Polarization resistance cathodic current density calculation

Figure 17. Dependencies of exchange current density jo at various electrodes on the ionic conductivity of various zirconia solid electrolytes (A) SDC (dashed, dash-dotted, and dotted lines) and Ru (0.5 mg/cm )-dispersed SDC anodes in humidified Hi. Reprinted from Ref. 45, Copyright (1997), with permission from The Electrochemical Society, (B) Pt cathodes in Oj, (C) LfCi jsSriu MnO, (LSM, dashed lines) and Pt (0.1 nig/cnr(-dispersed LSM cathodes in Oi. Each jo value was calculated from the polarization resistance (Rp, 2 tm ), since linear relationships were observed between 7 and j for rj < 0.1 V at all the electrodes and 7 ccii between 800 and 1000°C jo = (RT/nF)Rp. Reproduced by Ref. 46, Copyright (1999), by permission from The Electrochemical Society. 

The polarization curves shown in 4(d) are typical of systems in which anodes and cathodes are separated by corrosion products and/or solutions of some resistance. The corrosion rate (current density) depends on the average resistance between the anodes and cathodes and the potential which is measured depends on the position of the probe electrode in relation to the resistance separating the anode and cathode. In a corroding system of this type there is a current flowing through the solution which can be calculated by measuring the potential differences between two spaced electrodes in the solution and the resistance of the solution. Evans and his co-workers did this with iron in bicarbonate solutions and showed that all the corrosion could be accounted for by the currents flowing between anodes and cathodes, (2). This confirmed the electrochemical mechanism of corrosion. 

The overpotentials at the anode qAnode (oxygen overpotential) and cathode qcathode (hydrogen overpotential), also referred to as charge transfer overpotentials, result from the inhibition of electron transport in the separate electrochemical reactions (see Fig. 11.2). In order for current to flow through the electrolysis cell, the resistance polarization must also be overcome. It is caused by the ohmic resistance of the ceU (electrolytes, separator and electrodes). The ohmic voltage drop can be calculated from the current density i in A cm and the surface-specific resistance R of the ceU in Q cm. ... 

The anodic and cathodic Tafel coefficients are equal to jSg = 30 mV, = 50 mV. Calculate (a) the polarization resistance at the corrosion potential and (b) the corrosion current density. 

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