The inside of a fuel tank

[Aloha_float]

Originally Posted by Pelagic

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?

It's good that you have the system set up that you have to drain the tanks. I would suggest that you do open them up and perform and inspection. It's likely that you will find some pitting corrosion. If you know plate thickness, you can measure depth of pitting to determine integrity of the tank.

It would not hurt to coat the bottom of these tanks with an immersion epoxy designed for fuel service. If you are there, it pays to do the extra inspection.

Biocides do not prevent this corrosion problem. They simply kill bacteria and impede the growth of the biofilms that clog filters.

[s/v Jedi]

Originally Posted by Aloha_float

Biocides do not prevent this corrosion problem. They simply kill bacteria and impede the growth of the biofilms that clog filters.

But enzyme based fuel treatment does; the bacteria get burned in the engine while passing the filters after the enzymes break them down.

I guess the most well known enzyme treatment is Startron which is available anywhere.

[Aloha_float]

Originally Posted by s/v Jedi

But enzyme based fuel treatment does; the bacteria get burned in the engine while passing the filters after the enzymes break them down.

I guess the most well known enzyme treatment is Startron which is available anywhere.

I've looked at enzymes....appears to be 98% kerosene as the diluent. I also believe that we tested Startron. I will look through our test data. My recollection is that they do not fair well as an alternative to rid a system of sludge.

[shorebird]

Steel fuel tanks are very durable, but are prone to MIC. Newer fuel blends are regarded as responsible for the dramatic uptic in MIC in steel tanks used, for example, in home heating applications, but mobile tanks seem to have far less problems due to the sloshing of fuel and water within the tank inhibiting localized colonies of bacteria. I understand the fuel/water interface is the main problem, and keeping/removing water from the tank, as well as adding fuel additives seems to be the best thing to do for fuel and tank. Note that there are additives for killing microbial growth, and for inhibiting microbial growth, and I consider adding inhibitors normal routine on our vessel.
Steel tanks, at least the ones we manufacture, are epoxy lined only when containing water, or corrosive jet fuel, and many of our uncoated tanks are in constant use for 20 years or more, and still going.

[s/v Jedi]

Originally Posted by Aloha_float

I've looked at enzymes....appears to be 98% kerosene as the diluent. I also believe that we tested Startron. I will look through our test data. My recollection is that they do not fair well as an alternative to rid a system of sludge.

Then your tests must have gone wrong because I've seen many successful tests... not just published but right around me with dozens of boats where it completely cures the problem.

98% kerosine can be right; the product can be used like fuel so you can't overdose. Enzyme treatment is widely used in many applications and well proven and accepted. There is no question if this works or not and no magic involved.

[thinwater]

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.

It can be bacteria, but that is not the only thing. I am and API tank inspector and the corrosion pattern is very common. Much of the time bacteria play no part (tanks that are boiled, for example).

• Stray current.
• Salt in the fuel (fuel station flooded?).
• Condensation that extracts acid from the oil. The acid can exist in the fuel from the supply train, not just bacteria in your tank.

While a biocide can kill bacteria, additives like StarTron and Stabil will inhibit corrosion from a wide range of attacks. These 2 have been independently tested to be particularly effective on aluminum. Several other additives can actually make aluminum corrosion worse.

Polishing can help, but it will not remove dissolved water and the acid and salt in that dissolved water. I have done corrosion testing studies where the fuel was saturated with water containing corrosive elements and then polished. The polishing made less difference than you would think.

(While Star Tron's claims about helping prevent phase separation in e-10 where not supported by testing, it is NOT smoke an mirrors. It is moderately effective as a biocide and quite effective as a corrosion inhibitor. And I don't have a dog in the fight.)

[skipmac]

Originally Posted by Aloha_float

Facultative bacteria thrive in both environments...they can be aerobic or anaerobic.

Yes, I had looked up facultative and learned a new word.

Originally Posted by Aloha_float

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.

