Heterogeneous Mixed Electrodes and Cell Formation

There are no eurrent-density-potential eurves for mixed eleetrodes, only eur-rent-density-potential bands whieh ean be represented in a three-dimensional J-U-x 

The ratio of the areas of cathodes to anodes is decisive for the potential damage resulting from cell formation [16,17]. Using the integral (mean) polarization resistances 

The difference in rest potentials (see the practical potential series in Table 2-4) determines mostly the direction of the current and less of the level for these the resistances are significant. In particular can be neglected in the external corrosion of extended objects. In addition, the IJJJ) curve is usually steeper than the I U) curve (i.e., R. By introducing the surface areas of anode and cathode 

In electrochemical protection the necessary range of protection current is achieved by an appropriate arrangement of the electrodes. It follows that measures which raise the polarization resistance are beneficial. Coated objects have a coating resistance (see Section 5.2), which can be utilized in much the same way as the polarization resistance in Eq. (2-45). Therefore, the range in the medium can be extended almost at will by coatings for extended objects, even at low conductivity. However, the range is then limited by current supply to the object to be protected (see Section 24.4). 

Even for high-resistance media with x = 10 S cm , sufficient protection is obtained with only A0 = 0.1 V from the criterion of Eq. (2-40) -J = 0.14 7 

Note Rest potentials U in mV for common metals in (a) phthalate buffer at pH 6 and (b) artificial seawater [18], 25°C, air saturated and stirred. The rest potentials of the values in parentheses tend to become mote positive with time due to film formation. (The values ate dependent on the medium and operating conditions.) 

Sometimes difficulties arise because the cathode of the cell has a more positive potential than the anode (see Fig. 2-6). This is because the definition of anode and cathode is based on processes in the electrolyte, whereas potential measurement is based on events on the metal. This fact is illustrated in Fig. 2-8. If electrodes Pt and Fe are both in the same electrolyte with potential then from the metal point of view, Pt is more positive than Fe. U is the electromagnetic force (emf) of the cell. When the switch S is closed, electrons flow from Fe(-) to Pt(+). If both electrodes are initially connected and immersed in separate electrolytes, they both have the potential The electrolyte at Fe is now more positive than that at Pt. The voltage U can be measured between two reference electrodes in the electrolytes. When the tap H is opened, a positive current flows in the electrolyte from Fe(+) to Pt(-). 

---