engleski

EIS study of passive oxide films on aluminium in citric acid solutions

The influence of anodising time and pH value of solutions on the properties of passive oxide films potentiostatically formed at 1.0 V on aluminium (Al) in 0.05 M citric acid solutions (pH = 5-7), were investigated by the means of potentiostatic controlled current-time transients and electrochemical impedance spectroscopy (EIS) techniques. Electrochemical measurements were performed using a potentiostat PAR M273A and Lock-In Amplifier 5210, controlled by computer. From the impedance spectra of potentiostatically formed oxide films it is evident that the system shows at high frequencies capacitive behaviour and at low frequencies inductive behaviour. The capacitive behaviour can be related to the growth and dielectric properties of oxide barrier layer, while the inductive behaviour is probably due to the relaxation processes of adsorbed species at the oxide/solution interface. It is observed that during the anodising up to 1-2 h reciprocal capacitance (which is proportional to the thickness of oxide film) increase, after which the oxide film reaches the constant value of C-1 (constant film thickness). This behaviour suggests that subsequent ordering of the structure of anodic oxide films and/or surface coordination processes takes place during prolonged anodising. It is evident that the properties of oxide films depend on pH values of citric acid solutions. The higher pH values of solutions (pH = 7) stimulate formation of thicker and more resistive oxide film and at the same time decrease the influence of relaxation processes on the total impedance of the system.

aluminium; oxide films; electrochemical impedance spectroscopy EIS; citric acid

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