Kinetics of corrosion processes

Electrochemical Kinetics of Corrosion Processes Mixed Potential Model of Corrosion. 

In modeling the kinetics of corrosive processes and quoting numbers of kinetic constants it is usually inherently implied that we have steady-state conditions with a constant activity of the corrosive agent in the environment and an infinite reservoir of the material to be corroded. 

Corrosion tests are intended to identify materials that provide adequate corrosion performance and service life at a low materials cost. To ensure this, as noted earlier, corrosionists evaluate the performance of materials, the effectiveness of protection strategies (coatings, inhibitors, cathodic protection), and the mechanism and kinetics of corrosion processes in specific environments in service tests, pilot plant tests, and laboratory tests. Increasingly, materials costs are being evaluated in the context of component or structure service life. In this way, factors such as maintenance, repairs, replacement, rehabilitation, process downtime, and interest rates are added into the cost of providing adequate corrosion performance over the service life of the component or structure. Finding or producing the necessary data from corrosion tests for such... 

MIC refers to the influence of micro-organisms on the kinetics of corrosion processes of metals, caused by micro-organisms adhering to the interfaces (usually called biofilm ). A prerequisite for MIC is the presence of micro-organisms. If the corrosion is influenced by their activity, further requirements are (1) an energy source, (2) a carbon source, (3) an electron donator, (4) an electron acceptor and (5) water [6, 7]. 

The i-E curves used in this chapter are related to the Evans diagram, beloved by corrosion chemists, but follow the conventions used throughout the rest of this book. While they are greatly simplified polarization diagrams, their usefulness lies in their ability to predict and rationalize the kinetics of corrosion processes. The figures show, in a diagnostic fashion, the following features ... 

This is a simplified treatment but it serves to illustrate the electrochemical nature of rusting and the essential parts played by moisture and oxygen. The kinetics of the process are influenced by a number of factors, which will be discussed later. Although the presence of oxygen is usually essential, severe corrosion may occur under anaerobic conditions in the presence of sulphate-reducing bacteria Desulphovibrio desulphuricans) which are present in soils and water. The anodic reaction is the same, i.e. the formation of ferrous ions. The cathodic reaction is complex but it results in the reduction of inorganic sulphates to sulphides and the eventual formation of rust and ferrous sulphide (FeS). 

It is quite natural that the thermodynamic approach does not allow photocorrosion processes to be described comprehensively. In a number of cases, kinetic peculiarities of reactions play an important role (see, for example, Bard and Wrighton, 1977) these peculiarities are caused by the effect of crystalline structure, state of the semiconductor surface, etc. A detailed description of a complicated reaction with several particles in the solution and crystal lattice involved usually encounters considerable difficulties. Therefore, at this stage the kinetic approach is used to reveal purely qualitative regularities of corrosion processes. 

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

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. 

If one wants to obtain a comprehensive understanding of the interaction between a metal (or metal alloy) and a hydrothermal solution, then electrochemical kinetics and/or corrosion studies must be carried out. In particular, an electrochemical system capable of reliably operating at temperatures above 300 °C should be developed. It is a matter of fact that there are almost no data on the exchange current densities and the anodic and cathodic transfer coefficients for even the most fundamental electrochemical reaction in high-temperature subcritical and supercritical aqueous systems. Even the primary HERs and OERs have been poorly studied at temperatures above 100 °C. Therefore, the creation of a well-established method for measuring electrochemical kinetics and corrosion processes over a wide range... 

The applications of TGA are extensive and diverse and include oxy-salt decompositions, natural and synthetic polymer characterization, metal oxidation and corrosion analysis, compositional analysis of coals, polymers, and rubbers, study of glass materials, foodstuffs, catalytic materials, biological materials, and a wide range of chemical processing phenomena. It has been used very successfully to study the kinetics of chemical processes however, there is much controversy surrounding this application, particularly in terms of relating TGA data to reaction kinetics models. 

From the thermodynamic point of view, a redox process would preferably proceed if the redox potential Fredox is located above Fdecomp as illustrated in Fig. 8.16a and, conversely, the decomposition reaction should dominate if Fredox occurs below Fdecomp (Fig. 8.16b). Many experimental investigations have shown, however, that such a picture is far too simple because the kinetics of both processes play a dominant role. Accordingly, it is very difficult to predict whether corrosion or the redox process will dominate under given circumstances. 

Polarization behavior relates to the kinetics of electrochemical processes. Study of the phenomenon requires techniques for simultaneously measuring electrode potentials and current densities and developing empirical and theoretical relationships between the two. Before examining some of the simple theories, experimental techniques, and interpretations of the observed relationships, it is useful to characterize the polarization behavior of several of the important electrochemical reactions involved in corrosion processes. 

Inhibited films containing layers of high-barrier and metallized pol3uners display elevated protective properties. This is connected with their parallel capabilities of simultaneous kinetic and diffusive regulation of corrosion processes. 

The thermodynamics of corrosion processes provides a tool to determine the theoretical tendency of metals to corrode. Thus, the role of corrosion thermodynamics is to determine the conditions under which the corrosion occurs and how to prevent corrosion at the metal/environment interface. Thermodynamics, however, cannot be used to predict the rate at which the corrosion reaction will proceed [1—6]. The corrosion rate must be estimated by Faraday s law and is controlled by the kinetics of the electrochemical reaction. 

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]. 

Chapters 1 to 3 describe the theory of corrosion engineering and offer analyzed case studies and solved problems in the thermodynamics of corrosion processes, the relevance of electrochemical kinetics to corrosion, low field approximation theory, concentration polarization, the effects of polarization behavior on corrosion rate, the effect of mass transfer on electrode kinetics, and diffusion-limited corrosion rates. 

The main type of corrosion damages in liquid Pb, Bi and Pb-Bi is the dissolution of structural materials (steels) and their components in these coolants. The kinetics of dissolution processes can be of different nature. For example, in some cases the dissolution is localized on boundaries of grain, causing interstructure infiltration of liquid metal (Pb, Pb-Bi) into steel. 

The main mechanism is the formation of silica from SiC along with CH4, CO c C. The silica is then dissolved in H2O. The dissolution rate of silica will play a vital role in the kinetics of the process. Basically the attack should have active corrosion character (Eq. (6)). 

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. 

Since kinetics and mechanism of corrosion is controlled by electrochemical principles, the technique based on electrochemical methods is used to determine the corrosion rate and understand the mechanism of corrosion process. The testing methods are based on principle of accelerating the corrosion process without changing the environment and the corrosion rates can be measured without removing the test specimens. 

Figure 11.8 Simplified schematic to illustrate possible sources of fluctuations in corrosion current, /(-orr or corrosion potential measured at a distant reference electrode, for general corrosion with a diffusion-limited cathodic reaction such as oxygen reduction. Fluctuations leading to fluctuations in can be in (1), the transport rate of the cathodic reagent, leading to changes in diffusion-limited current (2) and (3), the relative areas of the anodic and cathodic processes, caused for example by detachment of surface scales or by changes in the electrode kinetics of these processes caused for example by the addition of corrosion inhibitors or change in surface concentration of such inhibitors (4), in the solution resistance between cathodic and anodic areas, if these are spatially separated, caused for example by fluctuations in local electrolyte composition itself linked to the occurrence of the corrosion reaction.

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