In the early and mid-60s of the last century, the hulls were mostly made of 2024 alloy. About 90% of the ship's hull is made of 5XXX series alloys, especially 5083.
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Corrosion of 2XXX Series Alloys (Duralumin) in Seawater The corrosion resistance of hard aluminum is relatively low, mainly due to the presence of Cu. Someone had a very severe pitting corrosion after 16 days of testing on a hard aluminum shell boat. After the Soviet scholars used the D16AT alloy to remove the aluminum layer and test the specimens for 45 days, the strength loss was shown in Table 1, and the loss of yield strength Rp0.2 became significant.
From the data in the table, it can be seen that the D16AT alloy plate without the aluminum coating is severely corroded in seawater, whereas the corrosion of the aluminum coating is much lighter. Because the clad alloy layer not only functions as protection, but also plays a role of electrochemical protection of the 2XXX alloys, 1XXX alloy clad layer is in the anode, ensuring 2XXX alloys from corrosion damage. Sometimes, even if the partial aluminum coating is peeled off or subjected to mechanical damage, it will not cause alloy corrosion, because the aluminum coating of other parts is still playing an electrochemical protective role. This means that the inherent deficiencies of the corrosion resistance of hard aluminum and super hard aluminum can be improved through the conditioning.
Corrosion in seawater of 5XXX series alloys Corrosion resistant to seawater in aluminum alloys is 5XXX series alloys. The corrosion rate in seawater is very slow, and their corrosion resistance is related to the magnesium content. Alloys with Mg content ≤ 5% have good corrosion resistance; alloys with Mg content > 5% suffer from intergranular corrosion and stress corrosion cracking when improperly heat treated (annealed).
The mechanical properties of the 5XXX series alloys, 2024 alloys and shipbuilding steels were fully immersed in 3% NaCl solution for 9 months. Table 2 shows the corrosion resistance of aluminum-magnesium alloys is much stronger than that of hard aluminum 2024 alloys and shipbuilding steels. The loss of mass is small, and the loss of mechanical properties is small, which shows that the aluminum-magnesium alloy manufacturing hull has obvious advantages.
American scholar CW Sivio's research pointed out that the 5083 alloy (containing 4.0% ~ 4.9% Mg, is an American alloy, stereotyped in the late 1930s) and 5086 alloy (containing 3.5% ~ 4.5% Mg, is also an American alloy , in the 1940s initially for type, then the grades for 86S) and some other alloys immersed in sea water, did not take any protective measures, six years after the great depth of 0.86mm 5086-H34 alloy specimens corrosion pits, but not great The strength loss; another test was to partially and completely immerse the unwelded and welded 5086 and 5083 alloy sheets in Tonabina Seawater in Florida for 5 years and 7 years. After 5 years, the strength loss of the fully immersed unprotected welds 5086 and 8083 samples was negligible, almost no corrosion pits were found, and the depth of the larger pits was only 0.38 mm. Partially impregnated in the sample, 3.2mm thick and 5.83 unwelded 5086 alloy sheet, regardless of the condition of immersion, the intensity losses are negligible, which fully explained 5086 and 5083 alloy retains stress inherent resistance to sea water corrosion. 
Marine Materials (44): Corrosion of Al-alloy Ships and Marine Facilities (2)