Abstract
Abstract The anodic polarization of zinc in aqueous solutions of oxalic acid was investigated. The polarizations were carried out in electrolytes with concentrations between 0.005 and 0.500 mol dm−3, at constant current densities (1 ÷ 40 mA cm−2). In the course of anodic polarization, induction periods were distinctly recorded, and their durations were found to be strongly dependent on both the electrolyte concentration and the applied current density. By employing XPS and XRD diffraction methods and performing ATR-FTIR measurements, it was established that the layers formed on the metal surface are composed of crystalline zinc oxalate dihydrate, covered with thin ZnO external layers. Furthermore, a conceptual model was developed which qualitatively explains the mechanism of appearance of the observed induction periods. This model perceives the roughness variation of the electrode surface during anodic polarization. The obtained AFM images and SEM micrographs confirm the assumptions made in the presented model. During the polarization of zinc in aqueous solutions of oxalic acid, high voltages are reached, which makes them suitable for forming ZnC2O4.2H2O coatings.
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