Pitting corrosion redox reaction

Others are the reduction of Fe + and [Fe(CN)6] in solution. These systems are often used for chemical corrosion tests. Pitted metals expose a small area of a few intensively dissolving corrosion pits that are not protected by a passive layer and a large cathode of the passive metal surface. Because of the large size of the cathode, a much smaller cathodic current density is required for the compensating reduction of the redox system in comparison to the active metal dissolution within the pits. However, electronic conduction is still required across the passive layer. Figure 3 depicts the existing sections of a pitted metal surface with the related electrode reactions, the very small metal dissolution /pass, and the redox reaction ha,pass via the protecting oxide film and... 

Fig. 3 Schematic diagram of a passivated metal showing the corrosion current densities of metal dissolution and redox reactions at the surface of the passive layer, a pit, and an electron conducting inclusion.

Pitting corrosion is initiated by chloride migration from the electrolyte to the metal-passive film interface. Chloride ions adsorb on the outer metal oxide surface, permeate and interact with the underlying metal [9,10]. Redox reaction at the interface increases... 

The scanning tip can also be used to detect variations in the concentrations of dissolved species near the substrate surface, such as dioxygen, hydrogen ion, or metal ions (species that are particularly important in corrosion processes), or to locally modify near-surface solution conditions (e.g., pH or Cr concentration) by means of a tip redox reaction, often to initiate pit formation. Numerous examples of these approaches were discussed in our previous review [8]. 

Any of various functions from which intensity or velocity at any point in a field may be calculated. The driving influence of an electrochemical reaction. See also active potential, chemical potential, corrosion potential, critical pitting potential, decomposition potential, electrochemical potential, electrode potential, electrokinetic potential, equilibrium (reversible) potential, free corrosion potential, noble potential, open-circuit potential, protective potential, redox potential, and standard electrode potential. 

A comparable effect to the sacrificial electrodes is provided by the direct supply of a cathodic current to the dissolving metal. A proper cathodic current applied to the metal structure sets the potential to a value where corrosion is prevented. A disadvantage is the requirement of a permanent cormection to a current supply (or even a potentiostat). However, with this approach, the metal construction may be tuned in for complicated situations. Metallic structures often consist of several metals with different corrosion properties. The environmental conditions may be very difficult. For example, passivation should be maintained but the potential should not become more positive than the critical pitting potential. In these complicated cases, a potentiostat with a CE and a RE is useful. The WE of this circuit is the metal construction. Protection by a current source is applicable to chemical reaction vessels or constructions with permanent location but not always to mobile devices like cars, ships, etc. Therefore both methods are useful, and the choice depends on the specific requirements for the construction in service conditions. In well-conducting electrolytes, one has to take care of the equilibrium potentials of the involved electrodes (metal/metal ion and redox electrode). If the environment has a low conductivity (wet soil), ohmic drops have to be taken into account in order to establish an appropriate protecting... 

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