wedge fuel tanks

[wooly350i]

Hi, i am hoping some knowledgeable sole out there can throw a light on the manufacturing process of how these petrol tanks were welded at the pipe spigots.The tank joints i know are roll welded by machine but the pipe spigots i assumed from the softness of the welds i thought were silver soldered but we have tried that and no sir they are not.So maybe a soft solder and flux was used but on steel tanks? Just got to repair one pipe joint before they go back on board. Replacing i know is an option but the priceno ta not at this stage,and anyway both tanks look pretty good in and out apart from the CRACK.
Any help would be appreciated so i can get on again,Cheers,John