Transport control, cathodic partial process

Figure 8 shows the electrode potential-current density behavior of a cathodic partial process demonstrating activation control, transport control and the transition region between them. The dashed line represents the extension of the Tafel line, i.e.. Equation 36. This dashed line predicts current densities which exceed the rate of mass transport - an impossibility. The vertical (electrode potential independent) line represents the limiting current density for the process. Although mathematical relationships have been proposed for the transition region, their utility is minimal since the limiting cases of activation and... 

Figure 8. Electrode potential—current density behavior of a cathodic partial process showing regions of activation control, transport control, and the transition...

Mixed potential systems with the cathodic partial process under transport control and the anodic partial process under activation control is typical of many corrosion systems. For the cathodic partial process to be under transport control. Equation 44 must be unity or larger. This occurs when the absolute value of the difference between the equilibrium electrode potential of the cathodic partial process and the corrosion is on the order of one volt. This condition prevails for most metals of interest in corrosion studies if oxygen... 

Equation 61 demonstrates that the corrosion rate for this class of systems is controlled uniquely by the by the rate of mass transport. Comparing Equation 61 with Equation 53 reveals that the corrosion potential is defined by the natures of the anodic and cathodic partial processes for Equation 53 while, in the case at hand, the corrosion potential is influenced by the magnitude of the mass transfer coefficient - a property of the convective mass transport condition. 

Although most corrosion systems can be described by the limiting models presented above, there are instances where control of the corrosion system is a combination of both types, viz., activation controlled anodic partial process with two cathodic partial processes - one under activation control and another under transport control. Examples are iron corrosion in acid solution with inorganic contaminants (, 18) and oxygen ( ). The corrosion current density in such systems is... 

Mass transport control of electrode reactions can be caused by one of two physical processes. In the first case, the interfacial concentration of the reactant drops to zero, i.e., the electron transfer reaction consumes the species as quickly as it arrives at the interface. In the second case, the interfacial concentration of the product reaches saturation. When these conditions prevail, the rate of mass transport is at its limiting (maximum) value. Limiting current densities, ij and in, for anodic and cathodic partial reactions, respectively, are... 

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