Best way to mothball a mild steel diesel tank?

yodave

yodave

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Hello,

We have two relatively large diesel tanks onboard our old GRP yacht. It would take us four years of typical usage to empty the tanks. Having just cleaned these out, I'm considering mothballing one of them. I don't want to make the mothballed tank redundant, as we may decide on another long trip one day.

Balance-wise, the tank I would keep in operation is on the opposite side from the water tank (which sits alongside the tank I'm considering mothballing).

Both tanks are mild steel. They've been cleaned to the best of my ability, but there are areas I can't reach particularly well. At a stretch I can just about reach all four corners of the bottom of the tanks with a paint scraper. The bottoms aren't in as in good nick as the rest of the tanks - they're rusty. I think they're still pretty solid, but far from perfect.

So the question is; keeping in mind that I want to be able to recommission the tank for future use, what is the best way of mothballing the tank?

  • If I leave it as it is, will condensation form inside and lead to water continuing the rusting process?
  • If I put a gallon or two of diesel in there, will that help, or will condensation form more quickly and lead to a layer of water under the diesel, which will again continue the rusting process?
  • Can I somehow treat the rust and then leave the tank empty, or perhaps add a gallon or two of diesel?
  • Should I avoid mothballing and just keep both tanks in service?
Any thoughts, advice or opinion greatly appreciated.

Many thanks
 
Ok, I'll try.


You will need to separate tank A from tank M for mothball, closing off all interconnecting pipes and return pipes from the engine(s). Then seal the breather pipe as well.

You now have M with only the only way in being the filler cap.

Hire a cylinder of nitrogen, and a plastic tube. Fill and overfill the tank with gas, until all the air is displaced.

The density of nitrogen is 1.251 g/L, which is heavier than air at about 7C upwards, so it will stay in the tank and not evaporate or float away. There's no water in cylinder nitrogen, so the inside of the tank cannot rust. However, condensation will take place on the outside, so you have to have scheme B for that.


If you are worried about the base of the tank being rusty, you could use POR15 before sealing the tank. and flushing with N.

http://www.frost.co.uk/por15-car-bike-motorcyle-fuel-tank-repair-sealer.html
 
JumbleDuck & Sarabande; thank you both for the pointer towards Frost fuel tank repair products. I hadn't heard of Frost before. I'll do some research on whether they're suitable for diesel as well as petrol.

Sarabande; I'm equally impressed and scared by your nitrogen solution. I have no knowledge in that area, but it sounds plausible. I'll give this some thought.

Many thanks!!!
 
sarabande said:
The density of nitrogen is 1.251 g/L, which is heavier than air at about 7C upwards ...
Click to expand...

Are you sure? A kmol of N2 has a mass of 28kg, a kmol of O2 has a mas of 32kg and both occupy the same volume, so I'd have thought that replacing the 20.95% by volume of air which is oxygen with nitrogen would reduce the overall mass, and therefore the density. What am I missing?
 
JumbleDuck said:
Are you sure? A kmol of N2 has a mass of 28kg, a kmol of O2 has a mas of 32kg and both occupy the same volume, so I'd have thought that replacing the 20.95% by volume of air which is oxygen with nitrogen would reduce the overall mass, and therefore the density. What am I missing?
Click to expand...

That the N2 wil come out of the cylinder and get cold via adiabatic expansion, this will make it more dense than ambient air.
There after it will diffuse in a gas-like manner as gases are wont to do.
The cooling will possibly condense some water on the inside of the tank?
 
lw395 said:
That the N2 wil come out of the cylinder and get cold via adiabatic expansion, this will make it more dense than ambient air.
Click to expand...

Ah yes, good point. Certainly filling the tank with inert gas seems like a good idea, as long as it can be completely sealed afterwards., and as long as that wouldn't cause problems with atmospheric pressure changes.

 
For a fuel tank of, say 200l, with steel walls perhaps 2mm, I can't see that kind of implosion or explosion taking place.


Approx figures for air. Coefficient of thermal expansion 0.0037/C

Vol of tank 0.2m3

Temp rise 5c to 20C.

Inc in volume of gas in tank 10.1l - which is rather more than I first thought. :(



Perhaps we can fill the tank with N, then stick a balloon over the breather so there's an autonomous balancing volume
 
A litre of cheap two stroke, block the holes and rotate. Every few months move the oil about, when you're ready to use clean out with petrol, wash with hot soapy water, dry and fill with fuel. It's what I do with my motorcycle tanks when they're in storage.

