Corrosion kinetics description

In most electrochemical measurements of corrosion kinetics a potentiostat is used. This description will cover the rudimentary operation of a potentiostat using the concept of an ideal operational amplifier (op amp) as a basis. An op amp is a three-terminal device as shown in Fig. 16 with two input terminals and one output terminal. A perfect op amp follows five basic rules (19) ... 

The description of corrosion kinetics in electrochemical terms is based on the use of potential-current diagrams and a consideration of polarization effects. The equilibrium or reversible potentials Involved in the construction of equilibrium diagrams assume that there is no net transfer of charge (the anodic and cathodic currents are approximately zero). When the current flow is not zero, the anodic and cathodic potentials of the corrosion cell differ from their equilibrium values the anodic potential becomes, more positive, and the cathodic potential becomes more negative. The voltage difference, or polarization, can be due to cell resistance (resistance polarization) to the depletion of a reactant or the build-up of a product at an electrode surface (concentration polarization) or to a slow step in an electrode reaction (activation polarization). 

Corrosion conditions typically are far removed from the reversible potentials for any of the reactions. Therefore, Tafel kinetics is usually an accurate description of corrosion kinetics in which mass transfer is not important. 

Corrosion is the deterioration of a material by reaction with its enviromnent. Although the term is used primarily in conjunction with the deterioration of metals, the broader definition allows it to be used in conjunction with all types of materials. We will limit the description to corrosion of metals and alloys for the moment and will save the degradation of other types of materials, such as polymers, for a later section. In this section, we will see how corrosion is perhaps the clearest example of the battle between thermodynamics and kinetics for determining the likelihood of a given reaction occurring within a specified time period. We will also see how important this process is from an industrial standpoint. For example, a 1995 study showed that metallic corrosion costs the U.S. economy about 300 billion each year and that 30% of this cost could be prevented by using modem corrosion control techniques [9], It is important to understand the mechanisms of corrosion before we can attempt to control it. 

The previous sections dealt primarily with phase transformations and corrosion in materials. Polymers also undergo phase transformations. For example, there are many polymers that utilize nucleation and growth kinetics to transform from amorphous to crystalline polymers. The kinetics of these transformations are very similar, in principle, to the preceding descriptions for glasses, so it is not necessary to duplicate that material here. Polymers also are susceptible to corrosion, but the term degradation is more... 

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. 

Solubility and phase behavior directly affect many of the key phenomena occurring within SCWO systems, including reaction kinetics, salt precipitation, and corrosion. A generic description of phase behavior and solubility in supercritical water was given previously in Section 2. This section provides a more specific discussion of phase behavior and solubility in SCWO systems. 

In the next layer of subjects we list the engineering sciences which are needed in various ways for understanding and further developing the core engineering subjects thermodynamics, chemical kinetics, electrochemical phenomena, and transport phenomena. These engineering sciences, which are themselves interrelated, form the basis for the analytical and numerical description of the chemical reactor and its peripheral equipment. For example, the subject of transport phenomena can be used to analyze difiiision-controlled reactions, separation schemes, transient processes in reactors, thermal processes, flow patterns in reacting systems, corrosion, difiusion in porous media, and other problems connected with reactor engineering. 

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). 

The complete determination of position and shape of overvoltage curves requires knowledge about the quantities io, it, be, and ba, of which the first one is given by Nemst s equation or the Pourbaix diagram. Through such knowledge, the best tool we have for description of the kinetics of electrode reactions is available. With a reservation for passivation effects (which we shall return to) we can with this background obtain a quantitative description of any corrosion process, in principle as shown in Section 4.9. 

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. 

There is a lack of fundamental understanding of the effect of elastic tensile stress or strain and plastic strain on dissolution by either thermodynamic or kinetic interpretations. The roles of stress and/or strain in aqueous corrosion reactions occurring at the atomistic level close to room temperature have been modeled for micrometer-scale descriptions of stress corrosion cracking (SCC) and hydrogen... 

On the basis of the author s approach have been investigated the impact of the injection of CO into the anode feed stream of a PEMFC on the long-term (at least 600 h) MEA performance degradation, under current-cycled operation representative of transport application conditions. First is proposed an elementary kinetic mechanism within MEMEPhys describing the transient behavioiu of a PEMFC MEA in the presence of CO at the anode, and accounting at the same time for the carbon catalyst-support corrosion phenomena in the cathode. In the anode side, competitive CO/H2 adsorptions and electro-oxidations on Pt are accoimted for, as well as the local anodic ORR and the detailed descriptions of the cathodic ORR and the cathodic carbon corrosion. ... 

Detailed kinetic studies of a commercial conventional-carbon-supported MEA were conducted to predict its lifetime. The carbon corrosion rates of conventional-carbon-support MEAs as a function of time at 80°C with potential hold at 1.1,1.2, and 1.3 V versus the RHE, respectively, are shown in Eig. 2. As might be expected, the carbon corrosion current increases as the potential increases. The response of the CO current versus corrosion time under these experimental conditions follows a linear log-log relation, which is consistent with the description of the corrosion current of carbon blacks in HjPO by Kinoshita (Kinoshita 1988 Kinoshita and Bett 1973). Detailed smdies of corrosion currents of conventional-carbon-support... 

The overall simulation of high-temperature corrosion processes under near-service conditions requires both a thermodynamic model to predict phase stabilities for given conditions and a mathematical description of the process kinetics, i.e. solid state diffusion. Such a simulation has been developed by integrating the thermodynamic program library, ChemApp, into a numerical finite-difference diffusion calculation, InCorr, to treat internal oxidation and nitridation of Ni-base alloys [10]. This simulation was intended to serve as a basis for an advanced computer model for internal oxidation and sulfidation of low-alloy boiler steels. 

As in chemistry in general, one may distinguish in electrochemistry between the equilibrium and the kinetics of processes. Both topics will be treated as a basis for corrosion. For the description of electrochemical equilibria, one needs thermodynamics for which a short introduction is given in this section. A more detailed description will be found in textbooks of physical chemistry [1]. 

The aqueous corrosion and passivation of metal surfaces involve electrochemical processes at the electrode-electrolyte interface. Like for all chemical reactions, the two aspects of equilibrium and kinetics have to be treated. Thermodynamic data give an important first insight into layer formation. They have been used to compose potential-plT diagrams for all elements and thus for all metals [26,27]. In Chapter 1 of this book on the fundamentals of corrosion, passivity of iron has been mentioned and the calculation of some of the lines based on thermodynamic data has been described. Here a more detailed description of foe potential-pH diagrams of iron and copper are presented. 

The Avrami equation is crucial in the description of crystallisation and other processes. The equation has been applied to areas as diverse as corrosion, reaction kinetics and the growth of micro-organisms. If applied correctly, it can give information on the type of nucleation (homogeneous or heterogeneous) as well as the geometry of crystallisation, for... 

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