Traud, corrosion

Fig. 1.29 Wagner-Traud method of representing (a) a single reversible reaction and (b) a corrosion reaction (note that E orr the potential when = 4)...

The main system chosen by Wagner and Traud themselves was the corrosion of zinc amalgam in aqueous HCl. They measured the current-potential curves of... 

Further, these anodic and cathodic reactions can occur spatially at adjacent locations on the stuface of a metal electrode rather than on two separated metal electrodes as shown in Fig. 11-1, where the anodic dissolution of iron and the cathodic reduction of hydrogen ions proceed simultaneously on an iron electrode in aqueous solution. The electrons produced in the anodic dissolution of iron are the same electrons involved in the cathodic reduction of hydrogen ions hence, the anodic reaction cannot proceed more rapidly than that the electrons can be accepted by the cathodic reaction and vice versa. Such an electrode at which a pair of anodic and cathodic reactions proceeds is called the mixed electrode . For the mixed electrodes, the anode (current entrance) and the cathode (current exit) coexist on the same electrode interface. The concept of the mixed electrode was first introduced in the field of corrosion science of metals [Evans, 1946 Wagner-Traud, 1938]. 

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. 

The corrosion behavior of semiconductors can, in principle, be described within the framework of the same concepts as for metals (see, for example, Wagner and Traud, 1938), but with due account for specific features in the electrochemical behavior of a solid caused by its semiconducting nature (Gerischer, 1970). One of the main features is photosensitivity related to a change in the free-carrier concentration under illumination. Photosensitivity underlies the phenomenon of photocorrosion. 

Wagner and Traud [141] developed the theory of mixed potentials in order to explain the corrosion of electrode surfaces. This theory assumes that the measurable current—potential curves for an electrode where more than one electrochemical reaction takes place simultaneously is represented by... 

The basic mechanism for the instability of ultrapure metals was suggested by Wagner and Traud in a classic paper in 1938.1 The essence of their view is that for corrosion to occur, there need not exist spatially separated electron-sink and -source areas on the corroding metal. Hence, impurities or other heterogeneities on the surface are not essential for the occurrence of corrosion. The necessary and sufficient condition for corrosion is that the metal dissolution reaction and some electronation reaction proceed simultaneously at the metal/environment interface. For these two processes to take place simultaneously, it is necessary and sufficient that the corrosion potential be more positive than the equilibrium potential of the M, + + ne M reaction and more negative than the equilibrium potential of the electronation (cathodic) reaction A + ne — D involving electron acceptors contained in the electrolyte (Fig. 12.8). 

Consider a system consisting of a metal corroding in an electrolyte. The corrosion process involves a metal-dissolution deelectronation (anodic) reaction at electron-sink areas on the metal and an electronation (cathodic) reaction at electron-source areas. (This picture is applicable to a metal s corroding by a Wagner-Traud mechanism provided one imagines the sink and source areas shrunk to atomic-sized dimensions and considers the situation at one instant of time.)... 

It follows that the corrosion potential on a heterogeneous metal corroding by local-cell action is virtually equal to the mixed potential at an electrode on which electronation and deelectronation reactions are occurring on spatially separated sinks and sources and is identical to a mixed potential when the metal is corroding homogeneously by a Wagner-Traud mechanism. The concept of the corrosion current /corr and the corrosion potential 40corr will now be treated quantitatively. 

C. Wagner and W. Traud, Z. Elektrochem. 44 391 (1938). The original formulation of the mixed potential concept and the basic theory of corrosion of a pure metal. 

T. P. Hoar, who was co-discoverer with U. R. Evans (in 1936) of the basic facts about the electrochemical mechanism of corrosion that led to Wagner and Traud s seminal theory, told about an episode from his early days as a corrosion consultant. Approached by an automotive concern for an inhibitor to stop the distressing breakdowns of its 1930s car radiators, he busied himself in his lab over a weekend and created (stumbled upon ) a potent organic inhibitor for the system concerned. The client wanted to pay a handsome fee, but Hoar wisely tempered his enthusiasm and humbly asked for just a few cents for every time the inhibitor was used. His decision, he says, provided him with a significant income for more than a decade. 

In 1939 Wagner and Traud used a flat and featureless plate to portray the basic theory of corrosion for a reason such extreme simplification of the modeled system made it possible to present a statement of the basic idea of why corrosion occurs. However, real, actual corrosion, corrosion in technology, in moist environments everywhere, does not occur uniformly over the entire area of flat plates. It is nearly... 

However, for the anodic reaction to occur, there must be a corresponding cathodic reaction—hence the reference to corrosion and the Wagner-Traud hypothesis. What is the counter-reaction that takes up the electrons rejected to the underlying semiconductor, thus setting up a mixed potential ... 

These examples (Farrington, 1996) illustrate the detail possible with these tools and the extremely heterogeneous nature of real corrosion in contrast to the image used in the Wagner-Traud model of corrosion, i.e., a uniform plane. 

