Pit Growth Study in Al Alloys by the Foil Penetration Technique

Abstract

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.

Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3

“…These observations are in stark contrast to the considerable effects that chromates can have at low concentration during exposure at open circuit. Other evidence has indicated that chromates act as cathodic inhibitors, which would not be seen under potentiostatically controlled conditions [14]. …”

Effect of applied tensile stress on intergranular corrosion of AA2024-T3

“…The sample was backed with a piece of filter paper and a Cu foil. Penetration was sensed by monitoring the resistance between the sample and the Cu foil using a detection circuit described previously [11]. A Pt counter electrode and saturated calomel reference electrode (SCE) were used in the foil penetration cell.…”

“…The nominal growth rate in the short transverse direction was slower because of the longer path length around grains in that direction. The intergranular nature of the attack was supported by X-ray microfocal radiography analysis of penetrated foils [11].…”

“…By measuring the penetration time for foils of different thickness, the depth of the fastest growing localized corrosion site as a function of time is determined. This powerful technique has been utilized by only a few investigators [10][11][12][13][14][15]. A recent study used the foil penetration technique to show that the pit growth rate in AA2024-T3 at open circuit was strongly reduced by the presence of a small amount of dichromate in solution, whereas the growth rate at a slightly elevated applied potential was practically unchanged, even in the presence of high dichromate concentrations [11].…”

“…This powerful technique has been utilized by only a few investigators [10][11][12][13][14][15]. A recent study used the foil penetration technique to show that the pit growth rate in AA2024-T3 at open circuit was strongly reduced by the presence of a small amount of dichromate in solution, whereas the growth rate at a slightly elevated applied potential was practically unchanged, even in the presence of high dichromate concentrations [11]. In another recent study, the foil penetration technique was used to show that the rate of localized corrosion growth in AA2024-T3 plate was anisotropic [12,13].…”