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 found to be present as an adsorbed specie at the surface of the Al oxide, as well as an incorporated specie within the passive oxide. The two species of Cl- were recorded by X-ray absorption near edge structure (XANES), using both 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 into the passive Al oxide film, prior to stable pit initiation. Cl- migration occurs once a critical anodic potential or critical adsorbed Cl- concentration 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 pitting or passive film dissolution. (C) 2000 The Electrochemical Society. S0013-4651(99)12-098-6. All rights reserved.