Stainless fuel tank repair

[trayle]

I have a 100 gallon stainless diesel fuel tank that is leaking. The tank has a rectangular finger that sticks down into the bilge. I can't see the outside of the tank, but the inside is corroded near the welds down in the bottom of the finger. I'm exploring ways to repair the tank without removal, through the inspection plate on top.

One option is to weld a rectangular plate in near the top of the finger, effectively cutting off the finger. This would only reduce the capacity of the tank by about 10-12 gallons (out of 100) and would keep the working area of the tank out of contact with bilge water.

Another cheaper/easier option is to pour something into the bottom of the tank to reseal it. So here's my materials question: What I could use that would bond to stainless and form a seal that withstands the pressure and possible flexing of the tank as well as exposure to diesel fuel?

I don't care if I have to pour something several inches thick, in fact that may be better to get above the level of exposure to bilge water. Someone suggested epoxy, or a few inches of concrete followed by epoxy. Will epoxy bond well to the stainless? Epoxy probably won't handle flexing, but perhaps concrete could be poured and cured with the tank full of water, locking the bottom of the tank into it's maximum flex position.

[gonzo]

Stainless tanks have a tendency to crack and leak at the corners if they are welded. The proper way to fix a tank is to weld a patch. Epoxy and cement are only going to make the repair more difficult and expensive when you have to remove the mess. If you can see corrosion, the only proper thing to do is to remove the tank and inspect it.

[trayle]

Removing of this tank means some pretty massive surgery on the boat. I only see corrosion in the bottom of this deep finger, the rest of the tank looks pretty clean.

I think it can be repaired in place. The welding option is kind of like amputating the whole cancerous finger. Maybe that's a better long term choice than covering the tip of the finger with something...especially if there's nothing that's going to bond with stainless like a weld does.

[gonzo]

If you have access to the side, you can always cut an ispection plate there.

[SamSam]

Isn't welding it risking an explosion?

[Frosty]

2x300 water --2x800 fuel under built in floor on both hulls. I sit up in a cold sweat some nights thinking what would I do if this happened to me.

I think I would replace them with bladders. If I had to rip out 14 feet of galley before I even got to the rose wood floor,-- damn I would be pissed

[trayle]

Yes, welding certainly is risking an explosion. But my welder has done jobs like this before and he says it can be done safely if the tank is cleaned well enough beforehand.

[Frosty]

Diesel tank will not explode even in my climate. You can weld a diesel tank with diesel in it.

But a parachute should be worn.

[gonzo]

It shouldn't be much of an explosion danger with diesel fuel. There will be a high risk of fire when the tank heats up and the spilled fuel gets to the flash point.

[hoytedow]

Why don't you off load the diesel fuel and fill the tank with water to displace remaining product, then weld?

[gonzo]

The tank was leaking from the bottom, so the boat is soaked in fuel

[hoytedow]

Understood.

[Ike]

If this tank has corrosion you can see then it also has corrosion you can't see. Stainless Steel is very prone to crevice corrosion which occurs primarily in the welds. Welding is the problem. That's why ABYC recommends stainless only be used where all sides of the tank are available for inspection, and that they be under twenty gallons and be cylindrical with domed ends. This minimizes the welds and allowsthe tank to stay completely dry. Stainless will corrode if it is constantly wet, which is very likely if it's in the bilge.

You need to empty this tank and have it tested. You should perform both a pressure drop test and a soapy solution test for other holes. If the leak you have already detected is the only problem then you may be able to repair it. But if it has more holes, even pinholes then the best solution is to replace the tank. This may be disruptive to the boat structure but it's better than having a bilge full of diesel fuel.

[hoytedow]

You can fix a small leak with a neoprene washer and a sheet metal screw.

[nukisen]

I know a method, please be careful!
If you dont have to much corrosion and the plate isn´t to thin.
you can tight the leak by pounch with a hammer and a dorn. As this will not hold for more the a few minutes, you will be able to dry the surface as much as you are possible to weld the leak. If you use to much heat and cause a hole in the tank at this moment. then I can tell you will have problem with quite fast leak. I possible you can also cause vacuum insdide tank to stop it temporarily. The big secret is to get the welding area clean and dry. If oil or camparable liquid is inside in bigger volumes the heat will only cause a small boiling at the point you do weld.
WARNING!!!!!!
If you do weld above the liquid upper line Then the gases is mixed with oxy. Oil gases and oxy mixed air isn´t a very good kombination with ignition.