Galvanic cells, mechanism

Figure 4.42 Galvanic cell mechanism. (From Perkins, R.N. (1972). Br. Corros. Jr., 7, 15. Reproduced by kind permission of British Corrosion Journal, London, UK)...

Semiconductor electrodes can be used in galvanic cells like metal electrodes and a controlled electrode potential can be applied by means of a potentiostat, if the electrode can be contacted with a suitable metal without formation of a barrier layer (ohmic contact). Suitable techniques for ohmic contacts have been worked out in connection with semiconductor electronics. Surface treatment is important for the properties of semiconductor electrodes in all kind of charge transfer processes and especially in the photoresponse. Mechanical polishing generates a great number of new electronic states underneath the surface 29> which can act as quenchers for excited molecules at the interface. Therefore, sufficient etching is imperative for studying photocurrents caused by excited dyes. 

FIGURE 12.15 The mechanism of rust formation, (a) Oxidation of the iron occurs at a point out of contact with the oxygen of the air, and the surface of the metal acts as an anode in a tiny galvanic cell, (b) Further oxidation of Fe2+ to Fe3+ results in the deposition of rust on the surface. 

Of the Mechanical Origin of Corrodibility , which appeared in 1675 in London.2 It was not until the turn of the 19th century3,4 that some of the basic principles were understood, soon after the discovery of the galvanic cell and Davy s theory on the close relationship between electricity and chemical changes.5... 

The mechanism of sulphation has been explained [11] by means of an electrochemical process described by a galvanic cell formed between CaCOa acting as a negative pole and the corrosive environment (SO2 -I-air -I- water vapour) activy as the electrolyte. 

Figure 18-3 shows the movement of various charge carriers in a galvanic cell during diseharge. The electrodes are connected with a wire so that the spontaneous cell reaction occurs. Charge is transported through such an electrochemical cell by three mechanisms ... 

The decrease of Gibbs free energy is therefore equal to the reversible non-mechanical work done by the system at constant temperature and pressure. In this book the only non-mechanical work considered is electrical energy. For a galvanic cell this has already been shown, page 99, to be equal to EnF, so that... 

Such reactions are common in inorganic chemistry and serve for the preparation both on a laboratory and on an industrial scale of a number of metals. If this reaction takes place in solution, as for example when zinc is immersed in a solution of copper sulfate, forming zinc sulfate and copper, while the formulation of the mechanism of the reaction is not quite as simple as in the first case, the oxidation-reduction changes are the same. Such reactions also are very common, and are the source of electric currents in galvanic cells using various combinations of metals and solutions, the electromotive forces under certain conditions being a measure of the affinity of the chemical reaction taking place. 

The unstable behavior of some solder-replacement adhesives has been attributed to galvanic corrosion. Similar to most corrosion mechanisms, condensed or absorbed moisture on the surface and dissimilar metals are required to form a galvanic cell. The silver filler acts as a cathode while the substrate metallization acts as an anode and is oxidized. In the case of tin-lead solder surfaces, the solder, which has a lower electrochemical potential (0.13 V) than silver (0.79 V), becomes the anode at which corrosion and oxidation occur. A smaller potential difference between a copper surface and silver accounts for some improvement in contact resistance over the solder-silver couple. 

Ingenious mechanisms have been proposed [50, 51, 106] by which neutral species can participate in the transfer of anode material across a galvanic cell. If one has an electrolyte material in which, for whatever reason, anions can migrate whereas cations cannot, and through which neutral species can diffuse, then it is simple to envisage a process whereby ... 

The transport of mass in the form of ions through the electrolyte can often be attributed to a chemical reaction or a transport process at an electrode. In this way reaction rates can be measured electrically. It is often possible to analyze reaction mechanisms in detail by a combination of rate measurements by means of the electrical current with measurements of thermodynamic quantities— in particular, chemical potentials—by means of the emf of the galvanic cell. More details on kinetic investigations using galvanic cells will be given in Section V.B. 

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