Formation of environmentally friendly protective Ce2O3-CeO2 conversion coatings on Al, modified by phosphate layers: chemical and electrochemical characterization

Abstract

Conversion ceria coatings were deposited on substrates of Al-1050 in a solution containing CeCl3×7H2O and CuCl2×2H2O. The post-treatment of as deposited coatings was realized in phosphate-containing solutions: 0.08 M Na3PO4 or 0.22 M NH4H2PO4. The XPS characterization of the surface of samples was carried out on an AXIS Supra electronspectrometer (KratosAnalytical Ltd.). The electrochemical investigations and determination of the basic corrosion parameters (Ecor, icor, Epit, EOCP, Rp, CR, etc.) for the studied systems were carried out on a Gamry Interface 1000 (EISSFR Analyzer EIS300). Based on the XPS investigations, it was ascertained that there is a substantial influence of the “pretreatment” and “post-treatment” operations on the chemical composition and chemical state of the elements on the samples` surface. It is expressed by a strong decrease in the concentration of Аl2O3 and Ce2O3+CeO2 components in the ceria coatings at the expense of formation of AlPO4 and AlOOH, CePO4, as well as PO3-, P2O5 and P4O10 compounds with Al and Се. The comparison of the results of Rp values and concentrations of Ce3+/Се4+, Al and P shows that the changes in Rp and quantity of Се4+ and P on the surface of the studied samples are directly related. The supposed and proved formation of low-soluble corrosion products on the surface of the studied systems and accomplished synergic effect of protective action by the formed cerium and aluminum oxides/phosphates layers determine the increase of the Rp and decrease of icor and CR, respectively. It was also concluded that the mixed conversion layers are a more effective barrier for the chloride ions diffusion to the metal surface and increase the corrosion resistance of Al 1050 to pitting and general corrosion.