Microstructural and electrochemical characterization of Ce conversion layers formed on Al alloy 2024-T3 covered with Cu-rich smut

Abstract

In the present work the microstructure of Ce conversion layers formed on Al alloy 2024-T3 covered with a Cu-rich smut was characterized by scanning electron microscopy (SEM), and its electrochemical behaviour in 0.5 M NaCl solution was investigated by open circuit potential (OCP) measurements, anodic polarization curves and electrochemical impedance spectroscopy (EIS). The results of the microstructural characterization have shown that the presence of Cu cathodic particles uniformly deposited on the electrode surface favours the homogeneous nucleation of the conversion layer, apparently reducing the role of intermetallics in the layer precipitation mechanism. Electrochemical measurements have shown that, during the first hour of immersion in the test electrolyte, the samples covered with the conversion layer exhibit a high interfacial activity, characterized by an active anodic behaviour, which was ascribed to an intense electrochemical activity associated with Al Cu Mg intermetallics not covered with the conversion layer. However, as the experiment proceeds an improvement on the corrosion behaviour of the sample was observed, which was interpreted on the basis of self-healing properties of the Ce conversion layer as well as on the blocking of the defective sites of the conversion coating by corrosion products. The electrochemical results have also evidenced the beneficial effect of the pre-treatment with benzotriazole in the corrosion response of Ce-coated samples.