Resistance anode interface

To address this type of corrosion mitigation problem, a specialized CP system was developed that can minimize the drying effect at the anode/concrete interface without direet water exposure. By creating a low-resistance anode interface using a semicon-ductive layer the effect of electrochemical osmosis at the anode/concrete interface can be minimized, so that the CP system is not dependent on water or moisture from the outside environment and can also operate at low voltage. In addition, the semicon-ductive layer would minimize acid generation if the anode comes into direct contact with saltwater present because of new water leakages. 

This sharp decline in cell output at subzero temperatures is the combined consequence of the decreased capacity utilization and depressed cell potential at a given drain rate, and the possible causes have been attributed so far, under various conditions, to the retarded ion transport in bulk electrolyte solutions, ° ° - ° ° the increased resistance of the surface films at either the cathode/electrolyte inter-face506,507 Qj. anode/electrolyte interface, the resistance associated with charge-transfer processes at both cathode and anode interfaces, and the retarded diffusion coefficients of lithium ion in lithiated graphite anodes. - The efforts by different research teams have targeted those individual electrolyte-related properties to widen the temperature range of service for lithium ion cells. 

Ionic path impedance through the ceramic phase and electronic path impedance through the metallic phase have been found to be dominant factors in determining the electrode characteristics.In order to provide a low sheet resistance anode, it is imperative that the metallic and ceramic phases have maximized continuous paths that allow ionic and electronic migration from the electrolyte/anode interface throughout the entire anode. 

Figures 4.3(a) and (b) are sections in the zx-plane showing the distribution of potential (( )) in the solution as cross sections of imaginary surfaces in the solution of equal potential (isopotentials) and the distribution of current as current channels with cross sections defined by traces of the surfaces. ..(n - l),n, (n + 1)... perpendicular to the isopotentials. These traces are located such that each current channel carries the same total current. Figure 4.3(a) applies to an environment of higher resistivity (e.g., water with specific resistivity of 1000 ohm-cm) and Fig. 4.3(b) to an environment of lower resistivity (e.g., salt brine, 50ohm-cm). The figures are representative of anodic and cathodic reactions, which, if uncoupled, would have equilibrium half-cell potentials of E M = -1000 mV and E x = 0 mV and would, therefore, produce a thermodynamic driving force of Ecell = E x - E M = +1000 mV. This positive Ecell indicates that corrosion will occur when the reactions are coupled. For the example of Fig. 4.3(a), the high solution resistivity allows the potential E"m at the anode to approach its equilibrium value (E M = -1000 mV) and, therefore, allows the potential in the solution at the anode interface, < )s a, to approach +1000 mV (recall that (j)s = -E"M). The first isopotential above the anode, 900 mV, approaches this value. The solution isopotentials are observed to decrease progressively and approach 0 mV at the cathode reaction site.

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

Finally, it is important to point out that although in localised corrosion the anodic and cathodic areas are physically distinguishable, it does not follow that the total geometrical areas available are actually involved in the charge transfer process. Thus in the corrosion of two dissimilar metals in contact (bimetallic corrosion) the metal of more positive potential (the predominantly cathodic area of the bimetallic couple) may have a very much larger area than that of the predominantly anodic metal, but only the area adjacent to the anode may be effective as a cathode. In fact in a solution of high resistivity the effective areas of both metals will not extend appreciably from the interface of contact. Thus the effective areas of the anodic and cathodic sites may be much smaller than their geometrical areas. 

Fig. 1.69 Effect of resistivity of solution on the distribution of corrosion on the more negative metal of a bimetallic couple, (a) Solution of very low resistivity and (b) solution of very high resistivity. Note that when the resisitivity is high the effective areas of the cathodic and anodic metals are confined to the interface between the two metals...

FIGURE 2.12 Effect of Ni content on the electrode/electrolyte interface conductivity (aE) and the ohmic resistance (Wohm) for Ni-YSZ cermet anodes prepared using NiO-YSZ powder mixtures precalcinated at 1400°C and sintered at 1500°C. (From Kawada, T. et al.,./. Electrochem. Soc., 137 3042-3047, 1990. Reproduced by permission of ECS-The Electrochemical Society.)... 

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