C. Cachet

EIS investigation of zinc dissolution in aerated sulfate medium. Part I: bulk zinc

Abstract The kinetics of anodic dissolution and corrosion of zinc are investigated by electrochemical impedance spectroscopy in sulfate medium for various surface preparation conditions and different experimental procedures. A new version of the reaction model, which was established previously, accounts well for the experimental results. It implies several parallel paths of dissolution and three adsorbed intermediates: Zn+ad, Zn2+ad and ZnOHad. From the simulation of impedance data are deduced the values of quantities such as the partial current intensities of each parallel paths or the surface coverages by three adsorbates. It is shown that the surface preparation conditions may affect the balance between the competitive dissolution paths. When zinc is immersed in the electrolyte, a protective layer is formed, which inhibits the direct dissolution of the metal, but which tends to be eliminated by a potential activation. These phenomena are time dependent and may be affected by the slow formation of a corrosion product layer.

EIS investigation of zinc dissolution in aerated sulphate medium. Part II: zinc coatings

The kinetics of anodic dissolution and corrosion of zinc coatings deposited onto steel sheet either by electrodeposition or by hot dipping are investigated by electrochemical impedance spectroscopy in aerated sulfate medium. The results are compared with those obtained previously on pure bulk zinc and interpreted on the basis of the model derived in the first part of this paper. It is shown that zinc coatings are less sensitive to corrosion than pure bulk zinc and changes of their behavior with time are not identical. Important differences are observed between the various coatings in the respective contributions of the three parallel paths of the dissolution process. Three kinds of oxidation products were identified by Raman spectroscopy. A compact non-stoichiometric zinc oxide was formed by surface reaction on zinc. Above it, a thick and porous layer made of zinc hydroxi-sulfate and stoichiometric ZnO, was formed by precipitation from a local saturation of the solution. A strong correlation was evidenced between the oxidation products and the various paths of the reaction model. It was assumed that the impurities, initially present in the metal, may affect the interfacial reactions, the increase of the micro-roughness, and may also reinforce the protective properties of oxidation product layers. The differences between the various zinc coating behaviors result mainly from their impurities. Their crystal preferred orientations have no significant influence.

Reaction mechanism for zinc dissolution in chloride electrolytes

Abstract A model of interfacial processes is developed so as to account for the polarization curves and impedance plots obtained for zinc dissolution in various electrolytes containing ZnCl 2 and/or NH 4 Cl. The slow electrode activation with increasing anodic potential is related to the removal of zinc hydroxide which acts as a protective layer at corrosion potential. The reaction patterns for zinc dissolution involves two parallel paths. The major path is catalysed by the Zn ads I intermediate and the minor path involves a second intermediate such as Zn ads II . The minor path is much more dependent on the volume diffusion of reacting species than the major one. Both paths are strongly stimulated by chlorides anions which appear to be responsible for zinc corrosion.

The kinetics of zinc electrode in alkaline zincate electrolytes

Abstract Impedance measurements in conjunction with steady-state polarization measurements are reported for the dissolution and deposition of zinc in 0.5 M ZnO electrolytes for various KOH concentrations. Four faradaic relaxation processes are pointed out, with low values for some proper frequencies. It is concluded that zinc deposition and dissolution involve at least four adsorbates, do not occur only by series reactions and particularly imply a slow and sharp electrode activation with increasing overpotential. In addition a much lower charge transfer resistance observed for dissolution than for deposition indicates a change in the electrode kinetics on either side of the equilibrium potential. The inhibition of zinc deposition and dissolution by an additive (tetrabutylammonium bromide) depends on the KOH concentration and results from changes in the rates of interfacial processes.

Electrode kinetics connected to deposit growth for zinc electrodeposition: Influence of surfactants

Abstract Impedance measurements and SEM examination of deposits show that the mechanism of zinc deposition in zincate alkaline electrolytes is highly sensitive to the electrolyte content in anodically dissolved zinc (ADZ). Two distinct electrode kinetics closely connected to the deposit growth are pointed out: (i) a low-frequency capacitive feature when spongy deposits are formed in the presence of ADZ and (ii) a low-frequency inductive feature for the formation of granular deposits. This correlation subsists in the presence of surfactants which inhibit the growth of zinc moss and dendrites: then the impedance plots are typical of the mechanism leading to granular deposits with various roughness and grain size. By changing the rates of interfacial reactions, the surfactants yield an increased charge transfer resistance. A fluorinated surfactant generates a stronger inhibiting effect which strengthens with increasing concentration of ADZ.

Mechanism of zinc electrodeposition in acidic sulfate electrolytes containing Pb2+ ions

Abstract The influence of Pb 2+ ions on the kinetics of zinc electrodeposition in acidic sulfate electrolyte can be interpreted in terms of a reaction model involving hydrogen adsorption and evolution, a multistep mechanism for zinc deposition and the overall reaction for zinc dissolution. From impedance data and polarization curves, it is shown that lead essentially adsorbs on the electrode and thereby strongly inhibits all the reactions taking place on the zinc electrode. In addition, the presence of Pb ad also decreases specifically the rate constants of the reactions for zinc dissolution, hydrogen evolution, and affects the slow renewal of kink sites on the deposit surface. The slow inclusion of Pb ad in the deposit progressively releases the sites for zinc deposition with increasing cathodic polarization.

The Ag/Ag+ system: An impedance model for nucleation and growth

Abstract An analytical expression for the electrochemical impedance spectra is calculated, based on the assumption of successive monolayers generated at a mean nucleation rate and governed by the same ageing law. From simulated complex plane impedance plots, it is shown that this model can be applied to highly reversible metal-ion systems such as Ag + ⇋Ag.

Influence of a perfluorinated surfactant on the mechanism of zinc deposition in acidic electrolytes

The influence of the non-ionic surfactant Forafac F1110 on the kinetics of zinc deposition in sulfate or chloride solutions is investigated by voltammetry and impedance spectroscopy. From a discussion of the results in terms of a reaction model, it is shown that the additive produces a significant inhibition of the corrosion process. In addition and in a specific way for each electrolyte, the additive (i) inhibits the different steps of charge transfer leading to the zinc deposit and (ii) modifies the kinetic parameters of the slow reactions involved in the formation and destruction of the active sites for zinc deposition.

The pore texture of zinc electrodes characterized by impedance measurements

Abstract Impedance measurements have been applied to the characterization of the pore texture of zinc electrodes used in electrochemical batteries. It is shown that these electrodes are equivalent to cylindrical pore electrodes. The electrode parameters (radius of pore, pore depth and surface density of pores) have been determined from the values of the electrode capacitance, the electrolyte resistance inside the pores and the electrode porosity.

Couple axial gradients of potential and concentration in a cylindrical pore electrode: an impedance model

Abstract The impedance of a cylindrical pore electrode in the case where the potential gradient due to the electrolyte resistivity is coupled to the axial concentration gradient of reacting species has been calculated semi-analytically from the approximate solution reported previously for the steady-state concentration and current profiles in the pore. Complex plane impedance plots, computed by an iteration technique for the transmission line, indicate: (i) a quasi-semi-circular diffusion loop at low frequencies due to diffusion control; and (ii) a high frequency loop in which the frequency dispersion is strongly dependent on the electrode parameters (electrolyte resistivity, diffusion coefficient of the reacting species, pore depth, Tafel coefficient of the electrochemical reaction and overall current flowing through the pore).