Electrodes polarisation resistance

Danielson, M. J., Analysis of Errors in Using The Two Electrode and Three Electrode Polarisation Resistance Methods In Measuring Corrosion Rates , Corrosion, 36, No. 4, 174-178, April (1980)... 

ASR may be divided into ohmic resistance, R, and electrode polarisation resistance, Rp. The ohmic resistance originates from the electrolyte, the electrodes materials and the current collection arrangement. This is very much dependent on geometric factors such as thickness of the cell components and the detailed geometry of the contact between current collection and electrodes, and between electrodes and electrolyte as current constrictions may be important [41]. The electrode polarisation resistance is further divided into contributions from the various rate-limiting steps. Thus, ASR can be broken down in five terms ... 

Fig. 1.20 Cell consisting of two reversible Ag /Ag electrodes (Ag in AgN03 solution). The rate and direction of charge transfer is indicated by the length and arrow-head as follows gain of electrons by Ag -he- Ag—> loss of electrons by Ag - Ag + e- —. (o) Both electrodes at equilibrium and (f>) electrodes polarised by an external source of e.m.f. the position of the electrodes in the vertical direction indicates the potential change. (K, high-impedance voltmeter A, ammeter R, variable resistance)...

Choice of an appropriate reference electrode remains an enigma for non-corrosion-aware personnel-although commercially available polarisation-resistance probes can be adapted. An interesting aspect concerns localised corrosion in that, for some materials, localised corrosion only occurs within characteristic potential ranges. 

Impedance Some of the errors arising from the use of linear polarisation resistance led to interest and development in a.c. systems.An early development used a fixed a.c. frequency and a commercial instrument was produced in the UK. Inaccuracies still occurred, however, and were due to the electrode impedance which is fequency dependent. Electrode reactions have a capacitance component, in addition to resistance, resulting in a requirement to measure the impedance. However, the total impedance comprises values for the reaction, solution, diffusion and capacitance. Measurements at different frequency are more reliable, particularly where high solution resistances occur. Simplifications for industrial monitoring have been developed consisting of two measurements, i.e. at a high (10 kHz) and low frequency (0-1 Hz). The high-frequency measurement can identify the... 

Suppose that the envisaged reaction is a corrosion reaction, which means that reactions of the type of Equation 2.47 occur simultaneously at the electrode surface but belonging to two different redox systems then Rct can be defined as a polarisation resistance ... 

Most of the reference electrodes embedded in concrete are used for control of cathodic protection (CP) systems. Potential stability is then less important, compared to corrosion state monitoring. Control of CP systems requires only short-term stability, maximum 24 hours. Corrosion rate measurement, like linear polarisation resistance (LPR) measurements, also requires short-term reference electrode stability. However, regardless of application, a reference electrode which is to be permanently embedded in the test solution, e.g. concrete, must have a long life when exposed to this environment. 

The solid oxide fue( cell (SOFC) have been under development during several decades since it was discovered by Baur and Preis in 1937, In order to commercialise this high temperature (600 - 1000°C) fuel cell it is necessary to reduce the costs of fabrication and operation. Here ceria-based materials are of potential interest because doped ceria may help to decrease the internal electrical resistance of the SOFC by reducing the polarisation resistance in both the fuel and the air electrode. Further, the possibility of using less pre-treatment and lower water (steam) partial pressure in the natural gas feed due to lower susceptibility to coke formation on ceria containing fuel electrodes (anodes) may simplify the balance of plant of the fuel cell system, and fmally it is anticipated that ceria based anodes will be less sensitive to poising from fuel impurities such as sulphur. 

At temperatures below 1000°C the doped ceria has a considerable larger polarisation resistance, Rp, even in case of cermet with nickel. (The reaction rate is proportional to 1/Rp). At the lower temperatures the reaction may be accelerated by addition of fine Ni particle to the surface of the doped ceria. Impedance spectra at 850°C of Ni/Ceo jGdo iO, 95 cermet electrodes with and without small amounts of fine Ni particles (probably of nanometer size) are given in Fig. 15.13. The impedance is significantly affected. The dominant low frequency arc (fsummit = 1 Hz)... 

According to ASTM G59-97 (2009), a small potential scan, AE(t), defined with respect to the corrosion potential (AE = E — corr), is applied to a metal sample and the resultant currents are recorded. The polarisation resistance, Rp, of a corroding electrode is defined from Eq. (7.33) as the slope of a plot of the potential... 

This last statement means that electrodes producing higher currents during polarisation show a more non-uniform current distribution, since high currents are correlated with low polarisation resistances. 

If the current lead is a noble metal, for example platinum, and if the working electrode is a metal in the active state, it is possible to expose the noble metal wire to the electrolyte provided its area is snnall compared with that of the working electrode and its polarisation resistance larger than that of the working electrode. 

When research results are subject to interpretation through impedance spectroscopy, one has to bear in mind, that measured quantities are a measure of analysed system s properties, which is composed of electrodes and located between them material. Hence, the measured quantities give a picture of the entire circuit in sinsuidally alternating electric field, including lead resistance and inductance, stray capacitance and phenomena related to electrode polarisation. 

Where 7 is the solution resistance, 7 p is the polarisation resistance and Cji is the double-layer capacitance. Various electrochemical phenomena at the metal solution interface causes a time lag and a measurable phase angle 9. These processes will be simulated by resistive and/or capacitive electrical networks. The impedance behaviour of an electrode may be expressed in Nyquis plot of Zj g (imaginary part of impedance) as a function of Z eai (real part of impedance) or in Bode plots of mod Impedance and 9 versus frequency, where co — 2 nf. 

Despite the number of reports on the structural and transport properties of many apatite electrolytes, their potential use in SOFC devices has not been widely studied (Brisse et al., 2006 Yoshioka et al., 2008 Bonhomme et al., 2009). In this sense,Tsipis et al. (2007) and Yaremchenko et al. (2009) reported the electrochemical behaviour of different cathode materials in contact with silicate apatite electrolytes and they found that silicon migration towards the surface layer blocks the electrochemical reaction zones increasing the electrode polarisation. In addition, high area-specific resistances were found (Brisse et al, 2006) with Ni0-La9SrSi< 026.5 cermets. However, a complete single fuel cell with apatite-type electrolyte, using different electrode materials, has not been reported before. Hence, further studies are still needed for a better characterisation of these potential electrolyte materials for SOFCs on several issues, such as the chemical reactivity and electrochemical performance between apatite electrolytes and different... 

Kinetic study of the chlorine electrode reaction on Ti/Ru02 through the polarisation resistance. Electrochim Acta 47 1129-1152. doi 10.1016/... 

DC polarisation tests took place in a three-electrode electrochemical cell where the area of the working electrode (modified Al-2024 T3) exposed to the corrosive solution was 1 cm. For these measurements, a platinum foil and a SCE were used as the counter and the reference electrodes, respectively. Potentiodynamic tests were carried out by scanning the potential between -1 V and 0 V vs SCE at a rate of 1 mV s. The resulting graphics were fitted to the Tafel plots allowing the corrosion current and potential, and the polarisation resistance to be extrapolated. Corrware software was used for fitting. 

Figure 2.20 presents the electrochemical behaviour of different nick-elate-based electrodes and it is clear that for the L 2Ni04+s Ln = La, Nd, Pr) series, relatively low polarisation resistances were observed. 

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