Corrosion electrode kinetics

Haynie, F. H. and Ketcham, S. J., Electrochemical Behaviour of A1 Alloys Susceptible to Intergranular Corrosion. Electrode Kinetics of Oxide-covered Al , Corrosion, 19,403t (1963) Ketcham, S. J. and Haynie, F. H., Electrochemical Behaviour of Al Alloys Susceptible to Intergranular Corrosion. Effect of Cooling Rate on Structure and Electrochemical Behaviour in 202A Al Alloy , Corrosion, 19, 242t (1963)... 

In this section the interaction of a metal with its aqueous environment will be considered from the viewpoint Of thermodynamics and electrode kinetics, and in order to simplify the discussion it will be assumed that the metal is a homogeneous continuum, and no account will be taken of submicroscopic, microscopic and macroscopic heterogeneities, which are dealt with elsewhere see Sections 1.3 and 20.4). Furthermore, emphasis will be placed on uniform corrosion since localised attack is considered in Section 1.6. 

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

The thermodynamic and electrode-kinetic principles of cathodic protection have been discussed at some length in Section 10.1. It has been shown that, if electrons are supplied to the metal/electrolyte solution interface, the rate of the cathodic reaction is increased whilst the rate of the anodic reaction is decreased. Thus, corrosion is reduced. Concomitantly, the electrode potential of the metal becomes more negative. Corrosion may be prevented entirely if the rate of electron supply is such that the potential of the metal is lowered to the value where it is found that anodic dissolution does not occur. This may not necessarily be the potential at which dissolution is thermodynamically impossible. 

The principle of electrochemical noise experiments is to monitor, without perturbation, the spontaneous fluctuations of potential or current which occur at the electrode surface. The stochastic processes which give rise to the noise signals are related to the electrode kinetics which govern the corrosion rate of the system. Much can be learned about the corrosion of the coated substrate from these experiments. The technique of these measurements is discussed elsewhere (A). 

In recent years, electrochemical charge transfer processes have received considerable theoretical attention at the quantum mechanical level. These quantal treatments are pivotal in understanding underlying processes of technological importance, such as electrode kinetics, electrocatalysis, corrosion, energy transduction, solar energy conversion, and electron transfer in biological systems. 

The introduction of 0 in the equations for current density need by no means refer only to the adsorbed intermediates in the electrode reaction. What of other entities that may he adsorbed on the surface For example, suppose one adds to the solution an oiganic substance (e.g., aniline) and this becomes adsorbed on the electrode surface. Then, the 0 for the adsorbed organic substance must also be allowed for in the electrode kinetic equations. So, in Eq. (7.149), the value of 0 would really have to become a 0, where the summation is over all the entities that remain upon the surface and block off sites for the discharging entities. Many practical aspects of electrodics arise from this aspect of the Butler-Volmer equation. For example, the action of organic corrosion inhibitors partly arises in this way (adsorption and blocking of the surface of the electrode and hence reduction of the rate of the corrosion reaction per apparent unit area).67... 

Current and potential distributions are affected by the geometry of the system and by mass transfer, both of which have been discussed. They are also affected by the electrode kinetics, which will tend to make the current distribution uniform, if it is not so already. Finally, in solutions with a finite resistance, there is an ohmic potential drop (the iR drop) which we minimise by addition of an excess of inert electrolyte. The electrolyte also concentrates the potential difference between the electrode and the solution in the Helmholtz layer, which is important for electrode kinetic studies. Nevertheless, it is not always possible to increase the solution conductivity sufficiently, for example in corrosion studies. It is therefore useful to know how much electrolyte is necessary to be excess and how the double layer affects the electrode kinetics. Additionally, in non-steady-state techniques, the instantaneous current can be large, causing the iR term to be significant. An excellent overview of the problem may be found in Newman s monograph [87]. 

In this chapter, we will review the fundamental models that we developed to predict cathode carbon-support corrosion induced by local H2 starvation and start-stop in a PEM fuel cell, and show how we used them to understand experiments and provide guidelines for developing strategies to mitigate carbon corrosion. We will discuss the kinetic model,12 coupled kinetic and transport model,14 and pseudo-capacitance model15 sequentially in the three sections that follow. Given the measured electrode kinetics for the electrochemical reactions appearing in Fig. 1, we will describe a model, compare the model results with available experimental data, and then present... 

As COR and OER occur simultaneously in the cathode, their kinetics are particularly important in evaluating carbon-support corrosion. The kinetics of OER is material-specific, dependent on catalyst composition and electrode fabrication.35,37 -39 A number of OER kinetics studies were done on Pt metal electrodes.37-39 However, there is a lack of OER kinetics data on electrodes made of Pt nano-particles dispersed on carbon supports. Figure 2 shows the measured OER current density with respect to the overpotential defined by Eq. (6).35 The 02 concentration was measured at the exit of a 50-cm2 cell using a gas chromatograph (GC). The 02 evolution rate (= 02 concentration x cathode flow rate) was then converted to the OER current density, assuming 4e /02 molecule. Diluted H2 (10%) and a thicker membrane (50 p,m) were used in the measurement to minimize H2 crossover from anode to cathode, because H2 would react with 02 evolved at the cathode and incur inaccuracy in the measured OER current density. Figure 2 indicates that the OER... 

