Studies on Atmospheric Corrosion Processes in AA2024

Atmospheric corrosion of aluminium alloy AA2024 was investigated using in situ synchrotron micro-tomography, which allows visualisation in a non-destructive manner in real time. The effect of atmospheric variables such as salt type, humidity, exposure time and salt deposition density on the corrosion rate was investigated. It was found that corrosion fissures grow along grain boundaries parallel to the rolling direction of the alloy, reaching a limiting depth, and then spread laterally. The volume of corrosion increases with salt density and relative humidity. Salt type has a limited effect on the volume of corrosion in microtomography measurements where the droplet is constrained at the top of a pin, but in parallel lab-based experiments on plate surfaces, it was found that NaCl and simulated ocean water droplets spread laterally, leading to increased corrosion owing to an increase cathodic area, whereas pure MgCh and CaCh droplets do not spread. Preliminary microtomography work on cycling the relative humidity showed transient increases in localised corrosion during wetting and drying phases, often associated with rapid growth of pmt of a localised cmTosion site, or initiation of new sites.