engleski

Anodic oxide films on aluminium in citric acid solutions

The mechanism, kinetics of growth and properties of thin anodic oxide films on aluminium, w(Al) = 99.999 %, in citric acid solutions of pH 5-7 were studied by the means of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. An EG&G PAR Model 273A Potentiostat/Galvanostat and EG&G Lock-In Amplifier Model 5210, controlled by PC, were used in electrochemical measurements. The influence of potentiodynamic anodising conditions on the mechanism, kinetics of growth and properties of anodically formed oxide films on aluminium in citric acid solutions have been examined. When aluminium is anodically oxidized in citric acid solutions (pH 5-7) thin high protective barrier-type passive films were formed. When the potential changes in anodic direction (from E(OCP) to 1.5 V vs. SCE) at different scan rates (10-100 mV s-1) aluminium shows increased current with the occurrence of a current shoulder after which the current remains constant ('current plateau'). The 'current plateau' is observed at potential region between 0.25 and 1.5 V. Growth kinetics parameters of potentiodynamically formed oxide films were determined by analysis of 'current plateau' values depending on scan rates. The analysis show that the potentiodinamically growth of the oxide films on aluminium in citric acid solutions is in accordance with mechanism of ionic conductivity under the influence of the high electric field through the oxide film. The impedance measurements were performed after potentiodynamic polarization at different potentials in anodic passive potential region. Parameters of equivalent circuit Rel(RQ) were evaluated using a simple least square fit procedure. Rel and R denote the resistance of the electrolyte and anodic oxide film respectively, and Q is a constant phase element representing the capacitance of the anodic oxide film (n &#8776 ; 1). The linear dependence of reciprocal capacitance with anodic potential was observed. The total thickness (d) of oxide films were determined from capacitance values. Current efficiency of oxide film formation was determined by comparing the oxide film thickness as determined by capacitance measurements and that determined by the quantity of charge used up per surface unit during the anodising process. The current efficiency is lower than 100 %, what means that anodic processes on aluminium in citric acid solutions can include other processes besides formation of the oxide film (dissolution of aluminium through the oxide film, chemical dissolution of the oxide, adsorption and surface complexation processes on aluminium oxide).

aluminium; oxide film growth; citric acid

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