Seal gas tank after welding?

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BACKGROUND:

I bought an aftermarket gas tank to use as an auxiliary tank since getting the OEM subtank seems like 🦄 and 💰. The tank has dimensions very similar to the OEM subtank but I need to relocate the filler inlet pipe and add a vent. I'm planning to MIG weld in the two new pipes and close the existing inlet (the tank is band new so no concerns about explosions). The tank is Ni-terne steel, which is nickel-plated then hot-dipped in lead/tin solder.

QUESTION:

How concerned should I be about the backside of the mild steel welds rusting from the inside of the tank? I can protect the outside welds with paint or epoxy, but I'm worried that welding with mild steel wire will destroy the excellent corrosion resistance of the Ni-terne coating on the inside of the tank. There are several pour-in gas tank liner products such as Red-Kote and POR-15, which seems to work reasonably well but sometimes fail after a time leading to a big mess. Is it worth it to use a tank sealer after welding, or should I just take my chances and leave the welds unprotected from rust on the inside? Alternatively I could try to solder in the pipes instead of welding, which may be less damaging to the original Ni-terne coating but maybe not as mechanically strong.

Anyone have experience with this?
 
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What procedure did the factory use to install the fill neck?
It is brazed or soldered on. Not sure if that was done before or after the coating was applied.

I would be hesitant to weld on it for a variety of reasons.
Care to expand? No risk of explosion, so I think the worst that could happen is it rusts after a few years. I'm not worried about creating pinholes and leaking (minus eventually from rust) since I can pressure test and can also apply gasoline-resistant epoxy to the outside of the welds.
 

flintknapper

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It is brazed or soldered on. Not sure if that was done before or after the coating was applied.


Care to expand? No risk of explosion, so I think the worst that could happen is it rusts after a few years. I'm not worried about creating pinholes and leaking (minus eventually from rust) since I can pressure test and can also apply gasoline-resistant epoxy to the outside of the welds.

Assuredly, it was hot dipped after the tank was complete.

IF you choose to weld...you'll want to grind down to bare metal. Don't try to just 'burn' your way to it. There is a lot of lead content to deal with. I imagine the thickness of the tank is going to present you a problem in terms of not blowing through with your weld.

You will be welding the 'edge' of any hole you create for a filler neck or vent. Even if you keep most of the heat on the parent metal and push the puddle to the vent/filler, I don't think you're going to get a very good weld on it. IF you are really careful *and experienced* you might be able to apply small/quick stitch welds around the circumference (as I did on the bung pictured below.


Supposing you are successful with the welding procedure, you'll need to coat the welds *inside and outside of the tank* with something....or you will eventually have corrosion.

TransPan6.jpg
 

flintknapper

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@flintknapper Do you think brazing/soldering would be better than welding, and would you still seal the inside of the tank?

@LandCruiserPhil I see you welded on your tank and AFAIK did not seal the inside (Factory Auxiliary Sub Tank Install - https://forum.ih8mud.com/threads/factory-auxiliary-sub-tank-install.818551/#post-9369592), any issues with corrosion after several years?

IF the vent and filler neck can be situated on a 'flat' on the tank, then I'd be more inclined to weld a 'flange' on those parts then attach them 'mechanically' with a sealant under the flange. Use self drilling/tapping screws and a fuel resistant sealant *possibly JB weld* under the flange.
 
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I was planning to silicon-bronze braze fittings on a tank. Primarily to reduce the heat I was putting into it, but I figure it also reduces the surface area for corrosion to start.
 
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This might sound like a silly thing to say, but can you use stainless welding wire? I’ve used stainless MIG wire with normal steel and everywhere I have welded hasn’t rusted, but on some welds I can see where the base steel has started to rust...

alternatively you could convert to diesel then you won’t have a problem 😊
 
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can you use stainless welding wire

This is a good idea, I have some stainless wire and it should help, though I don't think it will be totally rust proof since the weld pool will include regular carbon steel from the gas tank.

My current plan is to attach the filler neck and vent with a few MIG tack welds for strength (with the SS wire), then use lead-tin solder on the joint to seal everything real well. I'm not planning to seal the entire inside of the gas tank using a pour-in gas tank liner since sometimes those fail and make a huge mess. Hope it works, without much rust.
 

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