Galvanic cells factor

Conditions necessary for the onset of corrosion are quite often provided by heterogeneities. These heterogeneities may very well exist within the metal or alloy or may be imposed by external factors. These heterogeneities can give rise to variations in potential on a metal surface immersed in an electrolytic fluid. The galvanic cell thus formed gives rise to flow of current that accompanies corrosion [188]. 

What factors affect the cell potential of a galvanic cell ... 

Predict any other factors that you think might affect the voltage of a galvanic cell. Describe an investigation you could complete to test your prediction. 

It follows that the contact potential between the two metals (difference between the work functions) is the principal factor determining the e.m.f. of galvanic cells consisting of two metals it is generally decidedly larger than the difference between the two electrode-electrolyte potentials, V -Vs, and VP— Vs. 

We can use standard electrode potentials and the Nernst equation to calculate the potential obtainable from a galvanic cell or the potential required to operate an electrolytic cell. The calculated potentials (sometimes called thermodynamic potentials) are theoretical in the sense that they refer to cells in which there is no current. As we show in Chapter 22, additional factors must be taken into account if a current is involved. 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

If the oxidation process (or half-reaction) is physically separated from the reduction half-reaction, electrons can be made to traverse a circuit. A chemical reaction used to generate an electric current is called a galvanic cell, and a commercially important example of such a cell is the battery. A number of factors influence the nature and function of a galvanic... 

The distinguishing factor between the thermodynamic and practical conventions is the direction of current flow. In the thermodynamic convention, positive current always flows from the cathode, whether the ceU is galvanic or electrolytic. In the practical convention, positive current flows from the positive electrode of a galvanic cell, yet to the positive electrode of an electrolytic cell. Interconversion between the... 

Fig. 4 Graphical illustration of the polarization (red) and power (blue) characteristics of an ideal photogalvanic cell. Note that the fill factor as defined by Eq. 17 is 25 % for an ideal galvanic cell under discharge...

For certain types of recM tion it is possible to obtain a direct conversion of chemical energy into electricity and thereby into mechanical work. Processes of heat transfer are thereby avoided and the conversion factor, I be heat engine does not come into the picture. This can be achieved whenever the reaction has an ionic mechanism and can be set up as a galvanic cell. The electrodes act in much the same manner as the semi-permeable membranes of the equilibrium box. That is to say, the reagents can be added to the system, and the recK tion products withdrawn, under reversible conditions. The two great prcu tical advantages of this method of operation are as follows ... 

It is important to realize that galvanic corrosion effects can be manifested not only on the macroscopic level but also within the microstructure of a material. Certain phases or precipitates will undergo anodic dissolution under microgalvanic effects. Because the principle of galvanic corrosion is widely known, it is remarkable that it still features prominently in numerous corrosion failures. Figure 7.17 illustrates the main factors affecting the formation of a galvanic cell [14]. 

Generally speaking, the following factors should be considered in analyzing the micro-galvanic cell related corrosion of Mg alloys. 

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