The hull plates on all u-boats were not galvanized . If the plates were rolled to shape then galvanized you woulld have to grind it away around the edges to be welded , if you do not a deadly poision gas is created . Rust prevention was in the form of paint , inside the ballast tanks a silver aluminium paint was used . The only area that I know of that used galvanized metal was the outer skin to the conning tower . This is why most conning tower outer skins were rivited , the gas emitted from welding and most yards did not have enough trained workers that could weld such thin sheet metal without melting it .Paint does not stick to galvanized surfaces very well and if you look at photos of u-boats that were at sea for awhile you will notice a lot of paint is always chipped off and the tower has a mottled look to it , sometimes you can see the straight lines from the various sheets of metal and the rivits used to fasten them .