made that can be interpreted in terms of mixed potential theory. The most common of these measurements in electrochemical corrosion engineering is the polarization curve. 

The information required to predict electrochemical reaction rates (i.e., experimentally determined by Evans diagrams, electrochemical impedance, etc.) depends upon whether the reaction is controlled by the rate of charge transfer or by mass transport. Charge transfer controlled processes are usually not affected by solution velocity or agitation. On the other hand, mass transport controlled processes are strongly influenced by the solution velocity and agitation. The influence of fluid velocity on corrosion rates and/or the rates of electrochemical reactions is complex. To understand these effects requires an understanding of mixed potential theory in combination with hydrodynamic concepts. 

A common classification of inhibitors is based on their effects on the electrochemical reactions involved in the corrosion process. In the framework of mixed potential theory (see Chapter 1.3, this volume), these effects are most conveniently visualized by -log j I diagrams, such as shown in Fig. 1. For a freely corroding metal, the corrosion potential Eq and the corrosion... 

This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

In order to explain the corrosion process of metals, Wagner and Traud [54] developed the mixed potential theory, which assumes that the current-potential relationship is given by... 

Sedriks et al. [20] and Bakulin et al. [19] found that Ecggn of B/Al MMCs was active to that of their monolithic matrix aUojrs in aerated NaCl solutions. That behavior does not appear to comply with the mixed-potential theory. Bakulin et al. [79], however, found that hot-pressed stacks of aluminum foil processed in the same way as the MMC (but without the BFs) have Ecorr values that are active to the MMCs. The only difference between the monolithic aluminum and the hot-pressed stacks of aluminum foil was crevices in the diffusion bonds between adjacent foils. The crevices, which are sources of additional anodic sites, can polarize the stacks to active potentials. Thus, the B/Al MMCs are actually noble to the matrix material processed in the same way, emd the Ecorr values actually coincide with the mixed-potential theory. Sedriks et al. [20] found that increasing the volume fi ac-tion of BF caused anodic current densities (w.r.t. matrix area) to increase. This implies that BF-matrix interfaces, which increase with BF content, were also sources of anodic sites. Evans and Braddick [89] also reported that BF-matrix interface regions were severely attacked in an oxygenated NaCl solution. These reports indicate that the BF-matrix and foil-foil interfaces are major causes of corrosion. 

However, closer examination showed that the Cu-Ni system, immediately after the interruption of current, acquires an open circuit potential (or a mixed potential) which hes above the nickel reversible potential and below the copper reversible potential. In addition, the pH of these solutions, i.e. pH = 4, does not allow the formation of nickel oxides on the surface of the nickel. Therefore, in accordance with the mixed potential theory, copper continues to deposit at the expense of nickel dissolution, forming a classic galvanic corrosion cell. This process does not... 

This more generalized theory has some important experimental and practical implications. In particular it allows greater use of polarization data for the interpretation of corrosion phenomena and for the determination of the rate of corrosion during the corrosion process. For purposes of discussion we may designate this as the mixed potential technique. 

The anodic partial process. Equation 46, generates the electrons which are used in the cathodic partial process, Equation 47. This model of corrosion processes is based on the theory of mixed potentials (11) and is shown schematically in Figure 9. The original theory of mixed potentials was based on the "superposition" of polarization curves for the respective partial processes (11-13). However, since many mixed potential systems (particularly corrosion processes) involve interactions among the reactants, the presentation of mixed potentials given here will consider the more recent approach considering these interactions (14). 

This chapter is coniined to analyze the complex aqueous corrosion phenomaion using the principles of mixed-potential, which in turn is related to the mixed electrode electrochemical corrosion process. This theory has been introduced in Chapter 3 and 4 as oxidation and reduction electrochemical reactions. Basically, this Chapter is an extension of the principles of electrochemistry, in which partial reactions were introduced as half-cell reactions, and their related kinetics were related to activation and concentration polarization processes. The principles and concepts introduced in this chapter represent a unique and yet, simplified approach for understanding the electrochemical behavior of corrosion (oxidation) and reduction reactions in simple electrochemical systems. 

In his own paper, Frumkin also analyzed the effect of the sodium amalgam concentration on the rate of electrolysis of water, and arrived at the conclusirm that even for physically and chemically uniform surfaces, the processes of anodic metal dissolution and cathodic hydrogen evolution could occur simultaneously at the same potential. This same idea was later used by K. Wagner and W. Traud (1938) in their formulation of the theory of the mixed potential, the cornerstone of modem corrosion theory. 

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