A. Review of the Governing Electrode Kinetics in Corrosion Processes... 

Jan. 26, 1927, Farnborough, Great Britain - July 9, 2005, Ottawa, Canada) Canadian electrochemist, 1946-1949 Imperial College, London University, thesis on -> electrocatalysis and corrosion inhibitors (supervisor J.O M. Bockris), 1949-1954 Chester-Beatty Cancer Research Institute with J.A.V. Butler on DNA, 1954-1955 post-doc at University of Pennsylvania with J.O M. Bockris (among other subjects -> proton -+ mobility, the effect of field-induced reorientation of the water molecule), since 1956 professor at the University of Ottawa (Canada), more than 400 publications on physical electrochemistry, electrode kinetics and mechanisms, - electrochemical capacitors. 

July 13,1916 in Berlin, Germany - Dec. 12,1974 in Berlin, Germany) Study of chemistry and physics in Gottingen and Berlin from 1955 professor of physical chemistry at the Free University of Berlin. Cooperation with K. F. Bonhoeffer, - Gerischer, and J. W. Schultze, almost 100 publications (fundamentals of electrode kinetics, overpotential, redox reactions, ion transfer, passivity and corrosion, especially of iron), most important is his book [i] ... 

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. 

This book is recommended for scientists and engineers in chemistry, chemical engineering, materials science, corrosion, battery development, and electroplating. Graduate students in electrochemistry, electrochemical engineering, and materials science will also find it a challenging and stimulating introduction to electrode kinetics., ... 

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

Simple but pedagogically useful theories of electrode kinetics are presented in Chapter 3. This permits discussion of models for anodic and cathodic reactions at the metal/environment interface and for diffusion of species to and from the interface. Mathematical models of these theories lead to so-called kinetic parameters whose values govern the rate of the interface reaction. The range of values that these parameters can have and some of the variables that can influence the values are emphasized since these will relate to understanding the influence of such factors as surface conditions (roughness, corrosion product films, etc.), corrosion inhibitors and accelerators, and fluid velocity on corrosion rates. This chapter also introduces electrochemical measurements to determine values of the kinetic parameters. 

These materials are highly efficient as a means of corrosion inhibition due to their ability to realize almost all inhibition mechanisms of metal corrosion, namely the barrier mechanism connected with the impenetrability of polymers for most corrosion media the inhibition mechanism induced by a specific action of the inhibitors on the corrosion process kinetics the protecting mechanism related to the effect of the polarizing charge formed in the plastic upon distribution of electrode potentials within the corrosive system,... 

Anodic protection was developed using the principles of electrode kinetics and is difficult to understand without introducing advanced concepts of electrochemical theory. Briefly, anodic protection is controlled by the formation of protective passive film on metals and alloys using an externally applied potential. Anodic protection is used to a lesser degree because of the limitations on metal-environment systems for which anodic protection is viable. In addition, it is possible to accelerate corrosion if proper controls are not implemented during anodic protection. 

The action of pigments is often evaluated by measuring the change in the electrode kinetics in solutions of the pigments. It could be shown that chromates act as cathodic inhibitors on zinc surfaces thereby increasing the corrosion resistance... 

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]. 

The concept of polarization in a corrosion cell can be explained by considering a simple galvanic cell, such as a Daniel cell, with copper and zinc electrodes. The Evans diagram of a Daniel cell shown in Fig. 3.5 is the basis for understanding the underlying corrosion process kinetics [26,27]. 

The effect of mass transfer on electrode kinetics is shown in Fig. 3.12. Many useful kinetic rate expressions based on Tafel conditions, mass transport limitations can be developed from Eq. (3.59). Prediction of mass transfer effects may be useful in corrosion systems depending on the system s corrosion conditions. The mass transport limitations in corrosion systems may alter the mixed potential of a corroding system. Under Tafel conditions (anodic or cathodic), Eq. (3.59) can be written as ... 

The solution velocity in this case is one of the major factors that control the corrosion potential and the corrosion rate in the active state of the alloy. The effect of mass transfer on electrode kinetics is discussed in detail in Chapter 3. Figure 4.11 correlates the... 

Equation (5.36) is used to calculate the corrosion rate of a system without knowledge of electrode-kinetic parameters. This approximation may not always result in accurate corrosion estimates. However, this equation provides a basis for rapid corrosion evaluation studies. 

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]. 

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