STUDIES ON THE PASSIVATION BEHAVIOR OF AL-ZN-MG ALLOY IN CHLORIDE SOLUTIONS CONTAINING SOME ANIONS AND CATIONS USING ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

Potentiodynamic polarization and impedance measurements were performed on Al-Zn-Mg alloy in 0.01 M NaCl solution with various concentrations of nitrate, molybdate, sulphate, Cd(II) and Ce(III) ions. At 10(-2) M nitrate, molybdate and Ce(III) ions, both the open circuit and the pitting potentials were found to shift more in the noble direction as compared to lower concentrations. In the presence of molybdate and Ce(III) ions a wide passive region was observed, while in the presence of Cd ions, a higher corrosion current was noticed. The observed impedance spectra was interpreted in terms of change in characteristics of the already existing oxide film at the surface of the alloy. The low capacitance values measured in the presence of sulphate ions can be related to the morphological changes occurring in the film. The effect of the ions in healing and strengthening the existing oxide film of the aluminium alloy was found to increase in the following order: cadmium(II) < sulphate < molybdate < nitrate < Ce(III)

Corrosion and passivation behaviour of Al-Zn-Mg alloy - an EIS study

Aluminium alloys of 7 XXX series are widely used in aerospace industries because of their higher strength. These alloys contain Zn, Mg and Cu in their matrix which offer the desired mechanical properties. A protective, passive oxide film which is stable in the pH range of 4 to 8 is always present at the surface of these aluminium alloys. However, in chloride containing environments this oxide film undergoes dissolution at imperfect areas, causing pitting type of corrosion