I'm not a fan of sealers, they crack, lift, break up and block lines.
 
Yellow Ballad said:
A litre of cheap two stroke, block the holes and rotate. Every few months move the oil about, when you're ready to use clean out with petrol, wash with hot soapy water, dry and fill with fuel. It's what I do with my motorcycle tanks when they're in storage.

I'm not a fan of sealers, they crack, lift, break up and block lines.
Click to expand...

I'd agree with that.
The tank will probably rust from underneath before it goes from the inside.
Maybe spray the inside with oil now and then?
 
JumbleDuck said:
Can the tank be rotated? I presumed it was in the boat.
Click to expand...

I have no idea, if I were to mothballing a tank I would be removing it so I have extra storage space and storing it somewhere dark and dry. If the op wants to use the tank occasionally for long motors I wouldn't be mothballing it, I would be filling up the tank with diesel and fitting a couple of taps and alternating the tanks on each outing/tankful, but if it takes 4 years to empty both tanks of sailing at the moment I would be happy to think one tank would be good enough for most sailing, with a 20l jerry can as backup.
 
The absolutely best way to mothball tanks, piping tubulars and the like is with VCI powder http://www.corrosionvci.com/POWDERS.pdf Pipeline tubulars are protected with this stuff by making up plywood flange ends, blowing the powder in with an air line, then closing off the ends. Tubulars protected this way remain corrosion free for months or even years. Only problem seems to be that the smallest amount sold is 50 lbs but you might be able to obtain something smaller by googling VCI powder.
 
In complete sealing a mild steel tank would only allow a small amount of rusting inside as the rusting process would consume the oxygen in the air and once all the oxygen is consumed no more rusting can take place.

This is the same way that structural steel hollow sections are sealed so no corrosion protection inside the tubular structure section is needed.
 
JumbleDuck, sarabande, lw395, Yellow Ballad, vyv_cox, & Rogershaw - Many thanks for your input.

Looking back over your comments I've ruled out the nitrogen solution on the basis that it's a little scary and I can't be sure that I can seal the tank 100%. This uncertainty about sealing the tank completely also puts a question mark over (my ability to seal the tank and then) letting the rusting process remove the oxygen. I also can't rotate the tank, as it's large and fixed to the boat. Removing would require removing the engine and we're in the water early April.

Given vyv_cox's knowledge about metals in the marine environment, I have faith in his view that VCI powder is the best solution. I'm going to explore that. I have some niggling reservations about the sealing kit, with Yellow Ballad's comment about them cracking over time. I have contacted Frost for feedback on diesel tanks, as their product information seems to refer specifically to petrol tanks rather than diesel. The other option still in play is JumbleDuck's early mention of using grease. That may well have it's drawbacks, but it would be comparatively easy to do (access & process-wise). I'd need to figure out what to do with the grease when I recommission the tank, but that would also be the case for the powder.

Your help and advice is very much appreciated.

Many thanks!
 
Just a thought but what about aerosol foam filter oil, spray it into the nooks and crannies, top and bottom. It's very sticky tacky stuff so won't run off. I would have thought diesel would break it down if you needed to use the tank but it might be worth a jam jar test.
 
Yellow Ballad said:
Just a thought but what about aerosol foam filter oil, spray it into the nooks and crannies, top and bottom. It's very sticky tacky stuff so won't run off. I would have thought diesel would break it down if you needed to use the tank but it might be worth a jam jar test.
Click to expand...

I wondered about that as well and fogging oil is fairly easy to find. It should cover everything fairly easily and foams up a little initially. I imagine that there'd be a small amount of vapour left in the tank as well. It should be easy to mop out any excess before re-filling and doubt the film left on tank wall would be a problem.

I also suspect that the outside might corrode more quickly than the inside, especially near the supports underneath.
 
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Yellow Ballad & Mistroma; Thanks for this additional option. While I've still to fully explore the other options mentioned above, it seems that aerosol foam filter oil / fogging oil would most likely be the easiest option to execute. I'll do some homework on this, because even if I use something like the VCI powder solution for the mothballed tank ...I still have the other tank that I'm keeping in service to consider. I'll be filling that up with diesel at the end of March, and it makes sense for me to take steps to have some sort of barrier on the rusty bottom.

I still haven't heard back from Frost on the suitability of their products usage with diesel, and I'm a tad concerned about their solution failing at some stage (in part because I'm not sure that I can do all of the required preparatory work due to access restrictions). Any-which-way it may be that I grease or use thick sprayable oil on the tank that I'm keeping in service.

Thanks again!
 
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