Diesel Tank Bomb

[Katoh]

Colleges
I am in the middle of a full strip down and restoration of an aluminum 20' boat, I have removed the under floor fuel tank which sat in its own fully water tight well. The tank was bolted and foamed in which has now caused me a little problem, The tank was under water when removed and were in some places the foam had really taken to the aluminum sides, an amount of crevice corrosion is present.
I have removed the tank and cleaned up the outside with high pressure washer to expose all the corrosion spots, while hosing down the inside I also found a leek on the bottom of the tank, maybe others. The tank is out of the boat, all fittings have been removed including float and sender unit. I plan to fill the tank with hot soapy water and let it sit for a couple of days, then rinse with clean water.
I only have a mig with a spoolgun, that I was going to use to fill all the crevices with, and fix the crack, I know tig maybe better but I don't own one, nor do I have access to one.
The last thing I want to do is turn the tank into a bomb, being diesel I cant see it being explosive but you never know.
Katoh

[Bruce46]

The best advise I can give is throw that tank away and start with a new one. You might want to consider one of the new plastic tanks, no corrosion problems.

[Katoh]

The big problem is that tank was custom built to fit in that fuel well, there's no way I'm going to go altering that because of the hull design, just not going to happen. There is little chance of finding anything with that configuration and size pre-made, new tank would have to be custom, now were talking serious $. Just looking at the plastic tank OK you buy for 250-300 dollars but shipping to Au add an extra 1.5K, Ill fix the original first!

[michael pierzga]

Cut a good size inspection port on the top and carefully inspect the inside of the tank...tanks also corrode from the inside...no use wasting time.

[MarEng2]

Based on personal experience you should be fine welding this tank. I have had empty 3,000bbl tanks with diesel residue (clingage and scale) catch fire before. They burn sedately and are easily extinguished. With the washing you've already done I doubt you'd be able to get your tank to light.

You state that the access covers and sending unit are removed - keep them off while welding. I know I'm stating the obvious but you can't have a bomb if you can't build up pressure. Also, you need to have a relief path for the heated air from welding.

I speak only about the safety side of the repair - I don't weld aluminum and have no comments on the efficacy of the proposed repairs.

[bntii]

Maybe get throw a little pressure into tank to look for pin holes.
Soapy water outside and look for bubbles- look carefully along all the welds.
As mentioned a peek inside is worth the effort.

[tazmann]

Last aluminum truck tank I repaired I did the same washed it out several times then when I went to weld on it I ran an exhaust hose in it and let it purge for a while. then I welded it up with spool gun with the exhaust still blowing in there. took a couple passes because the exhaust does not enhance the weld on the first pass.
The best and safest bet would be build a new tank.
Tom

[DianneB]

I have done numerous fuel tanks and I usually start with a wash ("Marine Clean" is good). If I am concerned about vapors, I put the hose on the discharge end of the shop vac and leave it blowing into the tank while I am working.

There is a great product called "US Standard Fuel Tank Sealer" that puts a good fuel-proof coating over the inside of the tank - it looks like the old enameled cookware when you are done! It seals any small leaks and provides a nice clean surface and stops any dirt or rust from getting in the fuel. Every time I remove a fuel tank, it gets a coat of sealer as a matter of routine and I have never had a problem after.

[pdwiley]

I only have a mig with a spoolgun, that I was going to use to fill all the crevices with, and fix the crack, I know tig maybe better but I don't own one, nor do I have access to one.
The last thing I want to do is turn the tank into a bomb, being diesel I cant see it being explosive but you never know.
Katoh

Standard practice when I did my welding tickets 20+ years ago was to rotate a tank so the weld was uppermost and fill the tank with water to within 25mm or so of the top.

No or minimal air space, no fuel-air mix, no possibility of a big bang.

Back-purging with inert gas would work too.

There's a company on Ebay Australia advertising polyethylene diesel tanks, don't know if they have a suitable size/shape for your needs though.

PDW

[Katoh]

Gentlemen
Thanks for all the feed back, I used a plumbers camera or snake camera to have a look in the tank, and it looks in good shape, the pitting is all on the outside, were the foam was, I should elaborate its not all over the tank but a bit here and there, actually there's no pitting on the base at all.
I was planning to used compressed air to find the leaks, one I know off, and any others I dont. When I removed the tank from the boat there was no fuel leak present, I must have just popped it now with the high pressure washer. I do like the sounds of that "US Standard Fuel Tank Sealer" I have to research it little more.
As for plastic tanks I don't think so in my case, The tank is 4' x 2' x 10" approx, It bolts from either end into the well it has L shaped stringers welded to the top as supports for the checker plate floor above, the filling points are side mounted that run into their own well leading to the side of the boat. Its that tightly fit I cant even fit a well pump in there anywhere.
Many thanks
Katoh
Katoh

[Poida]

Watch out with the compressed air leaking method.

Most compressors will pump to at least 6 bar where a fuel tank will only need to withstand 0.1 bar it most probably will withstand more than that but not much more. I would certainly suggest a pressure regulator in line and not to exceed 1 bar.

