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Old 05-03-2013, 10:21   #1
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The inside of a fuel tank

Do you know what's going on inside your old boat's diesel tanks?

Well I found damp spots of diesel under my tanks so pulled them for inspection and found pinhole leaks in both. I cut out the bad spots and took a couple of photos. First cleaned and polished with fine grit sandpaper to show the damage.

One with a 1 inch ruler for scale. The ones with the bright spots are with a light below the metal shinning through the holes.

You can see how the aluminum was eaten through by the acids in the diesel sludge. The tanks are 1984, 5052 alloy, about 1/8" thick. The PO who owned the boat since new, swore he kept biocide in the tanks always, but when I got the boat I sucked out a pint or more of water and sludge from each of the two tanks. Obviously he never drained the tanks or polished the fuel in them.

By the way, I did inspect the tanks through a small port in the top but the corrosion was not visible in the staining and deposits on the inside of the tanks.
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Old 05-03-2013, 19:27   #2
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What you are looking at is known as microbially influenced corrosion. The anaerobic bacteria in the sludge are excreting organic acid that helps set up corrosion cells. The acid acts as the electrolyte to maintain these corrosion cells while the aluminum is sacrificed.
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Old 05-03-2013, 19:54   #3
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Re: The inside of a fuel tank

Quote:
Originally Posted by Aloha_float View Post
What you are looking at is known as microbially influenced corrosion. The anaerobic bacteria in the sludge are excreting organic acid that helps set up corrosion cells. The acid acts as the electrolyte to maintain these corrosion cells while the aluminum is sacrificed.
Yes, this I understand. Mainly posting the photos for informational purposes for those who may not realize what's going on inside their tanks.

I do find it interesting that the problem got so bad considering the PO faithfully used biocide in the tanks. My guess/theory is that the biocide was only soluble in the diesel so when there was phase separation and water settled to the bottom of the tank bacteria were able to grow in the water. Add the fact that the PO probably never drained or cleaned the tanks so the water, bacteria and the acid produced were left to work their magic.

Any other theories or ideas welcome but I guess at this point it's moot as far as my tanks are concerned. Plan for prevention is regular inspection and draining of the tanks.
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Old 05-03-2013, 20:01   #4
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Re: The inside of a fuel tank

Out of sight, out of mind sorta thing. All the more reason to run fuel polishers. Lack of maintenance is, and always will be, the end to all things marine. The ocean and it environments reclaims anything not looked after. And even then Murphy gets in the way.

Thanks for posting!
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Old 05-03-2013, 20:19   #5
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Re: The inside of a fuel tank

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Originally Posted by delmarrey View Post
Out of sight, out of mind sorta thing.
I have to confess that in this case I was guilty of that. Have such a long list of things to repair or replace that I was ignoring the fuel tanks. If I hadn't felt the leak while running lines for the new refer I would have passed over this one altogether.


Quote:
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All the more reason to run fuel polishers.
Which will be part of my maintenance schedule for the future. Which begs a question. Is polishing any more than running the diesel through a good filter/water separator? If I take a spare Racor 500 and pump the diesel from one tank to the other or recirculate through the Racor back into the same tank is that polishing or at least close enough to keep the tank problems down to a dull roar?


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Lack of maintenance is, and always will be, the end to all things marine. The ocean and it environments reclaims anything not looked after.
I sometimes think salt water is only slightly less corrosive than sulfuric acid.

Quote:
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And even then Murphy gets in the way.
Yea, that guy Murphy. Just when you think you've got things sorted out Murphy strikes. Working on what seems to be a simple repair, take the cover off and Murphy hands you a can of worms.

Since he has been such a constant companion I may have to name my boat after Murphy.
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Old 05-03-2013, 20:24   #6
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Quote:
Originally Posted by skipmac

Yes, this I understand. Mainly posting the photos for informational purposes for those who may not realize what's going on inside their tanks.

I do find it interesting that the problem got so bad considering the PO faithfully used biocide in the tanks. My guess/theory is that the biocide was only soluble in the diesel so when there was phase separation and water settled to the bottom of the tank bacteria were able to grow in the water. Add the fact that the PO probably never drained or cleaned the tanks so the water, bacteria and the acid produced were left to work their magic.

Any other theories or ideas welcome but I guess at this point it's moot as far as my tanks are concerned. Plan for prevention is regular inspection and draining of the tanks.
Biocides are themselves corrosive. They kill bacteria but do not dissolve and kill sludge. Biocides may slow down the fouling process but do not eliminate it. The sludge is actually layered with aerobic bacteria, anaerobic bacteria and facultative bacteria. The anaerobic reside on the bottom layer....they do not survive in aqueous environments. Facultative bacteria can survive in both aerobic and anaerobic environments.

