CORROSION AND PASSIVATION OF AL AND AL-SI ALLOYS IN NITRIC-ACID SOLUTIONS .2. EFFECT OF CHLORIDE-IONS

The electrochemical behaviour of aluminium and Al-silicon alloys in aq ueous solutions containing different concentrations of chloride ions w as investigated. Electrochemical impedance spectroscopy (EIS) and pola rization techniques were used. The electrodes were anodically passivat ed under galvanostatic control. The effect of formation voltage on the stability of the passive film was studied. The open-circuit impedance measurements have shown that the dissolution of the anodic oxide form ed on aluminium or aluminium alloys follows the empirical relation: C- 1 = C-0(-1) - Bt(1/2) C-1 is proportional to the oxide film thickness. B represents the rate coefficient of the oxide film dissolution. The results revealed that the presence of chloride ions even in the oxidiz ing medium (1 M HNO3) delays the passivation of the material. Complex plane analysis of the formed oxide film indicates that the corrosion r esistance increases as the concentration of chloride ions increases. F or naturally passivated electrodes, the corrosion resistance decreases as the concentration of chloride ions increases. This behaviour was e xplained by film repair-film removal equilibria.