?? Boats are made from aluminum. It is fine even with sea water. An oxide layer protects aluminum. (More here)

Aluminum should not corrode submerged in diesel fuel. But presence of water, crevices and lack of oxygen will cause corrosion. Being in contact with the steel tank would also be bad (there should be a rubber gasket).

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Last Fall, I replaced the aluminum diesel tank in my boat after 35 years service! It had corroded within the end seams below the normally full level. Much the same environment as benedict is finding. Now have nice sturdy plastic tank!

Don't discount bacteria being the cause of the pitting - anyone who has owned a diesel boat knows how bacteria can grow in diesel fuel. Key is to keep water out of system so bacteria have less chance of growing. Hard to do in some cases. Using the proper diesel additives will help.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5