Localized Corrosion Growth Kinetics in AA2024 Alloys

The kinetics of localized corrosion in AA2024 alloys were studied using the foil penetration technique. The growth kinetics for localized corrosion in AA2024-T3 exhibited a strong anisotropy, being slower in the short transverse or plate through-thickness direction than in either the longitudinal or long transverse directions. This growth rate anisotropy depended on the grain size and aspect ratio, and was different for different forms of AA2024-T3. The attack was primarily intergranular corrosion (IGC) at potentials of −610 mV SCE and higher in 1 M NaCl. The IGC growth rate was almost independent of potential over the range from −610 to +500 mV SCE, which suggests that only a small amount of the anodic dissolution current at the active IGC tip escapes from the IGC crevice. At lower potentials the attack was pit-like, and the rate of growth decreased with decreasing potential. Artificial aging of AA2024 to the T8 temper changed the attack from sharp IGC to selective grain dissolution, but the growth kinetics at high potentials were not strongly affected. The addition of nitrate increased the breakdown potential, and therefore decreased the growth kinetics in that region. At high potential, the nitrate had no effect. Sulfate addition had no significant effect at any potential studied.