Positron annihilation spectroscopy study of interfacial defects formed by dissolution of aluminum in aqueous sodium hydroxide

High-purity aluminum foils were examined using positron annihilation spectr oscopy (PAS) after dissolution for various times in 1 M NaOH at room temper ature. Measurements of the S and W shape parameters of the annihilation pho topeak at 511 keV show the presence of voids of at least nanometer dimensio n located at the metal-oxide film interface. The large S parameter suggests that the metallic surface of the void is free of oxide. Voids are found in as-received foils and are also produced by dissolution in NaOH, evidently by a solid-state interfacial process. Atomic force microscopy (AFM) images of NaOH-dissolved foils, after stripping the surface oxide film in chromic- phosphoric acid bath, reveal cavities on the order of 100 nm size. The aver age cavity depth is in quantitative agreement with the PAS-derived thicknes s of the interfacial void-containing layer, and the dissolution time depend ence of the defect layer S parameter closely parallels that of the fraction al coverage of the foil surface by cavities; thus, the cavities are believe d to be interfacial voids created along with those detected by PAS. The cav ity distribution on the surface closely resembles that of corrosion pits fo rmed by anodic etching in 1 M HCl, thereby suggesting that the interfacial voids revealed by AFM serve as sites for pit initiation. (C) 2001 The Elect rochemical Society. All rights reserved.