Study of the electrochemical behaviour of the 7075 aluminum alloy in the presence of sodium oxalate

The formation of oxide layers on aluminum and its alloys has a protective f unction. Protective oxide layers may be formed by various processes and may be promoted in the presence of chemical compounds present at the metal/wat er interface. In the present work, the electrochemical behavior of aluminum alloy 7075 in the presence of aqueous oxalate solutions in the concentrati on range between 10-80 mM was studied by open circuit potential, polarizati on and ac impedance measurements. It was found that the oxalate ions presen t in the solutions promoted passivation of the specimens surface. The passi vation was attributed to the formation of a resistive oxide layer which was thicker than the corresponding layer formed in distilled water. This findi ng was confirmed by XPS measurements. Since the interaction between the ani onic species, such as the oxalate ions, with the metal and or oxide surface s seem to play an important role, the influence of the duration of immersio n and of the concentration of the aqueous oxalate solutions on the surface oxide film formation was investigated. It was found that in a treatment pro cess in which both the immersion time and the oxalate concentration were in creased, a thicker, more resistive film was formed. In the treatment proces s of the aluminum alloy specimens with oxalate, the effect of the oxalate c oncentration on the electrical properties of the surface layer formed durin g the immersion in the oxalate solutions (increase of oxide resistance upon increasing oxalate concentration) was revealed when testing solutions of t he same concentration (10 mM) in sodium oxalate were used for conducting el ectrochemical measurements, regardless of the composition of the treating s olutions. The apparently low resistivity observed when the treating and tes ting solutions had the same, high, oxalate concentration (resulting in solu tions of high conductivity) was attributed to the porous structure of the s urface oxide layer. (C) 1999 Elsevier Science Ltd. All rights reserved.