Root cause analysis of pitting corrosion of AA2024 and AA7075 after exposure in salt fog environment

Two grades of anodised aerospace aluminium alloys, AA2024 and AA7075, were analysed to determine root causes of pitting corrosion after the samples were exposed to a salt spray test according to ASTM B177. Methods to investigate pitting corrosion behaviour of the aluminium alloys were divided into two steps: 1) characterisation of pre-exposed (intact) anodised film and stimulated pitting corrosion by polarisation; 2) comparison of samples that passed and failed the salt spray test. The surface properties were characterised by roughness measurements, scanning electron microscope (SEM) and energy dispersion X-ray spectroscopy (EDS). Pitting corrosion was stimulated by potentiodynamic polarisation prior to characterisation of pitting by SEM and EDS to determine pitting initiation point. The results reveal that thickness of the film does not prevent localised corrosion. Cu-bearing intermetallic particles are detrimental to anodic film formation and cause pitting in the film. Potentiodynamic polarisation tests alone cannot determine whether the film will pass or fail the salt spray test for 96 hours.