Leaking Diesel Tank

[Barking Mat]

I work with boats, I have a client who has a 1987 mild steel diesel tank which has just started to leak, so I pulled out the tank and standard problem is that the water is never drained from the drain valve at the bottom and systematically rots out the bottom of the tank, however this is not the case, exceptionally this diesel tank has been reguarly drained and it seems to me its the mig welds right at the bottom of the water trap which has rotted out. An inspection shows very little detoriation to the actual steel, was it badly done? An old mechanic once told me its always the welds which rot out first, myth or truth?

So im going to make a new water trap with drain, and weld it on, but mig or tig? I have both, I suspect the proper way to do it would be to tig and go over with the mig?

Heres some photos.....

 

[Wendelspanswick]

Ideally mig or tig and then if you think the welds are porous go over it with a plumber flux and solder or braze.

 

[Barking Mat]

Im going to try tigging it and if it all goes horribly wrong Ill mig it. Ive cut out the round, plug in the welder. Looks well wonky bottom photo.

 

[Rig Pig]

I did a few hydraulic tanks, had lots of fun with leaks from cold laps with the MIG so I used a stick & had no problems. Test the tank after you have welded it or it's bound to leak from the most awkward bit.

 

[KemppiFrog]

I did a few hydraulic tanks, had lots of fun with leaks from cold laps with the MIG so I used a stick & had no problems. Test the tank after you have welded it or it's bound to leak from the most awkward bit.

Been there too. If thick enough, then stick, if not, TIG.

 

[KemppiFrog]

Just had a prob with an ally tank. The rot was from the outside, where it rested against stringers. I managed to MIG in several discs at the worst pin hole areas. But a nightmare to chase the tiny leaks on weld overlaps. Def an AC TIG is the cure for that.

 

[Barking Mat]

Its done now, I apologise for the awful photos, At one point I got the tungsten stuck in the weld puddle, and had to snap it off, I tried to grind out the tungsten, and I went back over it. A couple of mini craters I went back over and one I had to bring up to the top piece of metal.

I did not purge the tank, the weld for the drain on top looked a little cold for me, I'm wondering if there should be more of a heaped stack of dimes look around the side of the drain, looks a little flat to me.

 

[pedrobedro]

If it does the job it doesn't need to look like a stack of dimes, that's just for bike frames and stuff on show, not a manky old diesel tank shut away in the bilges.