Abstract
The formation of oxide layers on aluminum and its alloys has a protective function. Protective oxide layers may be formed by various processes and may be promoted in the presence of chemical compounds present at the metal/water interface. In the present work, the electrochemical behavior of aluminum alloy 7075 in the presence of aqueous oxalate solutions in the concentration range between 10–80 mM was studied by open circuit potential, polarization and ac impedance measurements. It was found that the oxalate ions present in the solutions promoted passivation of the specimens surface. The passivation was attributed to the formation of a resistive oxide layer which was thicker than the corresponding layer formed in distilled water. This finding was confirmed by XPS measurements. Since the interaction between the anionic species, such as the oxalate ions, with the metal and or oxide surfaces seem to play an important role, the influence of the duration of immersion and of the concentration of the aqueous oxalate solutions on the surface oxide film formation was investigated. It was found that in a treatment process in which both the immersion time and the oxalate concentration were increased, a thicker, more resistive film was formed. In the treatment process of the aluminum alloy specimens with oxalate, the effect of the oxalate concentration on the electrical properties of the surface layer formed during the immersion in the oxalate solutions (increase of oxide resistance upon increasing oxalate concentration) was revealed when testing solutions of the same concentration (10 mM) in sodium oxalate were used for conducting electrochemical measurements, regardless of the composition of the treating solutions. The apparently low resistivity observed when the treating and testing solutions had the same, high, oxalate concentration (resulting in solutions of high conductivity) was attributed to the porous structure of the surface oxide layer.
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