Exchange-current density

Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

Faraday s constant (96,487 C/mol) overpotential total current current density exchange current density ratio of ohmic constriction to inter-facial resistance surface exchange coefficient volume-specific interfacial resistance in a composite thickness utilization length characteristic length of a porous microstructure... 

To evaluate polarizations and thus determine conditions of corrosion due to the macrocell, it is necessary to consider current densities exchanged at the anodic and cathodic surfaces as well as the macrocell current, I. 

Figure 15.4 Macrocell current density exchanged between a corroding bar of carbon steel in 3% chloride-contaminated concrete and a (parallel) passive bar of carbon steel in chloride-free concrete, 316L stainless steel in...

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. 

Critical current for the onset of passivation Current in the passive range Mass transport controlled limiting current Current density I i/A Exchange current density Exchange current density for the hydrogen evolution reaction... 

Surface impurities. Impurities adsorbed on the electrode surface usually affect its exchange current density. Exchange current density for the H+/H2 system is markedly reduced by the presence of trace impurities like arsenic, sulfur, and antimony. 

Exchange Current Density Exchange current density io is a very important parameter that has a dominating influence on the kinetic losses. It appears from Eq. (4.35) that activation polarization should increase with temperature. However, io is a highly nonlinear function of the kinetic rate constant of reaction and the local reactant concentration and can be modeled with an Arrhenius form as... 

For a more general reaction of the fonn Ox + ne Red, with differing concentrations of Ox and Red, the exchange current density is given by... 

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

Fig. 3. Hypothetical Evans diagram and polarization curve for a metal corroding in an acidic solution, where point A represents the current density, /q, for the hydrogen electrode at equiUbrium point B, the exchange current density at the reversible or equiUbrium potential, for M + 2e and point...

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

The exchange current density, depends on temperature, the composition of the electrolyte adjacent to the electrode, and the electrode material. The exchange current density is a measure of the kinetic resistance. High values of correspond to fast or reversible kinetics. The three parameters, a, a. ... 

Galvanic or impressed current anodes are used to protect these components. The anode material is determined by the electrolyte zinc and aluminum for seawater, magnesium for freshwater circuits. Platinized titanium is used for the anode material in impressed current protection. Potential-regulating systems working independently of each other should be used for the inlet and outlet feeds of heat exchangers on account of the different temperature behavior. The protection current densities depend on the material and the medium. 

If the areas of the electrodes are assumed to be 1 cm, and taking the equilibrium exchange current density /g for the Ag /Ag equilibrium to be 10 A cm", then /g will be 10 A, which is a very high rate of charge transfer. A similar situation will prevail at electrode II, and rates of exchange of silver ions and the potential will be the same as for electrode I. 

It is evident from these expressions that since in the Tafel region / (the current density actually determined) must be greater than /(, (the equilibrium exchange current density), the signs of the overpotentials will conform to equations 1.60 and 1.61, i.e. will be negative and will be positive. 

Fig, 1.24 Tafel lines for a single exchange process. The following should be noted (a) linear f-log I curves are obtained only at overpotentials greater than 0-052 V (at less than 0-052 V E vs. i is linear) b) the extrapolated anodic and cathodic -log / curves intersect at tg the equilibrium exchange current density and (c) /, and the anodic and cathodic current densities... 

Fig, 1,26 E Vi, log (curves for the corrosion of a metal in a reducing acid in which there are two exchange processes (c,f. Fig, L24) involving oxidation of M—are reduction of —vH2. Note that (o) the reverse reactions for exchange process are negligible at potentials removed from E, (b) the potential actually measured is the corrosion potential E , which is mixed potential, and (c) the E vs. (,pp curves (where ijppi is the applied current density) when extrapolated intersect at corr. 

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