Decomposition cathodic dissolution

Three anodic partial reactions are considered active dissolution of two metals M and M with different kinetics in the absence of their ions in bulk solution and decomposition of water with the evolution of oxygen. The kinetics of the latter process is so slow on most corroding metals that only at very negative potentials can oxygen present in the solution be electroreduced and this eventually becomes limited by mass transport due to the limited solubility of oxygen in water. At even more negative potentials, hydrogen evolution takes place on the electrode surface. The cathodic reduction of some metal ions present on the electrode surface as a consequence of corrosion is also considered in Fig. 13(b). 

Anodic decomposition is also possible in the presence of a suitable redox system in the solution without external voltage. Taking a redox couple of a very positive standard potential, such as Ce /Ce, then holes are injected from Ce ions into the valence band of the semiconductor. These holes are available for the anodic decomposition. The corresponding curves (with and without the redox system) for a p-type electrode are shown in Fig. 8.11. The cathodic reduction of the redox system sets in at Uiedox i.e. when the quasi-Fermi level of holes passes ,-edox (dotted line). In the cathodic range the current is diffusion-limited. The resulting j-U curve (dashed curve) passes the potential axis at a value at which cathodic and anodic currents are equal (compare also with Section 7.4.1). The rate of the dissolution current is controlled by the redox system. In the case of a very stable semiconductor, the decomposition current is much smaller and the two partial currents would be equal at more positive potentials. 

Hoar Ci J found that in the corrosion inhibition of iron in hydrochloric acid by g-naphthoquinoline, the corrosion potential increases monotonically with increasing inhibitor concentration, while in the case of o-tolylthiourea one observes first a decrease of the corrosion potential followed by an increase at higher inhibitor concentrations. A similar predominant inhibition of the cathodic partial reaction at small inhibitor concentrations is exhibited also by phenylthiourea according to Kaesche. Furthermore, in the series of the thiourea derivatives one often finds corrosion acceleration at small concentrations, as for instance in the case of phenylthiourea at concentrations of 10- moles per liter. This appears to be due to a small cathodic decomposition of thiourea and its derivatives in the course of which hydrogen sulfide is formed. As is well known, hydrogen sulfide tends to accelerate corrosion, in particular the anodic partial reaction of dissolution of iron, which has been demonstrated independently by other authors (17). 

In ECM, the higher the applied potential difference, the greater the rate of metal dissolution at the anode and hydrogen evolution at the cathode. The shape of the curve of applied potential difference against current is shown in Fig. 2.12. For initial values of potential difference the current is low along OA. When the values of the potential difference correspond to region A, the current rises sharply. The current increases appreciably along AB when the potential difference is further increased. Point A on the curve represents the onset of anodic dissolution of the process. The cmve AB is extrapolated back to the zero value current and meets the axis of the applied potential difference at point C. The potential at point C is known as decomposition potential [1]. 

Cell performance decay on cycling results from the following causes (1) anode crystal structure degradation [45-47], (2) cathode crystal structure degradation [22,48-50], (3) electrode/electrolyte interface properties degradation [51-53], (4) metal dissolution [54-56], (5) electrolyte decomposition [57-60], and (6) surface film formation [60,61],... 

The sustained dissolution or the surface transformation into passivating films of semicOTiductors in contact with electrolytes is limiting the lifetime of energy-converting devices and considerable efforts have been made to overcome this deficiency [65-69]. Rather early, criteria have been developed and recently been addressed again that allow to determine whether a semiconductor is thermodynamically stable [70, 71]. The method relates the position of the quasi-Fermi levels at the surface (see Eqs. 17 and 18) with those of the anodic or cathodic decomposition energies. The calculation of the decomposition levels is based on the respective corrosion reaction and a few... 

The enhanced cycling performance has been attributed to the thinner cathode electrolyte interface film originated from di(meth-ylsulfonyl) methane on the LiNii/3C0i/3Mn l/3O2. This not only results in a lower interfacial impedance, but also protects the decomposition of electrolyte and so prevents the cathode transition metal dissolution at high voltages (91). 

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