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

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

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.This article is from Journal of the Electrochemical Society 151 (2004): B227–B232, doi:10.1149/1.1666148. Posted with permission.</p

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.This article is from Journal of the Electrochemical Society 149 (2002): B108–B116, doi:10.1149/1.1455648. Posted with permission.</p

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.This article is from Journal of the Electrochemical Society 151 (2004): B22–B26, doi:10.1149/1.1631821. Posted with permission.</p

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.This article is from Journal of the Electrochemical Society 148 (2001): B92–B100, doi:10.1149/1.1341241. Posted with permission.</p