Cathodic partial process

If the cathodic partial current is larger than the anodic partial current, a total cathodic or reduction current will flow through the electrochemical interface, and vice versa. If both anodic and cathodic partial processes at an electrode are balanced, that is both partial currents are equal, no net reaction will take place at the electrode and no total net current will be observed through the external circuit. However, both... 

Figure 14. Model physical development cell in which the anodic and cathodic partial processes are separated.

Photoetching processes do not always consist of a simple superposition of an anodic and a cathodic partial process and may exhibit various types of complications. Firstly, even in the simple case of the photoetching of GaP single crystals in alkaline OBr solutions, the situation is actually more complex than depicted above, since at n-type crystals, it appears that the photoetching process itself induces a hole injection reaction and hence and electroless etching effect [24]. Initially, OBr is reduced at the GaP surface via the current-doubling mechanism (as is concluded from photocurrent measurements at p-type samples) ... 

Overall rate laws such as those discussed above are useful for obtaining information on which variables must be controlled more closely in order to maintain a constant deposition rate in practical electroless plating. However, overall rate laws do not provide any mechanistic information. Donahue and Shippey [14] proposed a method of deriving rate laws for partial anodic and cathodic processes in order to gain insight into the mechanism of electroless deposition reactions. If it is assumed that the anodic and cathodic partial processes may interact with each other, then the general rate laws for the partial reactions can be written as follows ... 

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...

The anodic partial process. Equation 46, generates the electrons which are used in the cathodic partial process, Equation 47. This model of corrosion processes is based on the theory of mixed potentials (11) and is shown schematically in Figure 9. The original theory of mixed potentials was based on the "superposition" of polarization curves for the respective partial processes (11-13). However, since many mixed potential systems (particularly corrosion processes) involve interactions among the reactants, the presentation of mixed potentials given here will consider the more recent approach considering these interactions (14). 

When there are two partial process in a mixed potential system and both are under activation control, the most probable forms of the current densities of the anodic and cathodic partial processes are Equations 33 and 35, respectively. For an isolated metal, the overpotential (since the corrosion potential represents the perturbed electrode potential in this case) is... 

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... 

The corrosion current density for this class of corrosion sytems (assuming that the limiting current density for the cathodic partial process is given by Equation 42) is... 

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. 

Equation 62 predicts Tafel behavior only for anodic (positive) polarization. Cathodic polarization is predicted to be potential independent at large negative polarizations. However, for most corrosion systems, this region of potential independence is small due to the presence of other cathodic partial processes, e.g., solvent decomposition to form hydrogen gas. 

While these other cathodic partial processes usually do not participate in the corrosion system per se, they are manifested in the experimental data and can cause difficulty in analyzing the data. Methods of compensating for these effects have been employed with success (16, 17). 

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... 

In the case of nitrobenzoates, for example, it has been claimed that an acceleration of the cathodic partial process by reduction of the nitro group may lead, in addition to the effect of oxygen in the thin electrolyte layer, to a complete passivation of iron or ordinary steels. Contributions from the two parts of the dissociated molecule to the inhibitive effect are very likely. 

Consider a freely corroding metal in an acid. The anodic partial process represented by M M ++2e intersects the cathodic partial process H2—> 2H -l-2e at Econ> the corrosion potential. The current corresponding to Econ is orr-The current between the local (microscopic) anodes and cathodes cannot be obviously measured by conventional means, such as by placing an ammeter. At orr (freely corroding potential), the current is icorr and the rate of forward process (if) is equal to the rate of reverse process if = What can, therefore, be done to measure the current An experiment can be designed to measure... 

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