No abstract

A model for the cathodic electrode activation is developed so as to account for the polarization curves and impedance plots obtained for zinc deposition in alkaline electrolytes. The reaction pattern involves the two-step discharge of zincate ions through an oxide-containing layer whose ionic and electronic conductivities are potential activated. The sharp electrode activation with increasing cathodic polarization is shown to be related to the spreading and thinning of the conductive layer. These phenomena and the concentration of the monovalent intermediate in the layer account for the three timeconstants distinguished in the inductive electrode impedance. The growth of granular compact deposits, favored by trace lead in the electrolyte, is associated with the exmtenee of a uniformly conductive layer on the whole electrode surface. The presence of a fluorinated surfactant (FI 110) inhibits the formation of spongy deposits in close connection with modifications to both the kinetic parameters of reaehons and the geometrical parameters of the conductive layer.

A model for the kinetics of active zinc dissolution is developed taking into account the presence of an interfacial layer. From electrode impedance spectroscopy, it is shown that the metal dissolution takes place essentially at the base of pores in a conductive layer of oxidation products which is progressively degraded by the anodic current. This model accounts for the four loops observed on complex plane impedance plots with decreasing frequency: (i) a capacitive loop generally highly depressed in connection with the current penetration within pores; (ii) an inductive loop corresponding to the presence of a monovalent intermediate ZnI in the reactive interphase; (iii) a capacitive loop resulting from the precipitation and escape of ZnII ions by diffusion from the pore bases; and (iv) an inductive loop consequent on the slow decrease of the pore length with increasing anodic polarization. Inhibition of zinc corrosion by an organic additive is shown to be related to changes in both the kinetic parameters of reactions and the layer properties.

a b s t r a c tThe present work describes a new methodology for contact free impedance of a solution in a polymer microchip taking into account the role played by the surrounding polymer on the impedance accuracy. Measurements were carried out using a photoablated polyethylene terephthalate (PET) microchannel above two embedded microband electrodes. The impedance diagrams exhibit a loop from high frequencies to medium frequencies (1 MHz-100 Hz) and a capacitive behavior at low frequencies (100-1 Hz). The impedance diagrams were corrected by eliminating from the global microchip response the contribution of the impedance of the PET layer between the two microband electrodes. This operation enables a clear observation of the impedance in the microchannel solution, including the bulk solution contribution and the interfacial capacitance related to the surface roughness of the photoablated microchannel. Models for the impedance of solutions of varying conductivity showed that the capacitance of the polymer-solution interface can be modeled by a constant phase element (CPE) with an exponent of 0.5. The loop diameter was found to be proportional to the microchannel resistivity, allowing a cell constant around 4.93 × 10 5 m −1 in contactless microelectrodes configuration.