Oxide outer boundary concentration

The concentration profile of fixed oxidized and reduced sites within the film depends on the dimensionless parameter Dcjr/d2, where r is the experimental timescale, i.e. RT/Fv in cyclic voltammetry, and d is the polymer layer thickness. When Dcix/d2 1, all electroactive sites within the film are in equilibrium with the electrode potential, and the surface-type behavior described previously is observed. In contrast, Dcjx/d2 <3C 1 when the oxidizing scan direction is switched before the reduced sites at the film s outer boundary are completely oxidized. The wave will exhibit distinctive diffusional tailing where these conditions prevail. At intermediate values of Dcjr/d2, an intermediate ip versus v dependence occurs, and a less pronounced diffusional tail appears. 

The rate of this and many other such phase boundary reactions depends upon the instantaneous state of the surface. For tarnishing processes, this generally means that the rate depends upon the instantaneous activities of the components at the phase boundary as well as upon the temperature. As long as diffusional equilibrium is maintained, and the outer phase boundary reaction alone is rate-controlling, the activity of the metal at the phase boundary between oxidation product and gas is constant and equal to one. There are indications [50] that the electronic defects can particularly influence the rate of dissociation of the gases at the phase boundary between oxidation product and gas. Use is made of this property of solid surfaces in the field of heterogeneous catalysis [4]. Since the defect concentration is determined by the activities of the components in the reaction product, it is understandable that the rate of the phase boundary reaction should, in general, depend upon the component activities in the reaction product at the phase boundary. 

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