Corrosion behaviour of aluminium alloys in deep-sea environment: A review and the KM3NeT test results

The interest in deep-sea environment is increasing both in the scientific and business community. Nevertheless, in situ corrosion studies are difficult to be executed due to high experimental cost and technical problems, consequently they are quite uncommon. After a summary of the scarce available literature about deep-sea environment studies on corrosion of aluminium alloys, results of an in situ exposure of Al 5083 H111, Al 6082 T6, Al 7075 T651, Al 8090 T81 alloys are presented. Specimens were exposed for 6, 12, and 18 months in deep-sea, at a depth of approximately 3350 m, off the Capo Passero (Sicily, Italy) in the KM3NeT project framework. Corrosion attack was evaluated by gravimetric weight loss measurements. Localised corrosion was investigated by using a digital camera and ImageJ software in addition to different microscopic techniques to better identify morphological/chemical details. Al 7075 T651 and Al 8090 T81 alloys performed much worse than Al 5083 H111 and Al 6082 T6 alloys. Al 6082 T6 alloy was the most resistant to corrosion, while Al 8090 T81 suffered the most relevant damage. Both Al 5083 H111 and Al 6082 T6 did not show any significant visible corrosion attack, while both Al 7075 T651 and Al 8090 T81 suffered severe localised attack. Al 8090 T81 experienced mainly pitting corrosion, while Al 7075 T651 was seriously affected by pitting, crevice, and exfoliation. The microstructural heterogeneity of both Al 7075 T651 and Al 8090 T81 alloys increases susceptibility to localised corrosion attack thus discouraging any deep-sea use of these alloys.