Dissolution under Open Circuit Conditions

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. 

The same reactions occur at the corresponding n-type electrode (Fig. 8.12). In the anodic range, the total current remains very small upon addition of the redox system. However, it is determined by two partial currents, namely the cathodic reduction current and the anodic decomposition current, as can be proved analytically. A description in terms of quasi-Fermi levels has already been given in Section 7.4.1. 

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In more recent work embrittlement in water vapour-saturated air and in various aqueous solutions has been systematically examined together with the influence of strain rate, alloy composition and loading mode, all in conjunction with various metallographic techniques. The general conclusion is that stress-corrosion crack propagation in aluminium alloys under open circuit conditions is mainly caused by hydrogen embrittlement, but that there is a component of the fracture process that is caused by dissolution. The relative importance of these two processes may well vary between alloys of different composition or even between specimens of an alloy that have been heat treated differently. 

Under -> open-circuit conditions a possible passivation depends seriously on the environment, i.e., the pH of the solution and the potential of the redox system which is present within the electrolyte and its kinetics. For electrochemical studies redox systems are replaced by a -> potentiostat. Thus one may study the passivating properties of the metal independently of the thermodynamic or kinetic properties of the redox system. However, if a metal is passivated in a solution at open-circuit conditions the cathodic current density of the redox system has to exceed the maximum anodic dissolution current density of the metal to shift the electrode potential into the passive range (Fig. 1 of the next entry (- passivation potential)). In the case of iron, concentrated nitric acid will passivate the metal surface whereas diluted nitric acid does not passivate. However, diluted nitric acid may sustain passivity if the metal has been passivated before by other means. Thus redox systems may induce or only maintain passivity depending on their electrode potential and the kinetics of their reduction. In consequence, it depends on the characteristics of metal disso-... 

Another example of a galvanic cell reaction is provided by open circuit corrosion of the metal deposit. Freshly deposited (and particularly finely-divided) metals are more active than their bulk, compact counter parts. Corrosion of the mixed electrode deposit may ensue if the cathode surface is left under open circuit conditions metal dissolution is balanced via reduction of species such as dissolved oxygen, protons or higher oxidation states of transition metal ions. Illustrative (simplified) examples of such oxidising agents include the following ... 

Nanoscopic Investigations of Dealloyed Surfaces Erom the background of competitive models of selective alloy dissolution as described above, a closer microscopic examination of this process with the ultimate objective of atomic resolution and chemical information on an atomic scale appears mandatory. Ex situ transmission electron microscopy (TEM) of thin, corroded alloy films provides lateral resolution at the nanometer scale, but suffers from poor depth resolution and from structural relaxation processes that may occur after termination of the anodic polarization and transferring the samples into high vacuum. Classical TEM investigations in this field were performed under open circuit conditions in oxidizing environments (that is, at > Eq) [51,... 

A deposit may be removed rapidly and selectively from the substrate by immersion in a suitable electrolyte, particularly in the reclamation of damaged or faulty electroplated workpieces. The technique may be carried out under open-circuit conditions or metal dissolution may be accelerated by anodic stimulation. The electrolyte may be chosen such that the substrate is immune or passive under the process conditions. In some circumstances, the rate of formation of this layer is enhanced by carrying out the coating electrochemically with the surfaces as the anode. Commonly, however, the layer is sprayed on and dried. 

Etching of the material may be carried out either chemically under open-circuit conditions (ie. controlled corrosion (Chapter 10)) or it may be elcctrochemically driven by applying a potential The former case is more common. It requires no power supply or auxiliary electrodes the electrolyte conditions are chosen such that the species to be removed is dissolved at a reasonable rale, courtesy of a simultaneous cathodic process. Taking the case of the dissolution of a metal M, the anodic process in reaction (9.1) is supported by a suitable electrorcduction ... 

Nonsteady behavior of electrochemical systems was observed by -> Fechner as early as 1828 [ii]. Periodic or chaotic changes of electrode potential under - gal-vanostatic or open-circuit conditions and similar variation of -> current under potentiostatic conditions have been the subject of numerous studies [iii, iv]. The electrochemical systems, for which interesting dynamic behavior has been reported include anodic or open-circuit dissolution of metals [v-vii], electrooxidation of small organic molecules [viii-xiv] or hydrogen, reduction of anions [xv, xvi] etc. [ii]. Much effort regarding the theoretical description and mathematical modeling of these complex phenomena has been made [xvii-xix]. Especially studies that used combined techniques, such as radiotracer (- tracer methods)(Fig. 1) [x], electrochemi-... 

This study was on a smooth platinum foil using gravimetric analyses and it was found that the equilibrium concentrations of platinum in the phosphoric acid were achieved rapidly, i.e., within one hour. Since it is to be expected that the kinetics of dissolution would be modified with high surface-area platinum crystallites, this equilibrium would be achieved more rapidly. Consequently, it is clear that, at the higher potentials, dissolution of the platinum is of great concern so that operating procedures must be established to prevent exposure under hot open-circuit conditions. 

Although the electrochemical nature of the processes involved in the formation of PS at open-circuit conditions (nonbiased) should be similar to that under anodic bias, there are several major differences in the formation conditions. The first is that at the OCP the driving force is provided by the oxidation agents, the reduction of which provides the anodic polarization of the electrode needed for silicon dissolution. Unlike the externally biased condition, the extent of polarization is limited by the oxidation power of the oxidation agents. The second is that the carrier supply at the open-circuit condition is localized and randomly oriented, while that at anodic potential is perpendicular to the surface. The anodic and cathodic sites in the chemical etching process must be in the vicinity of each other, and continuous alternations must occur between anodic and cathodic reactions on the surface at the pores tips. 

Dissolution of PS. The dissolution of PS during PS formation may be due to two proeesses a proeess in the dark and a proeess under illumination. Both are essentially eorrosion proeesses by which the silicon in the PS is oxidized and dissolved with simultaneous reduction of the oxidizing species in the solution. The corrosion process is responsible for the formation of micro PS of certain thickness (stain film) as well as the dissolution of the existing PS. The material in the PS which is at a certain distance from the pore tips is little affected by the extanal bias due to the high resistivity of PS and is essentially at an open-circuit condition (OCP). This dissolution process, which is often referred to as chemical dissolution, is an electrochemical process because it involves charge transfer across the interface. The anodic and cathodic reactions in the microscopic corrosion cells depend on factors such as surface potential and carrier concentration on the surface which can be affected by illumination and the presence of oxidants in the solution. 

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