The effect of prior Cl- ion incorporation into native oxide film on pure aluminium in neutral chloride solution on pit initiation

The present work deals with the effect of pre-immersion treatment of a pure aluminium (Al) specimen in chloride solution in reference to distilled water on the pit initiation analysing potentiostatic anodic current transient, electrogravimetric curve measured during the open-circuit potential (OCP), and potentiodynamic polarisation curves. Pre-immersion treatment in distilled water hardly changed the number of available pit initiation sites, suggesting that healing of defects in the surface film does not take place significantly during pre-immersion in distilled water. In contrast, the number of pit initiation sites was markedly reduced by the pre-immersion action in 1 M NaCl solution. This is explained by the occurrence of metastable pitting during pre-immersion in 1 M NaCl solution by which surface defects can be removed. The removal of surface defects during pre-immersion in 1 M NaCl solution is ascribed to chloride ion incorporation through the formation of defect-repairing oxide film, as validated by the electrogravimetric curve measured during the OCP. The reduced number of surface defect sites and formation of defect-repairing oxide film were found to contribute to the decrease of cathodic current density and OCP. (C) 2001 Published by Elsevier Science Ltd.