Finite-element simulations were used to calculate the influence of surface roughness of V-shaped grooves on the impedance of ideally polarized disk electrodes. The characteristic length associated with roughness was found to depend on the width or period of the roughness as well as the roughness factor. The characteristic dimension associated with a rough disk was shown to be the disk radius multiplied by the roughness factor. This work shows that for small roughness factors, while roughness causes frequency dispersion, the frequency dispersion is seen only at frequencies that are much higher than those associated with the disk geometry. Thus, a constant-phase element associated with a surface distribution of time constants cannot be attributed to surface roughness.
In this work, a rotating disk electrode was used to measure the cathodic kinetics on stainless steel as a function of diffusion layer thickness (6 to 60 μm) and chloride concentration (0.6 to 5.3 M NaCl). It was found that, while the cathodic kinetics followed the Levich equation for large diffusion layer thicknesses, the Levich equation overpredicts the mass-transfer limited current density for diffusion layer thicknesses less than 20 μm. Also, an unusual transitory response between the activation and mass-transfer controlled regions was observed for small diffusion layer thicknesses that was more apparent in lower concentration solutions. The presence and reduction of an oxide film and a transition in the oxygen reduction mechanism were identified as possible reasons for this response. The implications of these results on atmospheric corrosion kinetics under thin electrolyte layers is discussed.
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