Chloride ingress into aluminum prior to pitting corrosion - An investigation by XANES and XPS

Two distinct chloride (Cl-) species were detected on and/or in the passive oxides of polycrystalline Al samples, which were anodically polarized below the stable pitting potential in Cl--containing solutions. Chloride was fou nd to be present as an adsorbed specie at the surface of the Al oxide, as w ell as an incorporated specie within the passive oxide. The two species of Cl- were recorded by X-ray absorption near edge structure (XANES), using bo th an electron yield detector and an X-ray fluorescence detector, and by X- ray photoelectron spectroscopy (XPS). Electron yield XANES and XPS results indicate that adsorbed Cl- migrates from the solution/Al oxide interface in to the passive Al oxide film, prior to stable pit initiation. Cl- migration occurs once a critical anodic potential or critical adsorbed Cl- concentra tion is reached. The migration of Cl- is followed by a loss of oxidized Al from the passivating film, as determined by XPS, and can be attributed to ( i) metastable pitting events or (ii) oxide dissolution. The ingress of Cl- into the oxide appears to be a key factor for the onset of metastable pitti ng or passive film dissolution. (C) 2000 The Electrochemical Society. S0013 -4651(99)12-098-6. All rights reserved.