Anode polarisation characteristics

The anode material must have a more or less constant operating potential over a range of current outputs. Consequently the anode must resist polarisation when current flows the polarisation characteristics must also be predictable. 

The extent of galvanic effects will be influenced by, in addition to the usual factors that affect corrosion of a single metal, the potential relationships of the metals involved, their polarisation characteristics, the relative areas of anode and cathode, and the internal and external resistances in the galvanic circuit (see Section 1.7). 

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

The assumption of a position-independent local area specific resistance is an approximation, which is not always justifiable. Part of the anode polarisation resistance is dependent on fuel composition. However, often this part is small. If the cell is not isothermal, the local resistance will vary with position due to its temperature dependence. Also the actual flow pattern may be much more complex than just co-flow. Even so, if the fuel utilisation is large. ASRcor derived from Eqs. (4) and (5) will always be a better characteristic of a cell than a value derived neglecting the fuel utilisation (Eq. (1)). More precise evaluation of ASRcor requires a rigorous 3-D modelling of the cell test. 

Open Circuit Potential. Metal immersed in an aqueous solution develops an electric potential at its surface called open circuit potential (OCP) which is a characteristic of the metal solution system. The magnitude of OCP is measured with respect to reference electrode with the help of high impedance voltmeter and potentiostat is used to polarise or displace equilibrium potential of specimen in the negative (cathodic) or positive (anodic) direction with reference to OCP. This is manipulating the rates (ionic currents) of respective cathodic and anodic half-cell electrochemical reactions. The electrochemical potential of a metal in a certain solution is dependant on the type of the metal, the composition of the solution and its pH, oxygen content and temperature [104, 105]. 

In Fig. 10.6, the polarisation and power density curves obtained from the model are compared with the data from [1] in which the fuel considered was CH4 and the fuel composition for obtaining the static characteristic curves was fully reformed steam and methane mixture. It can be seen that the difference between the results is small because the principal gaseous species in the anode channel are still H2 and H2O, which is a valid assumption. A part of the small difference between the results can also be attributed to the difference in the calculations of the activation over-voltage between this model and [1]. 

Jones (1971,1972), based on kinetic considerations, postulated the adoption of a criterion for a permissible corrosion rate on the basis of a polarization characteristic of a given corrosion system determined beforehand. In order to determine the corrosion rate at polarisation potential CP> knowledge of the course of the partial curve of the anodic reaction is necessary. The first steps in the analysis of partial currents of corrosion systems were made by Stern and Geary (1957), who described the method of determination of the Tafel coefficient of the anodic reaction on the basis of direct current measurement results of the cathodic polarization curve. Extrapolation of the linear section of this curve to a corrosion potential Ej-orr allows reproduction of a fragment of the anodic polarization partial curve at chosen potential values, as a difference between the polarization current and the partial cathodic current. 

The basic concepts of composite or single-phase MIEC electrodes are equally applicable to anodes. Traditionally, however, the typical anode used to date has been a composite mixture of Ni and YSZ. The presence of YSZ not only suppresses the thermally induced coarsening of Ni, but it also introduces MIEC characteristics. Other anodes currently under investigation are based on cermets of copper, which are being explored for direct oxidation of hydrocarbon fuels [39]. These types of anodes are in an early stage of development and thus their polarisation behavior is not discussed here. In so far as single-phase anodes are concerned, some work has been reported in the literature, most notably on La-SrTi03 [40, 41]. Work on this as well as other perovskite-based anodes is in its infancy, and is not elaborated upon further. The discussion in this chapter is confined to Ni + YSZ cermet anodes. 

This latter reaction scheme does not depend upon the adsorption of fuel gas, while the former one does. The implication is that anodic activation polarisation would be independent of what the fuel is in the latter scheme, while it would be a function of the type of fuel in the former case. Recent work has shown that the total polarisation loss with CO as a fuel is much greater than that with H2 as the fuel, and the difference cannot be attributed to differences in concentration polarisation [42], It is possible that the differences may be due to differences in the adsorption characteristics of H2 and CO. Thus, the preliminary conclusion is that adsorption of fuel gas must be an important step. 

---