Impedance in a Confined Medium

Thin-layer cells are experimental devices of interest for simulation of various localized corrosion processes. For example, a thin moisture film covers the metal surface during atmospheric corrosion, and the thin-layer geometry is also relevant to crevice corrosion, and corrosion under delaminated protective films. Thin-layer cells are usually achieved by confining a thin electrolyte layer (the thickness 

In a thin-layer cell, the residual convection ( natural convection ) which always exists in a large volume of solution becomes negligible. When the migration processes of the electroactive species can also be neglected (for instance, in presence of a supporting electrolyte), the mass transport is purely achieved by diffiision and is determined by Pick s laws  

Prom a more general point of view, Keddam et al. also demonstrated that the impedance measured in a thin-layer-cell configuration is significantly modified by the existence of a radial potential 

As discussed above, if we assume that diffusion processes occur only normal to the electrode surface (one-dimensional model), the theoretical diffusion impedance associated with the transport of an electroactive species can be derived analytically in a thin-layer-cell configuration. For a thin-layer cell of thickness e, the expression of this impedance will depend on the condition considered at the boundary. 

In the case of a boundary impermeable to the diffusing species [i.e., dc/dx)x = 0], a one-dimensional spatially restricted difili-sion impedance Zid (S2 cm ) is obtained at the electrode  

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