engleski

Impedance and transient study of barrier films on aluminium and Al-Sn alloys

Corrosion and passivation of aluminium and aluminium alloys is subject of tremendous technological importance due to the increased industrial applications of these materials. These applications are often possible because of the natural tendency of aluminium to form a passivating oxide film. One of the attractive features of anodic alumina is the flexibility of its properties. By choosing a regime of anodic process and, especially, electrolyte composition, one is able to get oxide films with structure, hardness, chemical composition etc., varying in a large scale. If alien ions are built into the oxide, its properties may undergo certain changes. This work examines the growth kinetics and the properties of relatively thin barrier-type oxide films on high purity aluminium (99.999%) and Al-Sn alloys (with 0.02%, 0.09%, 0.2% and 0.4% Sn) in the borate buffer solution (pH = 7.8). This was done by following the decay of the anodic current upon step polarization into the passive region, and by measuring the impedance of Al (alloys)/oxide film/electrolyte systems. A characteristic decrease of current with time of anodizing was observed in all investigated cases. A decrease of the anodic current with time reflects the process of oxide film formation. As the current established during the oxide film formation process is a measure of the protective capability of the oxide, its decrease with time is due to the increased thickness of the oxide film and/or the ordering of the oxide film structure. By integrating the surface under the current-time responses the quantity of charge, used during film formation was determined. If the current efficiency for oxide growth was 100%, the thickness can be calculated from the anodic charge passed. It was observed a linear dependency of the barrier film thickness on the passivation potential for all examined samples. The oxide films for impedance measurements were formed using a potential step method by prepolarizing the electrode at the different potential value in passive region (from -0.6 to 2.4 V vs. SCE) for 60 min. For all examined samples, the response of the system was semicircle whose diameter increased with increased in potential and Sn content in the alloy. The capacitive time constant is attributed to the reactions involved at the formation of the oxide layer or the oxide layer itself. Namely, the oxide layer is consider to be a parallel circuit of a resistor due to ionic conduction in the oxide, and a capacitance due to the dielectric properties of the oxide. A mathematical analysis of the impedance diagrams has shown that the resistance and thickness (calculated from capacity) of barrier layers in all examined samples (for Al and all Al-Sn alloys) increase linearly with increase in the passivation potential, and that these magnitude increase with the increase of tin content in the alloy. It seems that presence of tin stabilizes the oxide films. The relations between the oxide thickness determined by capacity measurements and that determined by the quantity of charge, yields a straight lines whose slops can be used to determine current efficiency. The value of ~22 % indicates the formation of barrier film on aluminium to be a process that takes place at great current losses. Increasing of tin content in alloy increases the value of current efficiency. The current efficiency for oxide growth is 25%, 34%, 40% and 75% for Al - 0.02% Sn, Al - 0.09% Sn, Al - 0.2% Sn and Al - 0.4% Sn respectively.

Superpure aluminium ; Al-Sn alloys ; barrier films

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