Effect of Impurities on Interfacial Void Formation in Aluminum

The effect of impurities on formation of interfacial metallic voids, during uniform dissolution of aluminum in 1 M NaOH, was investigated. These voids are thought to act as initiation sites for pitting corrosion, and were previously shown to be formed by NaOH dissolution. Samples of three different bulk purities were compared: 99.98, 99.997, and 99.9995%. Positron annihilation spectroscopy and atomic force microscopy revealed that nanometer-scale voids were formed by dissolution in each foil. For each sample, the void volume fraction interpreted from these measurements increased to a maximum during dissolution, and then declined. As the purity increased, more extensive dissolution was required to produce voids. Accumulation of near-surface Cu and Fe impurities during dissolution was characterized using Rutherford backscattering spectrometry. The results suggested a possible general correlation of void volume fraction with copper surface concentration. Processes involving near-surface copper impurities may then at least partly control the formation of voids. © 2004 The Electrochemical Society. All rights reserved

Corrosion-Related Interfacial Defects Formed by Dissolution of Aluminum in Aqueous Phosphoric Acid

The mechanism was investigated by which pit initiation on aluminum foils during anodic etching is affected by the use of phosphoric acid as a pretreatment. Positron annihilation measurements, coupled with atomic force microscope images of foils with chemically stripped oxide layers, show evidence that the pretreatment introduces nanometer-scale voids in the metal, at or near the metal-oxide film interface. The location and morphology of voids compares favorably with those of pits, suggesting that voids act as pit initiation sites. The number of void sites was estimated to be 107 cm−2, the same magnitude as the maximum number of pits formed by anodic etching. Capacitance measurements further indicate that the treatment decreases the surface oxide thickness to about 2 nm. Formation of large numbers of pits during etching is promoted by either reduced oxide thicknesses or more positive etching potentials. It is suggested that the rate of initiation of pits at interfacial voids is determined by the electric field in the overlying surface oxide

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 spectroscopy (PAS) after dissolution for various times in 1 M NaOH at room temperature. Measurements of the S and W shape parameters of the annihilation photopeak at 511 keV show the presence of voids of at least nanometer dimension located at the metal-oxide film interface. The large Sparameter 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 average cavity depth is in quantitative agreement with the PAS-derived thickness of the interfacial void-containing layer, and the dissolution time dependence of the defect layer S parameter closely parallels that of the fractional coverage of the foil surface by cavities; thus, the cavities are believed to be interfacial voids created along with those detected by PAS. The cavity distribution on the surface closely resembles that of corrosion pits formed by anodic etching in 1 M HCl, thereby suggesting that the interfacial voids revealed by AFM serve as sites for pit initiation

Detection of Corrosion‐Related Defects in Aluminum Using Positron Annihilation Spectroscopy

Near‐surface atomic‐scale defects in aluminum foils of at least 99.98% purity were characterized with positron annihilation spectroscopy measurements of the Doppler‐broadening parameter S. Profiles of S vs. positron beam energy (i.e., vs. depth into the sample) were analyzed with a model for positron diffusion and trapping in order to characterize the defect layer structure. As‐received foils were shown to possess a defect layer within 10 to 100 nm of the oxide film/metal interface. Both dissolution in aqueous sodium hydroxide solution and anodic pitting corrosion in caused significant changes in the position spectra which were interpreted as increases in the defect population. On the basis of isochronal annealing, the defects were impurity‐complexed voids or vacancy clusters, or else interfacial voids at the metal/film boundary located at surface roughness features. Either case suggests a possible role for the defects as pit sites, since both near‐surface impurities and surface roughness are known to influence the number of pits on a surface. Defects found after pitting may be present in layers surrounding individual pits, and might have been produced in the process of pit initiation

A Mathematical Model for the Initiation of Aluminum Etch Tunnels

A mathematical simulation is presented which predicts the spontaneous shape evolution of cubic etch pits on aluminum as they develop into etch tunnels during anodic etching in chloride solutions. The simulation is based on a model for oxide passivation, according to which the rate of oxide film coverage increases as the potential at the dissolving surface is made more negative than the critical repassivation potential, which depends on the local chloride ion concentration. Mass transport calculations are used to predict the electrolyte concentration and potential in the pit, which in turn determine the rate of oxide advance and hence the shape change of the pit. The model predicts the pit‐tunnel transformation, as well as width expansion of tunnels near their mouths. The occurrence of these features is independent of the choice of the passivation rate constant, an adjustable parameter in the simulation. Tunnel width oscillations were found at relatively low values of the rate constant. In the model, the pit‐tunnel transformation is produced by rapid pit sidewall passivation, which is due to the relatively slower increase of the pit electrolyte concentration relative to the ohmic drop, during the pit\u27s initial growth. A fully quantitative comparison of the model and experiment is possible with independent experimental information on passivation kinetics

Positron Annihilation Spectroscopy Study of Interfacial Defects Formed by Anodic Oxidation of Aluminum

Positron annihilation spectroscopy (PAS) measurements were carried out to characterize open-volume defects associated with anodic oxidation of aluminum. The annihilation fractions with low and high momentum electrons (S and W spectral lineshape parameters, respectively) of the annihilation photopeak were determined, as a function of the positron beam energy. A subsurface defect layer, containing nanometer-scale voids in the metal near the metal/oxide film interface, was found after oxide growth, and was shown to contain new voids created by anodizing. Such interfacial voids in the metal are of interest because of their possible role as corrosion initiation sites. The Sparameter characterizing the defect-containing layer (Sd) was obtained by simulation of the S-energy profiles. On samples with two different surface conditions, Sd remained constant at its initial value during anodizing. Because Sd is related to the void volume fraction in the interfacial metallic layer containing the voids, that result suggests that formation of metallic voids, and their subsequent incorporation into the growing oxide layer, occurred repeatedly at specific favored sites. © 2003 The Electrochemical Society. All rights reserved