[Katoh]

Poida
You are dead right! I was not planning on more than 20psi, don't want to create more holes than needed.

[El Sea]

UL recommends round tank to be tested at 5 PSI & rectangle tanks at 2-3 PSI.
We find most alum. fuel tanks that are approaching 20 years of age to be problematic if encased in foam.

[rcwade]

Por 15 has saved me many a headache on restoring auto tanks. Not sure on the size of this tank but it may be something to look into. Seals the tank and you don't have to worry about welding up the holes.

[FAST FRED]

"You are dead right! I was not planning on more than 20psi, don't want to create more holes than needed."

5 psi is enough. 20 will blow every marine fuel tank!

Fill the tank with water cap it , and take a hose 10-11 ft up filled with water .

That's all the "test" you need.

FF

[dinoa]

Other fuel tank sealers would be ProSeal "http://www.aircraftspruce.com/catalog/appages/ps890.php"
and this formulation
"http://www.vansaircraft.com/cgi-bin/catalog.cgi?browse=misc&product=prosealand" that's supposedly superior to ProSeal. Very messy stuff to work with though. Usually used to to seal integral wing tanks made of riveted sheet aluminum alloy.


Dino

[Katoh]

I can see the 20psi is a bit much, that's what used last time on S/S boiler to check for leaks, Ill stick with 5psi. I have looked into that urethane sealant (por15) and I will use that, after I weld in the spots that look a little dodgy. Would I be advised to paint the outside of the tank as well?
Now I know your not meant to use foam anymore is there something else you put around the tank, I was think of welding on a couple of right angle tabs on the sides of the tank and using pop rivets to hold them in place on the sides of the fuel well.
The tank is approx 4' x 2' x 10" 144liters.
Katoh

[Poida]

Unfortunately dieseine will flow through holes that water wont. We test welds with a kero soaked rag and test for holes.

Sometimes when ali corrodes it forms a crater on the inside. This crater may collapse and block the hole if you use water.

Air would be best in my opinion, if you don't think your tank will take 1 bar wind your pressure regulator up a few lbs at a time.

Squirt the outside of the tank with soapy water and it will bubble where you have a leak.

And, air will go through a hole dieseline or water won't.

[Katoh]

I was going to test with air, I can also do a test with vacuum.
Just a thought how will those sealers go with Bio? I know that some materials will dissolve in bio but are safe with diesel. Might be a question for the manufacturer.

[SaltOntheBrain]

We used to use coal-tar epoxy to coat aluminum tanks that would be in contact with foam or similar materials. It's tough and it's got a little give to it, so it won't chip off if it gets bumped. Be careful, though. One gouge and all your corrosion will attack the one bare spot.

Diesel tanks are easy...just take a second argon or co2 cylinder and regulator and hose and purge the tank while you are welding...even if the tank still has diesel in it. (Just don't weld below the level of the fuel, or it won't weld right and will blow out at the weld) The 2 no-no's on diesel tanks are pressure and oxygen. Make sure all caps and vents are open, and use inert gas to purge the oxygen. Personally, I wouldn't use exhaust, because there's no guarantee you aren't getting some unburned fuel and/or oxygen in it.

4 psi is what we used to test for leaks. 20 psi WILL crack the seams and/or blow the tank.

I welded up a lot of diesel tanks that had cracks or leaks on vertical seams while they were still in the boat and while they still had fuel in them. There's no great trick to it.

All that said, I wouldn't weld the bottom of the tank you described. you don't want any thru-welds or stress points on the bottom of a fuel tank that could become cracks and then leaks. I'd cut the bottom and top off and replace the bottom with new material and put the top back on. (you need access to the inside of the sides and baffles to weld the bottom properly...that's why the top has to come off too).

LF

Lance.

[SamSam]

Diesel tanks are easy...just take a second argon or co2 cylinder and regulator and hose and purge the tank while you are welding...even if the tank still has diesel in it. (Just don't weld below the level of the fuel, or it won't weld right and will blow out at the weld) The 2 no-no's on diesel tanks are pressure and oxygen. Make sure all caps and vents are open, and use inert gas to purge the oxygen. Personally, I wouldn't use exhaust, because there's no guarantee you aren't getting some unburned fuel and/or oxygen in it.


That sounds practical. And keeping the purge going while welding is essential.

This would seem to be pumping oxygen into the tank...If I am concerned about vapors, I put the hose on the discharge end of the shop vac and leave it blowing into the tank while I am working.

