A critical look at interpretation of electrochemical impedance spectra of sol-gel coated aluminium

Abstract

A proper assignment of time constants is important for the correct interpretation of the electrochemical impedance spectra of coated substrates. In this work, several model aluminium samples with different anodic oxide thicknesses were prepared and characterised. The aluminium samples were coated by a hybrid sol-gel formulation and studied by electrochemical impedance spectroscopy (EIS) during immersion in NaCl solution. A hierarchical model containing three R-C elements adequately described the impedance spectra of the coated samples. The R-C elements described the properties of sol-gel coatings, aluminium oxide layers and corrosion process. The contribution of the aluminium oxide layer to impedance was unambiguously proven to be at middle frequencies on the spectra. Furthermore, the “real” capacitances of oxide layers were obtained from two capacitance–CPE equations and their applicability was scrutinized. The obtained results presented an opportunity to determine the adhesion behaviour of the sol-gel coatings to the oxide layers, which was studied by analysing the ratio of the oxide layer capacitance of the sol-gel coated samples to the capacitance of the uncoated ones. It was demonstrated that the exposed surface area of the oxide to the electrolyte was about 60–68% and was not changing during immersion. Based on these findings, a degradation model of the sol-gel coating on anodised aluminium was proposed.