Metals are obtained by the treatment of oxides and sulfide ores found in the earth. However, there is an initial difficulty—the desirable ores are often mixed up with those of little commercial value, and the problem is to obtain the desired ore free from those of lesser worth. For many years now, largely due to the initiative of Australian workers, it has been possible to find organic substances which, when added to a suspension of mixed ores, pick out the desired one, and (when air is bubbled into the system) float it to the surface, from which it can be raked off, i.e., separated and made available for chemical or electrochemical extraction of the metal. It turns out that the basis of this mineral flotation technology involves the Wagner and Traud mixed-potential concept and is thus indirectly related to corrosion theory. 

Although the author believes that the generalized concept was originally responsible for the electrochemical treatment of corrosion processes by the early workers, it appears that Hammett and Lorch (23) and Frumkin (24) were among the first to specifically describe metallic dissolution according to this concept. Wagner and Traud (16) showed that the electrode kinetics for hydrogen evolution are not affected by the simultaneous dissolution of the metallic ions. 

Solid materials, in general, are more or less subject to corrosion in the environments where they stand, and materials corrosion is one of the most troublesome problems we have been frequently confronted with in the current industrialized world. In the past decades, corrosion science has steadily contributed to the understanding of materials corrosion and its prevention. Modem corrosion science of materials is rooted in the local cell model of metallic corrosion proposed by Evans [1] and in the mixed electrode potential concept of metallic corrosion proved by Wagner and Traud [2]. These two magnificent achievements have combined into what we call the electrochemical theory of metallic corrosion. It describes metallic corrosion as a coupled reaction of anodic metal dissolution and cathodic oxidant reduction. The electrochemical theory of corrosion can be applied not only to metals but also to other solid materials. 

Equation (41) is identical in form to Eqs. (18 and 24). The curve is centered around Ecorr rather than and the current density at zero overpotential is icorr instead of io- This expression, along with the theory for mixed potentials, was derived by Wagner and Traud, and therefore will be referred to as the Wagner-Traud equation. As described in the Chapter 7.3.1.2 on experimental techniques, the Wagner-Traud equation is used in software analysis packages that accompany modem computer-controlled potentiostats. A nonlinear least squares fit of this equation to the experimental data provides values of corr. corr. ha. and he vvith the assumption that perfect Tafel behavior is observed for both the anodic and cathodic reactions, and that the extrapolations of the Tafel portions of the curves both intersect at the corrosion potential. 

Corrosion Rate Measurement by Fitting Polarization Curve to Wagner-Traud Equation... 

Based on low-field approximation, a simple procedure for the evaluation of corrosion currents and corrosion rates was developed in 1938 by Wagner and Traud [22]. Stern and Geary [23] and Stern [24,25] developed an experimental procedure for measuring the corrosion rates known as the linear polarization technique. This technique wiU be discussed in detail in Chapter 5. 

L. I. Antropov, Theoretical Electrochemistry, Translated from Russian, MIR Publishers, Moscow, 1972. C. Wagner, W. Traud, On the interpretation of corrosion processes through superposition of electrochemical partial processes and on the potential of mixed electrodes, Z. Elektrochem. 44 (1938)... 

Cathodic protection (CP) is defined as the reduction or elimination of corrosion by making the metal a cathode by means of impressed current or sacrificial anode (usually magnesimn, aluminum, or zinc) [11]. This method uses cathodic polarization to control electrode kinetics occurring on the metal-electrolyte interface. The principle of cathodic protection can be explained by the Wagner-Traud mixed potential theory [12]. 

Polarisation methods involve changing the potential of the WE and monitoring the current which is produced as a function of time or potential. One of the most relevant physical quantities measured by DC polarisation methods is linear polarisation resistance (LPR). Its definition is based on the mixed potential theory proposed by Wagner and Traud [4], that explains the corrosion reactions by assuming that cathodic and anodic partial reactions occur at the metal-electrolyte interface at a certain corrosion (or mixed ) potential,... 

Wagner, C., and W. Traud. 1938. The interpretation of corrosion phenomena by super-imposition of electrochemical partial reactions and the formation of potentials of mixed electrodes. Zeitschrift finer Elektrochemie und Angewandte Physikalische Chemie 44 391. 

Electrode kinetics is the study of reaction rates at the interface between an electrode and a liquid. The science of electrode kinetics has made possible many advances in the understanding of corrosion and the practical measurement of corrosion rates. The interpretation of corrosion processes by superimposing electrochemical partial processes was developed by Wagner and Traud [1]. Important concepts of electrode kinetics that wifi be introduced in this chapter are the corrosion potential (also called the mixed potential and the rest potential), corrosion current density, exchange current density, and Tafel slope. The treatment of electrode kinetics in this book is, of necessity, elementary and directed toward application of corrosion science. For more detailed discussion of electrode kinetics, the reader should refer to specialized texts Usted at the end of the chapter. 

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... 

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