[Katoh]

Hi lance
lots of good Info, I have removed and washed the tank out, quite thoroughly. I have only got a pin hole in the bottom, the rest are crevices on the sides and top no leeks but need a bit of material in them before they become leeks. The tanks going back in with no Foam! I will look into that coal tar epoxy.
Hopefully this time it wont be sitting under water for its entire life.
cheers
Katoh

[viking north]

Katoh, I don't want to be the dark side here but from what you describe it sounds like your tank has suffered from what is commonly known as poultice corrosion. A chemical interaction between the wet foam and the aluminium causing the aluminium to break down. This often occurs between aluminium when in constant contact with wet wood. This type of corrosion can be very aggressive and while you might see only pinholes the actual metal itself can change it's characteristics due to leaching and becomes generally weaker(brittle) while still looking ok. My recommendation is to cut out the damaged area and weld in a filler. Personally I would never trust this tank again unless this Proceedure is followed. Too many years in the repair and boat building buisness and having learned the hard way has instilled this wisdom in my head regarding working with alum. I realize you are concerned over costs but if you do the prep work and take it to a pro to tig weld your cost should not be more than the min. 1 hr charge. Say $60.00. --Geo.

[Katoh]

viking north, from what you say it's looking pretty bleak for my tank, if it was only a spot hear and there Ok, but buy the time I cut and weld in new sections that's the whole tank, or upper tank. I thought about welding a new plate over the old, making like a double tank, but I don't think I have the room.
The other option is to make a new tank.
what about a Stainless tank?
Katoh

[viking north]

Katoh, stainless possibly is more of a challenge to fabricate than alum. Under Boat design go to page 4 then scroll down to Reverse Engineering page 20 post # 297 by SAM SAM. He has some interesting info on stainless fuel tank fabrication. I would reconstruct in marine alum if I were doing the job. It's hard for me to asses your old tank, is it really that corroded . When I recommend to cut out a section i am referring to an area if it's full of pin holes and the general area has flaked to a thinner thickness. You'll possibly also notice a change in the alum. colour shade. Is there anyway you can post a few photos of the corrosion. You could possibly do the repairs plugging one pin hole at a time, apply an interior sealing compound and coat the outside with coal tar epoxy and temp. solve your problem but since you have the tank out why not repair it proprerly or if it's beyond it's life build a new one. Is there an option of installing another off the shelf tank in another location ?---Geo.

[Katoh]

Viking North, There's no chance of relocation of the tank, the builder built a well fully enclosed all round as part of the hull structure. It can be done with major works to the hull. That is not going to happen.
The tank has only one little hole, There are a Lot of crevices in the sides and top, I was going to fill these in one by one with the mig, I have time. The hole down the bottom maybe drill out and give it someone to tig, Or I just mig that as well! Then urethane the tank inside, maybe even paint the outside too. My biggest problem was, there was no drain in the fuel well and the tank was under water. I would like to add a pic but don't have a website, cant really work that one out.
katoh

[Katoh]

I think Ive got this Picture thing sorted.
First shot shows the only hole, that's a toothpick in it.
Second shot is about 100mm directly right and 100mm down from the toothpick, Those craters are say 3-10mm dia ( 1/8" - 3/8") That's just a typical spot, on the tank. Funny thing there's none on the base at all.
Happy now at least I can get some pics on here.
Katoh

[viking north]

Thats basically the vision i had. The typical problem is any area of concentrated corrosion as shown in the right photo usually is honeycombed with pinholes not imediately visible or leaking but on the verge of doing so. If this is the case only on a few locations i would cut them our or reweld a patch over the area say a couple of inches beyond the affected area but if it as you say typical thruout the upper area of the tanks surface thats not pratical. Looks like you are left with either trying the one hole at a time and a sealant type coating or a new tank. For liability reasons if the boat was in my shop i would have to recomment a new tank. I wouldn't try a repair if there were multible locations of corrosion as shown on the right photo. Just in interest, TEMPO sells an off the shelf plastic tank very similar in size to your dimensions, i am installing two of them in my new build in a location similar to yours. Another alternative cut the top off your tank and use it as a base for a fuel bladder style tank. I seem to recall these are custom made to size and very reasonable in price and easy to ship anywhere in the world. I can post the Tempo Tank info for you if you wish and maybe another forum member can supply additional info on the fuel bladder style but i will look for that info.
--Geo.

[Katoh]

Ill have a look at the tempo tanks, I have looked at plastic tanks and nothing suits, The feeder enters from the side, also you cant see them on the photo but there's stringers welded to the top, to hold down the floor plate. The bladder sounds interesting.
At the moment I have nothing to loose, I will have a go at filling the holes, one by one, after the first few, I will definitely know if I'm wasting my time or to keep going. The way I see it at the moment I have only 3 real options, have a go at mine, buy plate and try and make a new one, or look down the barrel of about $800 and have one built.
Cheers
Katoh

[viking north]

Katoh, if as you say the tank will be partially submerged in water don't build one out of stainless. Being submurged in water it will suffer from oxygen starvation and as such is very sustiable to another type of corrosion with the same outcome. Good luck on the repairs---Geo.

[Katoh]

The tank was under water when removed, there was no drain added to the fuel well only a breather line. I plan to fix this with a bit of piping and a stop cock. The builder must of thought that he had it water tight, but pop rivets and silicone have a lot to be desired.
I found a company by chance just now that make custom plastic tanks, and their in Aus.
Ill give them a ring in the morning.
Many Thanks
Katoh

[viking north]

Katoh, thats great, I think in the long run this is the answer.---Geo