Transitions between pitting and intergranular corrosion in AA2024

The pitting and intergranular corrosion (IGC) behavior of various tempers of AA2024 was investigated in 1 M NaCl. The breakdown potentials associated with pitting or IGC were determined. The breakdown potentials were found to be almost independent of sample orientation for any given temper. Artificial aging had a strong effect on polarization behavior and localized corrosion morphology. The anodic polarization curves of AA2024 in the solution heat treated and water-quenched condition, T3, and T3+ tempers exhibited two breakdown potentials, whereas overaged AA2024-T8, T8+, and solutionized and furnace cooled AA2024 exhibited only one breakdown potential. When two breakdown potentials were observed, the more active one was found to be related to the transient dissolution of S phase Al2CuMg particles leading to pitting while the noble one was thought to result primarily from initiation and growth of IGC. The breakdown potentials decreased with increasing aging time at 190 degreesC, and only one breakdown potential was measured for T8 and T8 + tempers. Unlike the T3 temper, no sharp IGC was found for these tempers. Selected granular attack from breakdown of the copper-depleted matrix was believed to be the cause for localized corrosion in the T8 and T8+ tempers. The effect of nitrate and sulfate ions on the localized corrosion behavior was also studied. (C) 2003 Elsevier Science Ltd. All rights reserved.