Observations of the Early Stages of the Pitting Corrosion of Aluminum

Gessmann

et al. 2001

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High-purity aluminum foils were examined using positron annihilation spectroscopy ͑PAS͒ 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 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 ͑AFM͒ 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.

Section: Resultsmentioning

confidence: 97%

Section: Methodsmentioning

confidence: 99%

Positron Annihilation Spectroscopy Study of Interfacial Defects Formed by Dissolution of Aluminum in Aqueous Sodium Hydroxide

Gessmann

et al. 2001

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“…Results are shown for tunnels at 70°C with initial pit half-widths from 0.15 to 0.5 m. This range corresponds to the spectrum of pit sizes found in etching experiments at this temperature. 12,24 The tunnel width and tip concentration of short tunnels with parallel walls are not affected significantly by the initial pit size. However, the pit size influences the tapering-width regime of tunnel growth: larger pits are associated with smaller rates of taper ͑larger ͒.…”

Section: Resultsmentioning

confidence: 99%

“…These pits are formed within milliseconds after the application of current. 24 The pit bottom is parallel to the outside surface. The electrolyte concentration in the pit is initially C b , and the entire pit surface is uniformly dissolving.…”

Section: Mathematical Modelmentioning

confidence: 99%

A Mathematical Model for the Growth of Aluminum Etch Tunnels

2001

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A simulation of the growth of pits on aluminum during anodic etching in hot chloride solutions was developed. The simulation is based on equations for mass transport and for the potential-controlled removal of chloride ions from the dissolving surface. The latter process initiates oxide passivation. Etch pits are found to transform into tunnels which at first maintain parallel sidewalls and then begin to taper. The predicted tunnel shapes agree quantitatively with those measured experimentally. Tunnel formation is possible only when the potential at the tunnel entrance during etching is within 20-30 mV of the repassivation potential; as a result, the size of the dissolving surface is nearly constant during pit growth. In the tapered-width regime of tunnel growth, the AlCl 3 concentration at the end of the tunnel is near saturation, despite the absence of precipitation from the model equations. The model shows that this condition derives from the low conductivity of the concentrated solution, coupled with the sensitivity of the rate of surface chloride removal to changes in the potential at the dissolving surface.

“…After nucleation of etch pits, there is a stage of rapid dissolution in which they grow to sizes of 0.1-1 p.m in a few milliseconds; afterward, their dissolution rate (or dissolution current density) is apparently constant and approximately the same as in tunnels. 3 The dissolution rate in etch pits is uniform along their walls (Fig. 3).…”

Section: Infroductionmentioning

confidence: 95%

A Mathematical Model for the Initiation of Aluminum Etch Tunnels

Zhou

1998

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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's initial growth. A fully quantitative comparison of the model and experiment is possible with independent experimental information on passivation kinetics. 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's initial growth. A fully quantitative comparison of the model and experiment is possible with independent experimental information on passivation kinetics.