Corrosion behavior of 5052 and 6061 aluminum alloys in deep ocean environment of South China Sea

Aluminum alloys have been found ever-increasing applications in marine environments. The study on the corrosion of aliminum alloys using field test was started in USA in 1940s. Such studies, however, were carried out in China untill 1980s. Although the corrosion behaviours of 11 kinds of aluminum alloys were investigated exposed to tide, splash and full immersion zones at Qingdao, Zhoushan, Yulin and Xiamen area of China, the corrosion behaviour of materials in deep ocean environments is different from that in shallow marine environments. In this work, the corrosion behavior of 5052 and 6061 aluminum alloys in 800 and 1200 m deep ocean environments of South China Sea was studied using field test. The morphology and composition of corrosion products were investigated using SEM, EDS and XRD. The results indicated that severe local corrosion took place in the aluminum alloys. The corrosion products were composed of Al₂O₃, SiO₂ and a small amount of Mg₃(SO₄)₂(OH)₂ and NaCl. The crevice corrosion perforation occurred on the edge of 5052 and 6061 samples. And the groove corrosion pits formed on the cross section of 5052 sample. Pitting corrosion took place on the main area of 5052 and 6061 samples. The size and density of pits formed on 6061 aluminum alloy were higher than those on 5052 aluminum alloy. And the pitting corrosion perforation formed on 6061 aluminum alloy in 800 m deep ocean. Comparing with the data of literatures, the maximum pit depths of 5052 and 6061 aluminum alloys decreased first and then increased with depth increased. The maximum value of pit depth appeared at about 800 m deep ocean. This is due to the amount of dissolved oxygen is the lowest in 800 m deep ocean, which promotes local corrosion.