[Gaffer]

So....I’m guessing the rivets were ferrous and eroded by the salt water?

[Paladin2]

"the rivets were ferrous"

Unlikely.

[grobdriver]

The rivets would have been aluminum, like the majority of the airframe.
Some steel rivets are used where more strength is needed - a patch over a window opening (an odd modification) would not be such a case.