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The foil penetration technique was used to study pit growth in AA1100-O and AA2024-T3. Preliminary work on AA1100-O foils of different thicknesses indicated that the pit growth rate increased with increasing applied potential, suggesting that pit growth was not under transport control. Foil penetration experiments were also carried out on AA2024-T3 foils of a given thickness, at open circuit as well as anodic potentials. Dichromate ions and other oxidizing agents were added to some test solutions. Dichromate ions were shown to have little influence on the pit growth rate at controlled anodic potentials, even when added in large concentrations. However, dichromate ions effectively inhibited pitting at open circuit when present in very small amounts. Polarization curves of AA2024-T3 in 1 M NaCl with various additives show a large effect of dichromate ions in the cathodic region and no effect in the anodic region. These observations suggest that chromate (or its reduction product) acts as a cathodic inhibitor. Examination of penetrated samples was performed by optical and scanning electron microscopies, as well as by microradiography.The foil penetration technique is a simple and direct method of measuring pit growth rate in materials.1 One can vary the sample thickness, sample orientation, test environment, and applied potential, and determine the kinetics of the fastest growing pit regardless of the pit growth path. In the original experiment by Hunkeler and Böhni, 1 pit penetration times were determined for Al foils of different thicknesses (0.05-0.20 mm) that were potentiostatically held at anodic potentials. Their Al foil working electrode was mounted onto the cell wall, and was backed by a Cu foil maintained at +12 V relative to the working electrode (WE), with a piece of paper separating the two metal foils. The instant of pit penetration, through the Al foil, was detected as the pit electrolyte wetted the paper, thus reducing the resistance between the Al and Cu foils and triggering a timer.Hunkeler and Böhni found the pit growth rate (the inverse slope of the penetration time/foil thickness relation) increased linearly with increasing potential, suggesting ohmic or mixed ohmic/charge-transfer control of pitting.

The effect of uniaxial tensile stress on intergranular corrosion (IGC) of AA2024-T3 was studied using the foil penetration technique. Standard ASTM G49 fixed-displacement jigs were modified to allow the use of sheet samples, which were then attached to an electrochemical cell as in the foil penetration setup. The time for IGC to penetrate samples of varying thickness was monitored. This method provides a new approach to bridge the gap between IGC and intergranular stress corrosion cracking (IGSCC). Samples with various orientations relative to the rolling direction were studied in 1.0 M NaCl at controlled anodic potentials. Potentiodynamic polarization measurements indicated that the two breakdown potentials typically observed for AA2024-T3 were lower for stressed samples than for unstressed samples, and the current at a given potential was higher. The penetration rate depended on potential and was higher for stressed samples than for unstressed samples. The primary form of attack above the higher breakdown potential was IGSCC, whereas pitting dominated between the two breakdown potentials. Stress had a larger effect on penetration rate at higher applied potentials, indicating that pitting is less susceptible to the effects of stress than a properly oriented IGC crevice. The effects of stress on the penetration rates in various orientations were strongly linked to the anisotropic microstructure. X-ray microfocal radiography and optical microscopy of cross-sections were used to characterize IGSCC defects in thin penetrated foils. In certain orientations, crack faces were parallel rather than perpendicular to the stress direction as a result of the constraints of the microstructure on the orientation of the IGC. Implications for the mechanisms of IGC and IGSCC are discussed.

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A specially designed setup was used to apply a constant load to a thin sheet sample of AA2024-T3 and, using microfocal X-ray radiography, to observe in situ the resulting intergranular stress corrosion cracking (IGSCC) from the exposed edge of the sample. The growth of and competition between multiple IGSCC sites was monitored. In many experiments twin cracks initiated close to each other. Furthermore, the deepest crack at the beginning of every experiment was found to slow or stop growing, and was then surpassed by another crack that eventually penetrated through the sample. These observations cannot be explained by the theory of fracture mechanics in inert environments. The possible mechanisms underlying the competition between cracks are discussed.