Well previously you had posted "The anaerobic reside on the bottom layer....they do not survive in aqueous environments." which is why I posted the question since in my experience at least some anaerobic bacteria can and do live in aqueous environments.

Originally Posted by Aloha_float

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

That is what I found in my tanks. A layer of sludge that I assume was produced by bioactivity and possibly other mechanisms, was collected in the bottom of my tanks and eating holes in the Al.

[s/v Jedi]

Originally Posted by skipmac

A layer of sludge that I assume was produced by bioactivity and possibly other mechanisms, was collected in the bottom of my tanks and eating holes in the Al.

And that is exactly what the enzyme based treatments prevent

[skipmac]

Originally Posted by thinwater

It can be bacteria, but that is not the only thing. I am and API tank inspector and the corrosion pattern is very common. Much of the time bacteria play no part (tanks that are boiled, for example).

• Stray current.
• Salt in the fuel (fuel station flooded?).
• Condensation that extracts acid from the oil. The acid can exist in the fuel from the supply train, not just bacteria in your tank.

Hi Thinwater and thanks for the post. API = American Petroleum Institute?

Without testing the sludge I pulled out of my tanks there's of course no way to tell what produced it. I read an article that claimed modern diesel fuels IE produced since oil prices climbed contain a lot of fractions from production process that were not present back in the good old days. Over time these fractions will break down into asphaltenes without any water or bacterial activity and produce nasty deposits, more or less like highway asphalt in your tank. I forget if these were low pH and contributors to metal corrosion but certainly part of the overall problem with fuel and tanks.

In my case I'm pretty sure that stray current was not an issue since the tanks are isolated from other metals and the corrosion was localized in the bottom with the deposits. Salt in the fuel, probably not a factor either so based on the type of corrosion and location it was definitely caused by something acidic. Source seems like it could have been from a number of causes but cure will be to keep the water and sludge from collecting in the future.

The PO regularly used biocides of some variety and seemed to think that was all he needed to do so never drained the crap from the bottom of the tanks. Obviously his plan was not adequate.

Originally Posted by thinwater

While a biocide can kill bacteria, additives like StarTron and Stabil will inhibit corrosion from a wide range of attacks. These 2 have been independently tested to be particularly effective on aluminum. Several other additives can actually make aluminum corrosion worse.

Polishing can help, but it will not remove dissolved water and the acid and salt in that dissolved water. I have done corrosion testing studies where the fuel was saturated with water containing corrosive elements and then polished. The polishing made less difference than you would think.

Will definitely be using something that will inhibit corrosion but your comments about polishing are interesting. Are you referring specifically to diesel or other fuels? I wouldn't think that the amount of water that would dissolve in diesel would be enough to be a serious factor in corrosion. The fact that the inside of my tanks showed no visible corrosion except in the sump where the water and sludge collected leads me to think that, at least in my data sample of one, I should focus on the stuff that separates and not worry too much about what's dissolved in the fuel.

Originally Posted by thinwater

(While Star Tron's claims about helping prevent phase separation in e-10 where not supported by testing, it is NOT smoke an mirrors. It is moderately effective as a biocide and quite effective as a corrosion inhibitor. And I don't have a dog in the fight.)

I think phase separation is an issue for gasoline and ethanol as the ethanol is hygroscopic and absorb enough moisture to drop out of solution but have not heard of it as a problem for diesel commercially. Of course for boaters with water in their tanks there is a definite separation. I think I still have a couple of jars of what I drained from my tanks to look at.

[skipmac]

Originally Posted by Pelagic

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

In general, most acids react with most metals and produce a metallic salt. In the case of a fuel tank an acid will react with the metal atoms from the inner walls of the tank to form the salt and leave pitting or holes where the metal used to be.

Steel or iron is very susceptible to most common acids so it just as critical to inspect a steel tank as Al.