I have an old aluminum tank and will be replacing it....probably this spring.
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Old 05-03-2013, 20:44   #7
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Re: The inside of a fuel tank

Every spring with this device I drain my tanks into another and back again. If there is anything in there this gets it out.

.
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Old 05-03-2013, 20:50   #8
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Re: The inside of a fuel tank

So glad my tanks have built in sumps with bottom drains. All I have to do is open a valve to drain any water or sludge out of the tank, and if there is water in the tank instead of covering the whole bottom of the tank it is concentrated in just the sump.
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Old 05-03-2013, 20:56   #9
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Re: The inside of a fuel tank

Quote:
Originally Posted by Aloha_float View Post
Biocides are themselves corrosive. They kill bacteria but do not dissolve and kill sludge. Biocides may slow down the fouling process but do not eliminate it. The sludge is actually layered with aerobic bacteria, anaerobic bacteria and facultative bacteria. The anaerobic reside on the bottom layer....they do not survive in aqueous environments. Facultative bacteria can survive in both aerobic and anaerobic environments.

I have an old aluminum tank and will be replacing it....probably this spring.
Facultative bacteria was a new one on me but then biology is not my specialty. However I will have to question one point, that is anaerobic bacteria do not survive in aqueous environments. Perhaps you refer specifically to the strain of anaerobic bacteria in diesel tanks but from what I understand strains of anaerobic bacteria can live and thrive in aqueous environments.
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Old 05-03-2013, 21:03   #10
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Re: The inside of a fuel tank

Quote:
Originally Posted by minaret View Post
So glad my tanks have built in sumps with bottom drains. All I have to do is open a valve to drain any water or sludge out of the tank, and if there is water in the tank instead of covering the whole bottom of the tank it is concentrated in just the sump.
Well fortunately my tanks do have a sump and that's the area with the corrosion. So am welding a new bottom on the the sump area and I should be good to go. Then down the road, drain whatever's in the sump on a regular basis.
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Old 05-03-2013, 21:05   #11
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Re: The inside of a fuel tank

Quote:
Originally Posted by delmarrey View Post
Every spring with this device I drain my tanks into another and back again. If there is anything in there this gets it out.

.

I've got two 40 gallon tanks so my idea is to occasionally run one dry and pump the diesel from the other, through a Racor into the empty tank. Alternate the next time.
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Old 05-03-2013, 21:14   #12
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Re: The inside of a fuel tank

Scientists have found out that there is a range of bacteria in fuel tanks. They are all of a common family, some feed on certain things and then some feed on them. Here is a link to a simple explanation. They recommend good fuel hygiene and an on board polishing system which is basically filtering but by moving a large amount of fuel using separate inlet and outlet pipes and pumps and filters that are separate from the engine fuel system.

Diesel fuel contamination - What it is and how to treat it
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Old 22-03-2013, 21:13   #13
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Quote:
Originally Posted by skipmac

Facultative bacteria was a new one on me but then biology is not my specialty. However I will have to question one point, that is anaerobic bacteria do not survive in aqueous environments. Perhaps you refer specifically to the strain of anaerobic bacteria in diesel tanks but from what I understand strains of anaerobic bacteria can live and thrive in aqueous environments.
Facultative bacteria thrive in both environments...they can be aerobic or anaerobic. Most anaerobic bacteria can not survive in aqueous environments but do survive I the very bottom layer of biofilms resting at the bottom of your tank. They are the culprits producing organic acid that contributes to the corrosion issue. They constantly replenish the electrolyte used in anodic corrosion cells known as MIC
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Old 22-03-2013, 21:45   #14
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Re: The inside of a fuel tank

Here is my diagram again, it has everything needed:
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Old 22-03-2013, 22:53   #15
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Re: The inside of a fuel tank

Quote:
Originally Posted by Aloha_float View Post
What you are looking at is known as microbially influenced corrosion. The anaerobic bacteria in the sludge are excreting organic acid that helps set up corrosion cells. The acid acts as the electrolyte to maintain these corrosion cells while the aluminum is sacrificed.
Wow Skipmac… Nasty surprise and informative thread!

Aloha...Is this acid problem specific to Alloy or same danger with Steel?

I am presently redoing my sound insulation and have option to open vertical manholes and climb in for a look.
However I have good seals on vertical manholes and don’t want to create a problem.

My 3 integral steel tanks (1984) also have built in sumps with bottom drains and they are drained regularly.
Not treated with biocide in last 10 years of my ownership.
I go thru many hours of Gen and ME before primary duplex fuel filters with vacumn gauges need changing
Sandblased hull and ultra-sound testing 3 years ago showed no corrosion problems.

So I am still a bit undecided if this extra work is necessary

…..Should I open and inspect them?
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