Corrosion process electronic electrode potential

Chemical and electrochemical processes that cause materials corrosion usually involve both reduction-oxidations and acid-base reactions. The reduction-oxidation reaction is dependent on the electron energy level of the particles involved in the reaction, and hence managing the electrode potential of corroding materials may control the corrosion reaction. The acid-base reaction, on the other side, is determined by the HSAB characteristics (hard and soft acids and bases) of the particles involved in the reaction. It is mainly through the acid-base property that the environmental substances such as aggressive salts affect the corrosion of solid materials. 

Crevice corrosion occurs as a result of a non-uniform concentration of eletrolyte solution. The local electrode potential varies with the concentration of electrolyte corrosion currents (electron and ion migration) that occur in the metal and solution. For instance a piece of copper immersed in copper sulphate solution of varying composition, tends to be more positive in the region of high copper sulphate concentration due to the enhanced rate of deposition of Cu ions. A flow of electrons through the metal towards this region occurs and the ion deposition is a continuous process. As a result corrosion occurs in the metal where the solution concentration is at its lowest, and Cu ions pass into solution. 

The equilibrium of processes occurring at the electrode are disturbed when a net reaction occurs and produces current in the external circuit. The current induces a potential change and subsequently causes polarization of the electrode. The principle of charge conservation requires that the total rate of oxidation must be equal to the total rate of reduction for any corrosion process. To avoid the accumulation of charge on the electrode, the sum of anodic currents must equal the sum of cathodic currents. The electrochemical reaction at the anode is the oxidation (loss of electrons) of the metal, corrosion. 

Electrode Potentials and Potentlometry Controlled-Potentlal Electrolysis and Voltammetry Electron-Transfer Processes Electrochemical Characterization of Molecules Industrial Electrosynthesis Batteries and Fuel Cells Corrosion Cathodic Protection... 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

Considerable progress has been made during the past decade toward a better insight into the basic concepts and mechanism involved in metallic dissolution and corrosion. More emphasis has been placed on the "fundamental particles (metallic ions, electrons, and electron acceptors) and on the use of current-voltage characteristics. The wide recognition of dissolution and corrosion as electrode processes, and the idea of a polyelectrode exhibiting a mixed potential, have augmented the use of electrochemical techniques in the study and interpretation of corrosion phenomena. There is even some evidence that the phenomenon of passivity may soon be clarified. 

The dry cell battery is a typical example of galvanic corrosion, or two metal corrosion as it is otherwise called. When two dissimilar metals are immersed in a conductive or corrosive medium, there is always the potential for a change in them. Once these metals are connected this difference induces electron flow between them. The less corrosion resistant metal is attacked more than the more resistant metal. This is an electrochemical process. In the case of a dry cell battery, the carbon electrode acts as the cathode (the more resistant materials) and zinc as the corroding anode. The natural phenomenon of corrosion is used in this case for producing electricity. 

Related to the corrosion problems was a recent SECM study, which demonstrated the possibility of eliminating typical experimental problems encountered in the measurements of heterogeneous electron transfer at semiconductor electrodes (27). In this experiment, the redox reaction of interest (e.g., reduction of Ru(NH3)s+) is driven at a diffusion-controlled rate at the tip. The rate of reaction at the semiconductor substrate is probed by measuring the feedback current as a function of substrate potential. By holding the substrate at a potential where no other species than the tip-generated one would react at the substrate, most irreversible parasitic processes, such as corrosion, did not contribute to the tip current. Thus, separation of the redox reaction of interest from parallel processes at the semiconductor electrode was achieved. 

If two or more electrochemical half-cell reactions can occur simultaneously at a metal surface, the metal acts as a mixed electrode and exhibits a potential relative to a reference electrode that is a function of the interaction of the several electrochemical reactions. If the metal can be considered inert, the interaction will be between species in the solution that can be oxidized by other species, which, in turn, will be reduced. For example, ferrous ions can be oxidized to ferric ions by dissolved oxygen and the oxygen reduced to water, the two processes occurring at different positions on the inert metal surface with electron transfer through the metal. If the metal is reactive, oxidation (corrosion) to convert metal to ions or reduction of ions in solution to the neutral metal introduces additional electrochemical reactions that contribute to the mixed electrode. 

A schematic representation of the ideal electron-transfer rate and transfer coefficient as functions of potential for a semiconductor electrode is shown in Figure 18.2.6. Although there have been numerous studies with semiconductor electrodes, such ideal behavior is rarely seen (45, 47, 49, 57-59). Difficulties in such measurements include the presence of processes in parallel with the electron-transfer reaction involving dissolved reactant at the semiconductor surface, such as corrosion of the semiconductor material, effects of the resistance of the electrode material, and charge-transfer reactions that occur via surface states. 

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