[skipmac]

Originally Posted by s/v Jedi

And that is exactly what the enzyme based treatments prevent

Yep. Which will be part of my multi pronged attack to prevent recurrence of the problem.

[Pelagic]

Quite an education….

Definitely will open and inspect and take pictures. Just hope my Corten steel is more resilient to fuel enhanced corrosion

[thinwater]

API=American Petroleum Institute. A part of my work is internal and external inspection of the 375 oil tanks my company owns under standard API 653.

Stray current corrosion is almost always limited to the bottom and very low side wall areas, since that is where the electrolyte is. It looks like what you illustrated. Most resently I found some like that on a tank where some junky had stolen the grounding rods for the copper scrap (never mine they are only copper plated).

Has the diesel formulation change made a difference. Opinions vary but my feeling is yes. Some even blame it on ethanol cross contamination.http://www.clean-diesel.org/pdf/ULSD...mCorrosion.pdf
I'm not sure they've made the case, but this may be a part of it.

I have found salt (NaCl) in most boat diesel bottom water samples. It is in fact very likely. Salt is a very good electrolyte and encourages exactly that sort of attack. I would not rule it out without proof. The trouble may be that may boats have filler leaks or mist getting in the tank vent, and some fuel docks flood and don't clean their tanks so well. Any salt will naturally concentrate in the bottom over the years. Less than a pinch is very dangerous in this sort of environment. Even though 23,000ppm is common in the ocean and aluminum can manage that well, sealed systems are very different. Just 100 ppm has been implicated in automotive radiator failures, for example (ASTM limit is 25 ppm).No-Rosion Products Technical Questions and Answers

Yes, that corrosion pattern is useually found at either the water/oil interface or the water/sludge interface.

Acid is a matter of testing for pH.

Polishing does help, but the 0.05% remaining in solution can come out in cool weather and do some damage. But primarily what I meant was that unless it can remove all of the sludge layer and all of the free water on the bottom, how clean it makes the free fuel is not pivotal.

And yes, condensation in diesel is possible:
http://sail-delmarva.blogspot.com/20...densation.html
Not as dramatic as E-10, but for some reason the separated water is far more corrosive. The other difference is this:
* When e-10 separates the engine dies and the owner is forced to replace the fuel.
* When they replace the fuel the new dry fuel will absorb the last bits of water, drying the system completely. This does not happen in diesel systems; the last bit stays in the tank bottom for years.

-----

Just sayin', don't rule things out unless there is analytical proof. There are several things that can cause this. It can also be more than one of these; most often it is.

We've gotten away from aluminum tanks and we epoxy line the bottom 6 feet of our steel tanks now, as a matter of policy.

[daddle]

Standard operating procedure here, before a major sail, is to remove the inspection cover and suction out the very bottom of the tank. There is about 2 liters of fuel below the dip tube so running the tank to "dry" is useless. I have yet to see solid water, but the fuel at the bottom is sometimes a bit cloudy and off-color. Suck it out and send it ashore.

Never have used Biocide. Seems unnecessary as I use the fuel in a few months time ... small tank.

[shorebird]

Removing water from a tank is problematic, since the dip tube will normally terminate well off the bottom, and few tanks have the sump/drain, and fuel tanks with any openings other than at the top are becoming obsolete. Preventing water from entry is essential, and I recently replaced my deck-fill cap, for that reason. (75$ grrr) Checking for water requires dipping the tank with water finding paste, and removing water is a matter of putting a tube right to the bottom of the tank, at its lowest point if possible, and using either a hand or electric pump to pump out the water until it becomes clear fuel. Some of the pumps used to pump motor oil through the dipstick tube could be suitable, and use a semi-rigid pickup tube that will work in more confined spaces. Bevel the end of the pickup tube so it can touch bottom and still pickup liquid. You won't get all the water, but if you keep most standing water from accumulating, the rest will usually be suspended into the fuel from agitation, and removed at the filtration system.