Interaction of Chromate Ions with Surface Intermetallics on Aluminum Alloy 2024-T3 in NaCl Solutions

The aim of this paper is to analyze the interaction of chromate ions with surface intermetallics on Al 2024-T3 in NaCl media under open circuit conditions at pH 3 and 6. These interactions are probed by Auger electron (AES), X-ray photoelectron, and energy dispersive X-ray spectroscopies, as well as scanning electron microscopic techniques. In acidic NaCl solution, S-phase particles undergo bulk dealloying through preferential dissolution of magnesium and aluminum, which leaves the particles enriched in copper. In contrast, no bulk dealloying can be observed on the intermetallics. Copper enrichment is also observed on the bulk matrix. At pH 6, S-phase particles are again subject to bulk dealloying through preferential dissolution of magnesium and aluminum, but the release of copper is also plausible since the particles are often surrounded by copper deposits. Similar to the effects in pH 3 solution, no bulk dealloying is observed on the intermetallics, though the surface has developed a rough iron oxide corrosion product layer. AES analyses show no sign of copper enrichment on the bulk matrix in neutral NaCl media. When chromate is added to the sodium chloride solution, a chromium (III,VI)-rich film forms on the entire surface. The extent of deposition increases in the following order in both acidic and near neutral solutions: bulk matrix intermetallics -phase particles. Our results indicate that chromate strongly inhibits magnesium dissolution, retards the copper enrichment on the S-phase particles, and prevents copper redeposition on the surface. In the presence of chromate, no corrosion product layer develops on the intermetallics, and copper enrichment on the bulk matrix is not observed.