Experimental and Computational Evaluation of the Protection Provided by an Aluminum Cladding to AA2024-T3 Exposed at a Seacoast Environment

The effective throwing power of a common metallic cladding was investigated by analyses of samples of scribed Alclad, AA2024-T3 (UNS A92024), which were exposed outdoors at Daytona Beach, Florida, for between 2 months and 18 months. Both optical microscopy of the exposed plane surfaces and cross-sectional metallography were performed to assess the effects of scratch width on the level of protection afforded. For scratches 5 mm wide or smaller, the exposed AA2024-T3 substrates were generally well protected by AA1230 (UNS A91230) of the clad with only small corrosion pits, likely due to diurnal cycles that generated isolated electrolyte islands on exposed substrates. Conversely, for scratches between 6 mm and 10 mm wide, the exposed substrate was only partially protected by the clad, and the percentage of protection appeared to decrease with scratch size according to simple quadratic behavior. The clad exhibited significant pitting corrosion over its entire surface, independent of the size of the scratch. A computational framework was adapted to study the system described above. Computations showed that water layer thickness (WL), scratch size, [Cl^–], passive current density (i_p) of clad, and diffusion-limited current density (i_dl) of AA2024 all have a significant impact on the throwing power of clad. With [Cl^–] = 1 M, WL = 25 μm, i_p = 0.002 A/m², and i_dl = 1.6 A/m², the model produced results that compared well to the experimental observations.