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 wat er 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 distil led water hardly changed the number of available pit initiation sites, sugg esting that healing of defects in the surface film does not take place sign ificantly 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 pitti ng during pre-immersion in 1 M NaCl solution by which surface defects can b e 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 for mation of defect-repairing oxide film were found to contribute to the decre ase of cathodic current density and OCP. (C) 2001 Published by Elsevier Sci ence Ltd.