M. Cortés

Potentiodynamic passivation of zinc in aqueous KOH solutions

Abstract The potentiodynamic oxidation of an upward-facing, horizontal zinc electrode in unstirred KOH solutions at concentrations in the range 1.0–5.0 M has been studied. Besides electrochemical treatment of the experimental data, the morphological evolution of the electrode surface during the oxidation has been followed by means of a scanning electron microscope. In the initial dissolution region, the Tafel slopes are near 40 mV dec−1(25°C) and the reaction orders with respect to the OH−ion about 3, these results being consistent with the mechanisms already proposed by Bockris and by Chang and Prentice. The experimental results suggest that the passivation of zinc takes place by a dissolution-precipitation mechanism. However, the rate determining step for passivation appears to be the resistance of the solution in the pores formed by the precipitation and spreading of a film presumably composed of Zn(OH)2, which is supposed to be the result of a local supersaturation with Zn(OH)2−4.

Potentiostatic passivation of zinc in alkaline solutions

The potentiostatic passivation of upward-facing horizontal zinc electrodes in quiescent 0.4–4 M KOH solutions has been studied according to different models and by SEM, coulometry and ICP. An initial flat maximum was found for all the potentiostatic transients except in 0.4 M KOH. The greatest part of the transients consisted in the initial maximum followed by a current plateau and a sigmoidal sharp decrease. The remaining curves were gradual modifications of the latter. The first part of all the transients could be interpreted according to a diffusion-controlled growth of a porous film coupled to metal dissolution from the electrode surface. In this model, the metal is dissolved as its ions and part of them react with the anions already present in the solution to form the anodic film, the rate of electrocrystallization being determined by the instantaneous concentration of the metal ions at the electrode surface. However, the second phenomenon appearing in practically all the potentiostatic transients, i.e., the current plateau and further sigmoidal sharp decrease must be explained by a model of 2D ohmic-controlled film growth.

Cathodic reduction of the anodically formed zinc species as a contribution to the study of the potentiodynamic passivation of zinc in alkaline media

Abstract The cathodic part of the potentiodynamic curves obtained for upward-facing horizontal 99.9% zinc electrodes in KOH solutions 0.4, 1.0, 2.0 and 3.0 M and sweep rates in the range 1–100 mV s −1 have been systematically analyzed in order to assign the possible species formed and contribute to the study of the potentiodynamic passivation of Zn in alkaline media. The anodic limit of the potentiodynamic cycles was changed and set for significant points of the total curve (between hydrogen and oxygen evolution). Also, the anodic sweep was interrupted at the potentials corresponding to the anodic limit and the cathodic sweep applied immediately from a potential near that of zero current of the cathodic half-cycle. Only two cathodic peaks have been found for the non-interrupted cycles. The assignation of peaks according to the equilibrium potentials of the reduction of the possible species implied, ie Zn(OH) 2− 4 , Zn(OH) 2 and ZnO, is not possible because local pH changes are expected and the zincate concentration near the electrode is unknown. The peak placed at more positive potentials for KOH concentrations 0.4 and 1.0 M is assigned to zincate and that at more negative potentials, to the reduction of the film. Just the opposite assignation has been found for 2.0 and 3.0 M KOH solutions. The experimental results can be interpreted assuming that the product formed at the passivation potential consists of the same chemical species as those corresponding to the first anodic peak, probably Zn(OH) 2 or hydrated ZnO. From calculating the maximum film thickness according to the charge passed and taking into account the recent theoretical analysis made by Chang and Prentice, it is concluded that the direct formation of ZnO on the electrode at the passivation potential as a consequence of local pH changes is not probable.

Magnetic CoPt (60–70 wt%Pt) microstructures fabricated by the electrochemical method

CoPt microstructures, in the form of both discontinuous layers and patterned arrays, of 60–70 wt% Pt and nanometric thickness have been grown by electrodeposition through a resist mask prepared directly onto a glass/ITO substrate. This substrate was selected because its conductive ITO layer permits the electrodeposition process but does not show magnetic response. The lack of magnetic response of the substrate enables the magnetic properties of the microstructures deposited over it to be measured directly. Test microstructures of the different aspect ratio were successfully prepared, which confirms the suitability of the used bath; a good definition of both has been attained in spite of the significant hydrogen co-evolution. The deposition conditions have been adjusted to obtain a highly distorted hcp crystalline structure. Differences in the magnetic behaviour of the microstructures were observed depending on the orientations of the magnetic field applied. This work demonstrates the capability of the electrodeposition method to grow well-defined nanometric thick microstructures of hcp magnetic CoPt alloy with modulable magnetic properties as a function of the orientation of the applied magnetic field, microstructures which could be directly incorporated in magnetic microelectromechanical systems.