Salt, impedance measurements

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

Simplicity and reliability of operation make AC impedance measurements attractive as a technique in the evaluation of coating integrity. As opposed to classical salt spray test, analysis times are shorter with the AC impedance technique and quantitative data are obtained permitting relevant mechanistic Information to be derived. Impedance test methods are likely to find many applications in the resolution of unsolved practical problems ( .) ... 

Impedance measurements taken on specimens after 96 hours exposure to salt spray show a combination of R-C and Warburg diffusion behavior. This Is in agreement with the observation elsewhere (9,11). 

It Is clear on analyzing the results obtained from specimens 18 and 19 as well as from 20 and 21 that film degradation and coating Integrity can be followed more efficiently by Impedance measurements than by salt spray testing (Table II and Figures 1 and 2). 

ASTM G53-77, Salt Spray and Electrochemical Impedance Measurements). The results indicate that electrochemical impedance measurements provide a satisfactory correlation with the behaviour of paint coatings as evaluated by visual examination. In addition, it appears that, in certain cases, data obtained by this technique will allow prediction of the metallic corrosion underneath the paint coating when no changes in the appearance of the coating can be externally observed. 

Figure 20.5 shows the temperature dependence of the ionic conductivity for zwitterion 10 with and without an equimolar amount of lithium salts. Neat zwitterion 10 shows low ionic conductivity of about 10 S cm at even 200°C from the ac impedance measurement. This is because there are no mobile ions in the system. However, zwitterion 10, which is mixed with an equimolar amount of lithium salt. 

Although simple impedance measurement can tell the existence of an anodic film, electrochemical impedance spectroscopy (EIS) can obtain more information about the electrochemical processes. In general, the anode/electrolyte interface consists of an anodic film (under mass transport limited conditions) and a diffuse mobile layer (anion concentrated), as illustrated in Fig. 10.13a. The anodic film can be a salt film or a cation (e.g., Cu ) concentrated layer. The two layers double layer) behave like a capacitor under AC electric field. The diffuse mobile layer can move toward or away from anode depending on the characteristics of the anode potential. The electrical behavior of the anode/electrolyte interface structure can be characterized by an equivalent circuit as shown in Fig. 10.13. Impedance of the circuit may be expressed as... 

To avoid artifacts in impedance measurements when modulating the input voltage of the potentiostat at high frequencies (>10 kHz), the reference electrode should be short-circuited via a capacitance of 10 nF and a Pt wire dipped into the solution in the main part of the cell (Fig. 4.2). The surface of the counter electrode should be sufficiently large so that its interface with the electrolyte does not influence the current-potential curve. Usually a platinized Pt sheet is used as a counter electrode. The electrolyte is made conductive by adding an inert salt of a concentration in the range of 10-3-10- M. 

It is relatively easy to get measurements of good precision for impedances between 1 and 10 Q at frequencies below 5 x 10 Hz. However, for lower and higher impedances, distortions may be observed. Very high impedances are found, for example, in measurements of protective coatings on metallic surfaces, and very low impedances are found in molten salts. The errors for high-impedance measurements originate from the finite potentiostat input impedance. Such resistance should be at least 100 times larger than the measured impedance if not, a calibration procedure is necessary. 

The interface between two immiscible liquids is used as a characteristic boundary for study of charge equilibrium, adsorption, and transport. Interfacial potential differences across the liquid-liquid boundary are explained theoretically and documented in experimental studies with fluorescent, potential-sensitive dyes. The results show that the presence of an inert salt or a physiological electrolyte is essential for the function of the dyes. Impedance measurements are used for studies of bovine serum albumin (BSA) adsorption on the interface. Methods for determination of liquid-liquid capacitance influenced by the presence of BSA are shown. The potential of zero charge of the interface was obtained for 0-200 ppm of BSA. The impedance behavior is also discussed as a function of pH. A recent new approach, using a microinterface for interfacial ion transport, is outlined. 

The addition of salt to a polymer may lead to formation of dipoles between ions and chain molecules. These dipoles have a (longitudinal) component along the chains as it is sketched in Figure 25. The impedance measurements reflect relaxation of these longitudinal dipole components since they relax sufficiently slowly. Dipole relaxations occur as maxima in plots of Z as a function of frequency. It is analogous to complex viscosity sketched in Figure 15. Real and imaginaiy part of complex impedance (Equation 53) read ... 

Laboratory exposure tests are usually conducted in chambers similar to that described in ASTM B 117, Test Method of Salt Spray (Fog) Testing. Because the environment is more corrosive than the salt fog described in ASTM B 117, the variations described in ASTM G 85, Practice for Modified Salt Spray (Fog) Testing, are often used. Some investigators have used a modified procedure in which the test coupons are mounted over holes on the outside wall of the exposure chamber so that they can be subjected to a thermal gradient [35]. Outside mounting of the coupons also affords the opportunity to conduct in situ electrochemical impedance measurements. 

The corrosion resistance of various Al- and Al/Zn-coated AZ91D Mg alloys has been evaluated by salt spray and electrochemical methods. In the following, the results from salt spray test, polarization curve and electrochemical impedance measurements manifesting the benehcial effects of Al and/or Al-Zn coating on AZ91D Mg alloy are demonstrated. 

Ionic liquid (electrolyte), such as water with added ionic salts, is one of the common samples investigated by EIS. An average ion electrophoretic migration velocity in aqueous solution is 10 mm/sec. As was shown above, the resistance to ionic migration current in the aqueous bulk solution within the frequency range of a typical impedance measurement can be simplified by a... 

The impedance measurement is shown in Fig. 14.14. A comparable interface resistance is observed with graphite anode vs. Li, between ionic liquid and reference electrolyte EC/DEC-LiPFs at 80 However, EC/DEC-LiFSI based salt showed a lower interface impedance at 65 In the diffusimi part, the ionic liquids show higher resistance (20 and it increases in the order given in Eq. (14.5). Due to the high viscosity of the IL, the diffusion resistance is consequently higher. 

Most of the polymer electrolytes that have been studied are solid solutions of salts in polymers. There is a possibility that both the cation and anion are mobile in such electrolytes. The ionic transport number in polymer electrolytes is a very important parameter in terms of the conduction mechanism of ions in polymers and of their practical application. Cationic transport numbers have been measured in the polymer electrolytes, especially in those of PEO using various methods, including nuclear magnetic resonance (NMR) [49,50], complex impedance measurements [51,52], tracer diffusion... 

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