Elaboration and microstructural characterization of calcareous/ceria based composite on zinc substrate

Abstract

Cerium conversion coatings were prepared by cathodic electrodeposition from a low concentrated aqueous solution of Ce(NO₃)₃ with KNO₃ addition to insure the conductivity of the electrolyte. The cerium oxide film was characterized by Scanning Electronic Microscopy (SEM). Although the deposit is uniform with current density of 2 mA/cm², it shows cauliflower morphology with a crack network, giving rise to bad mechanical and electrochemical behaviour. Elaboration of a calcareous deposit inside crack network of the cerium coating by cathodic polarization from artificial seawater is investigated at different applied potential, in order to increase the quality of the cerium coating formed. For cathodic potentials lower than–1.5 V/SCE, zinc corrosion products (gordaite) were observed inside the cerium oxide film instead of calcareous deposit although the current density decrease during the deposition suggesting a partially blocked surface. A pure calcareous compound was observed at–1.5 and–1.6 V/SCE. SEM and EDX cartography of the cross section revealed that open cracks in the cerium oxide structure are filled by calcium. Calcium was also detected inside the CeO₂ film not only all around the cracks but also in all the porosity of the CeO₂ film. It has precipitated as CaCO₃ (aragonite form) as revealed by micro-Raman spectroscopy and XRD.