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Investigation of Long-term Corrosion of AA2024 and AA5119 Aluminum Alloys under chromate coating in Marine Atmosphere by Electrochemical Impedance Technology

This study was focused on the long-term corrosion performance of coated AA2024 and AA5119 aluminum alloys in the atmosphere and coastal environment after 4, 8 and 12 years of exposure. Structural analyses of samples using SEM and XRD analyses revealed that the corrosive surface area of alloys increased with increasing the atmospheric exposure time, and formation of Al(OH) 3 as corrosion product on the corroded surface layer of both atmospheric exposed 12 years samples. Comparison between the exposed 12 years samples showed that the higher level of impurities, deeper pits and exfoliation corrosion were observed for exposed AA2024 sample which related to the destruction of anodic oxide film due to pitting attack and decrease of protection capability of the epoxy coated layer, and lower level of impurities and corrosion on the AA5119 sample can be related to the presence of lower Si/Mg molar ratio in the alloy. Electrochemical impedance studies showed the decrease in corrosion resistance with the increase in the duration of atmospheric exposure. Results also showed that deposition of corrosion products on the surface of AA5119 alloy can act as a protective layer through preferentially filling the pores/defects. Therefore, the observations indicate the corrosion resistance of aluminum alloys is significantly dependent on the composition and created crystal defects in the manufacturing process of alloys. Coated AA5119 aluminum alloys show a higher corrosion resistance than that AA2024 in long-term atmosphere exposure in the coastal environments.