The Effects of Chloride Implantation on Pit Initiation in Aluminum

Abstract

High-purity aluminum samples were implanted with 35 keV Cl ϩ then polarized in both Cl Ϫ -containing and Cl Ϫ -free electrolytes in order to ascertain corrosion behavior as a function of Cl Ϫ content in the oxide. Implant fluence between 5 ϫ 10 15 and 2 ϫ 10 16 Cl ϩ cm Ϫ2 resulted in little or no localized attack. Implant fluences of 3 ϫ 10 16 and 5 ϫ 10 16 Cl ϩ cm Ϫ2 resulted in significant pitting in a Cl Ϫ -free electrolyte with the severity scaling as a function of implant fluence. The low variability in the pitting behavior of the 5 ϫ 10 16 Cl ϩ cm Ϫ2 sample suggests that this implant dosage results in a critical Cl Ϫ concentration in the oxide for pit nucleation. The passive current density (i pass ) decreased with increasing implant fluence. A space-charge effect is proposed to account for this phenomenon, although effects from defect interactions and possible oxide thickening are still under consideration.

Chloride Ion Interactions with Oxide-Covered Aluminum Leading to Pitting Corrosion: A Review

“…Pitting was not observed in the control cases (unimplanted and Ar-implanted) or in the low fluence implants (1 × 10 16 and 2 × 10 16 ions cm −2 ). Wall et al 94 suggested that these experiments demonstrated that a sufficient Cl − concentration in the oxide film or in the substrate metal would result in the nucleation of pits and that pit nucleation was independent of the solution composition as E pit measured in sulfate solutions were similar to those reported for Cl − containing solutions. The authors noted that pit initiation in the implanted electrodes due to the presence of Cl o in the underlying metal could not be discounted.…”

“…Wall et al 94 utilized ion implantation of a chlorine species, Cl + , into oxide covered Al to determine the effects of the ion-implanted chlorine on the pitting behavior and properties of the oxide film in Cl − free and Cl − containing electrolytes. The Al was 99.999% pure and test solutions were 50 mM KCl or K 2 SO 4 .…”

“…Serna et al, 92 Brown and Macintosh, 116 and Skeldon et al 96 showed that the chlorine species that resides in the oxide film after ion implantation was chloride, Cl − . Wall et al 94 etched the ion-implanted Al for fixed times to change the concentration of Cl − . The influence of the etch time, which in turn affects the Cl − concentration, is shown in Figure 18.…”

“…The data showed that the higher the concentration of the implanted Cl − , the lower the value of E pit . In a second set of experiments, 94 the Cl − concentration was changed by altering the implantation fluence (1, 2, 3, or 5 × 10 16 ions · cm −2 ). The electrolyte in this set of experiments was 50 mM K 2 SO 4 instead of NaCl to avoid any influence of solution Cl − on the polarization behavior.…”

“…The responses of four samples are shown to indicate reproducibility. 94 Reproduced by permission of ECS-The Electrochemical Society.…”