In Situ X-ray Tomography Observations of Initiation and Propagation of Pits during Atmospheric Corrosion of Aluminium Alloy AA2024

In-situ synchrotron X-ray tomography has been used to observe the initiation and propagation of pits during atmospheric corrosion of aluminium alloy AA2024 under droplets of NaCl and simulated seawater at 21 ± 2 °C and a relative humidity of 85% over a period of 120 min. Each complete tomogram took 300 s to collect (pixel size 0.74 μm). Two metastable pits initiated at intermetallic particles, forming a crevice around the edge of the particles. Three other metastable pits formed at sites with no obvious micron-sized features. Only one pit, which had initiated under a fragment of debris, continued to stable growth. Metastable pits initiated sequentially, and all initiation events were accompanied by the formation of one or more bubbles, presumed to be hydrogen. Pit propagation was spatially non-uniform, with rapid growth in local regions while the remainder of the pit remained passive. The growth rate was highest parallel to the rolling direction of the plate. The highest local current density was estimated from the loss of metal between successive measurements to be ∼0.5 A cm^-2. This is the first time that the transition from initiation to propagation of individual localised pits in an aluminium alloy has been characterised in situ.