Anodic Activation Polarisation

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. 

Although the basic concepts of anode reaction are similar to the cathode, the details may be different, and are not well understood at the present time. The overall anodic reaction may be given by  

In the preceding, the Kroger-Vink notation has been used. Similar to the cathodic overpotential, the anodic activation overpotential also depends upon material properties, microstructure, atmosphere, temperature and current density that Is, 

Assuming a phenomenological model, anodic polarisation can be described using the Butler-Volmer equation, and its low current density (linear) and high current density (Tafel) limits. Experimental results for some selected cases can be 

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. 

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Cathodic and anodic activation polarisations, in light of MIEC electrodes, are discussed below. 

For passivation, the passivation current density ipjs must be applied either by means of an anodic current (polarisation) or in the reaction vith an oxidant at passivation potential Upas. In the active and passive range, trivalent chromium (Cr ) is dissolved. Above the transpassive breakthrough potential Ua, i.e. after the transition to the transpassive range, the current density, and with it the rate of corrosion, rises once again, since at this high oxidation potential chromium then dissolves in hexa-valent form (Cr ) as chromate. 

The final terms in Eq. (7), T ca, ilcc. ilAa and tiac- are the cathode activation, cathode concentration, anode activation, and anode concentration polarisations, respectively. In general, their dependence on the current density is nonlinear, although at low polarisation they may be approximated by linear relationships. 

Passivity of a metal lies in contrast to its activity, in which the metal corrodes freely under an anodic driving force. The passive state is well illustrated by reference to a classical polarisation curve prepared poten-tiostatically or potentiodynamically (Figure 1.39). As the potential is raised... 

Fig. 1.39 Schematic anodic polarisation curve for a metal. Region AB describes active dissolution of the metal. BC is the active/passive transition, with passivation commencing at B. Passivation is complete only at potentials higher than C. The metal is passive over the range CD...

Fig. 1.40 Schematic anodic polarisation curve for a passivatable metal (solid line), shown together with three alternative cathodic reactions (broken line). Open-circuit corrosion potentials are determined by the intersection between the anodic and cathodic reaction rates. Cathode a intersects the anodic curve in the active region and the metal corrodes. Cathode b intersects at three possible points for which the metal may actively corrode or passivate, but passivity could be unstable. Only cathode c provides stable passivity. The lines a, b and c respectively could represent different cathodic reactions of increasing oxidizing power, or they could represent the same oxidizing agent at increasing concentration.

Fig. 1.41 Schematic anodic polarisation curves for a passivatable metal showing the effect of a passivating agent that has no specific cathodic action, but forms a sparingly soluble salt with the metal cation, a without the passivating agent, b with the passivating agent. The passive current density, the active/passive transition and the critical current density are all lowered in b. The effect of the cathodic reaction c, is to render the metal active in case a, and passive...

Note that Reference" draws attention to the possibility of an increase of anodic polarisation of the more negative member of a couple leading to a decrease in galvanic corrosion rate. There can also be a risk of increased corrosion of the more positive member of a couple. Both these features can arise as a result of active/passive transition effects on certain metals in certain environments. 

It is convenient to consider three stages of anode polarisation with regard to temperature effects, (a) under film-free conditions, (b) under film-forming conditions and (c) at the active-passive transition. 

The general form of the anodic polarisation curve of the stainless steels in acid solutions as determined potentiostaticaiiy or potentiodynamically is shown in Fig. 3.14, curve ABCDE. If the cathodic curve of the system PQ intersects this curve at P between B and C only, the steel is passive and the film should heal even if damaged. This, then, represents a condition in which the steel can be used with safety. If, however, the cathodic curve P Q also intersects ED the passivity is unstable and any break in the film would lead to rapid metal solution, since the potential is now in the active region and the intersection at Q gives the stable corrosion potential and corrosion current. 

Polarisation from an external source may also affect the range of passivity. Cathodic polarisation may depress the potential from the passive to the active region (see Fig. 3.14) and thus care should be taken to avoid contact with any other corroding metal. Anodic polarisation, on the other hand, can stabilise passivity provided that the potential is not increased into the range of transpassivity (see Fig. 3.14) and anodic protection is quite feasible. 

Tin when made anodic shows passive behaviour as surface films are built up but slow dissolution of tin may persist in some solutions and transpassive dissolution may occur in strongly alkaline solutions. Some details have been published for phosphoric acid with readily obtained passivity, and sulphuric acid " for which activity is more persistent, but most interest has been shown in the effects in alkaline solutions. For galvanostatic polarisation in sodium borate and in sodium carbonate solutions at 1 x 10" -50 X 10" A/cm, simultaneous dissolution of tin as stannite ions and formation of a layer of SnO occurs until a critical potential is reached, at which a different oxide or hydroxide (possibly SnOj) is formed and dissolution ceases. Finally oxygen is evolved from the passive metal. The nature of the surface films formed in KOH solutions up to 7 m and other alkaline solutions has also been examined. 

The electrochemical examination of fusion joints between nine pairs of dissimilar metal couples in seawater showed that in most cases the HAZ was anodic to the weld metals" . Prasad Rao and Prasanna Kumarundertook electrochemical studies of austenitic stainless steel claddings to find that heat input and 5Fe content significantly affected the anodic polarisation behaviour under active corrosion conditions whilst Herbsleb and Stoffelo found that two-phased weld claddings of the 24Cr-13Ni type were susceptible to inter-granular attack (IGA) as a result of sensitisation after heat treatment at 600°C /pa was unaffected by heat input. 

Polarise all cathodic areas to open circuit potential of most active anode areas. 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

The use of the potentiostatic method has helped to show that the process of self-passivation is practically identical to that which occurs when the metal is made anodically passive by the application of an external current" . The polarisation curve usually observed is shown schematically in Fig. 19.37a. Without the use of a potentiostat, the active portion of the curve AB would make a sudden transition to the curve DE, e.g. along curve AFE or AFD, and observation of the part of the curve BCDE during anodic polarisation was not common until the potentiostat was used. 

Active Loop the region of an anodic polarisation curve of a metal comprising the active region and the active-passive transition. 

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