Corrosion kinetic aspects

The three elements necessary for corrosion are an aggressive environment, an anodic and a cathodic reaction, and an electron conducting path between the anode and the cathode. Other factors such as a mechanical stress also play a role. The thermodynamic and kinetic aspects of corrosion deterrnine, respectively, if corrosion can occur, and the rate at which it does occur. 

The kinetic aspect must be investigated as well because corrosion may be thermodynamically possible but is so slow that it can be ignored in practice. 

It is of importance to predict the lifetime of a metallic component in a specific corrosive system. For uniform corrosion (also called homogeneous corrosion), the lifetime can be calculated if the kinetics of the reactions are known and if no localized attack or corrosion cracks could appear. In this chapter, the kinetic aspects of the homogenous corrosion process will be considered. Only electrochemical corrosion mechanisms are regarded, not chemical and physical ones. 

The parameters that determine time of wetness and composition of surface electrolyte have been surveyed by Kucera and Mattson [8.1]. They present also a thorough description of the mechanism, with thermodynamic and kinetic aspects of corrosion on various materials. For instance, they consider potential-pH diagrams as a useful thermodynamic basis for understanding atmospheric corrosion. 

The corrosion product plays an important role in the corrosion resistance of zinc, particularly in aqueous and certain atmospheric environments (cf. low-alloy or weathering steels). In such cases, the corrosion kinetics may be parabolic in nature, the rate decreasing with time (cf. aluminum), an aspect that has been extensively investigated. Taylor and Tolley [184] have examined the characteristics of sprayed zinc coatings and concluded that the decrease in permeability was... 

Viktor I. Korobov (korvikS8 mail.ru) - Head of the Department of Physical and Inorganic Chemistry, Dnepropetrovsk National University (http //www.dsu.dp.ua). Research interests - the kinetics of electrochemical reactions, protection of metals against corrosion, methodological aspects of the use of mathematical packages in chemical education. 

Mixer-settlers have been the more common type of equipment and, with the development of hydrometallurgy over the past 20 years, designs have improved considerably. To select the appropriate equipment, a clear understanding of the chemical and physical aspects of the process is required. Also the economics must be considered relative to the type of equipment to suit particular conditions of given throughput, solution and solvent type, kinetics and equilibrium, dispersion and coalescence, solvent losses, number of stages, available areas, and corrosion. 

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... 

Today, a large number of important technologies are based on or related to electrodes reactions. Besides the chlor-alkali and aluminium industries, energy conversion in batteries and fuel cells, electrodeposition, electrorefining, organic electrosynthesis, industrial and biomedical sensors, corrosion and corrosion protection, etc. are amogst those technologies. In many of them, kinetic, catalytic or specificity aspects of electrode processes are of enormous importance. 

After 14 years on the faculty of Imperial College, Jacobs moved from London, England, to London, Ontario, where his research program focused on the optical and electrical properties of ionic crystals, as well as on the experimental and theoretical determination of thermodynamic and kinetic properties of crystal defects.213 Over the years his research interests have expanded to include several aspects of computer simulations of condensed matter.214 He has developed algorithms215 for molecular dynamics studies of non-ionic and ionic systems, and he has carried out simulations on systems as diverse as metals, solid ionic conductors, and ceramics. The simulation of the effects of radiation damage is a special interest. His recent interests include the study of perfect and imperfect crystals by means of quantum chemical methods. The corrosion of metals is being studied by both quantum chemical and molecular dynamics techniques. 

The heart of corrosion science has been identified as electrochemical science coupled with the thermodynamic and kinetic values. Other limbs are oxidation and high-temperature oxidation of metals, protective coatings, passivity, inhibitors, microbial-induced corrosion, corrosion fatigue, hydrogen embrittlement and corrosion-resistant alloys. Having identified the limbs of corrosion science, it is instructive to examine how the various aspects came into existence over a period of time. 

The Frumkin epoch in electrochemistry [i-iii] commemorates the interplay of electrochemical kinetics and equilibrium interfacial phenomena. The most famous findings are the - Frumkin adsorption isotherm (1925) Frumkin s slow discharge theory (1933, see also - Frumkin correction), the rotating ring disk electrode (1959), and various aspects of surface thermodynamics related to the notion of the point of zero charge. His contributions to the theory of polarographic maxima, kinetics of multi-step electrode reactions, and corrosion science are also well-known. An important feature of the Frumkin school was the development of numerous original experimental techniques for certain problems. The Frumkin school also pioneered the experimental style of ultra-pure conditions in electrochemical experiments [i]. A list of publications of Frumkin until 1965 is available in [iv], and later publications are listed in [ii]. 

Solid-state electrochemistry is an important and rapidly developing scientific field that integrates many aspects of classical electrochemical science and engineering, materials science, solid-state chemistry and physics, heterogeneous catalysis, and other areas of physical chemistry. This field comprises - but is not limited to - the electrochemistry of solid materials, the thermodynamics and kinetics of electrochemical reactions involving at least one solid phase, and also the transport of ions and electrons in solids and interactions between solid, liquid and/or gaseous phases, whenever these processes are essentially determined by the properties of solids and are relevant to the electrochemical reactions. The range of applications includes many types of batteries and fuel cells, a variety of sensors and analytical appliances, electrochemical pumps and compressors, ceramic membranes with ionic or mixed ionic-electronic conductivity, solid-state electrolyzers and electrocatalytic reactors, the synthesis of new materials with improved properties and corrosion protection, supercapacitors, and electrochromic and memory devices. 

The rate of corrosion will depend on a number of factors, including thermodynamic, kinetic, and mass-transport-related aspects. The first few sections of this volume will describe these influences on corrosion. 

The information about the tendenqf for corrosion to occur that can be obtained from thermodynamic calculations is important and useful. However, most of the science and engineering aspects in the field of corrosion focus on knowing and reducing the rate of corrosion. The rate of corrosion is not addressed by thermodynamics it falls instead within the purview of kinetics. So the kinetics of electrochemical reactions in general, and corrosion reactions specifically, are at the heart of the subject of corrosion. This chapter will introduce electrochemical kinetics at a simple level, with sufficient detail to develop the concept of mixed potential theory. The interested reader is referred to other volumes of this encyclopedia and to textbooks in corrosion [1-9] for a more detailed description. The kinetic underpinnings of some of the electrochemical techniques for determination of corrosion rate will also be presented. The influence of transport on the rates of electrochemical reactions will be discussed in the next chapter (see Chapter 1.4). 

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]. 

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