Why don't you ask "someone" the reason for their recommendation?

Metals (and other materials) crack when the load exceeds the strength, or when cumulative fatigue damage exceeds the fatigue tolerance of the design. You need to understand this before you start fabricating things. Especially things filled with inflammable liquids used on public roads.

Many fuel tanks are made of aluminum.

Many fuel tanks are made of stainless steel.

Many fuel tanks are made of steel with various coatings inside and out.

Many fuel tanks are made of aluminized steel (which I guess counts as a "various coating").

Many fuel tanks are made of plastic.

Fuel tanks are cheap and easy to buy in many different shapes. It seems unlikely that you "have to fabricate" one.

There are probably laws and regulations about fuel tanks that you need to comply with.

Please refrain from duplicate posting

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Apologies, I have deleted the duplicate post.

Aluminum is generally superior. Thicker plate can be used which requires fewer bulkheads for reinforcement.

Stainless steel also has issues with road salts and dirt. It requires frequent washing to maintain it's "stainless" properties. High concentrations of salt on the surface can initiate stress corrosion cracking. Aluminum has similar sensitivities to dirt but is not as severely affected by salt.

If this is for a mobile application - and I don't care that the fuel is going to be used for a generator mounted on the trailer as opposed to for propulsion, it can still leak and catch fire and it can still be involved in motor vehicle collisions - your first step is to consult the relevant regulatory bodies in the place where you are going to be using this trailer, and establish what standards apply and what those standards have to say about it.

Buy, if at all possible. Don't make.

If you have to ask the question then you should be outsourcing this to a supplier with experience designing onroad fuel tanks.

And ask TBE what happens when you get salt water inside of tanks.
I would think that you can find a pre-manufactured Al tank that will work fine.
Providing very good support is critical.
The dynamics of liquid sloshing inside of a tank while being moved can impart some very high forces.
I am a SS guy but the risks of external corrosion on SS concern me.
I have used SS tanks in past, and the outsides were always coated with very good products.

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P.E. Metallurgy, consulting work welcomed

The good news is that with the modern ultra-low sulphur diesel fuels the microbial induced corrosion isn't so much of a problem anymore. I wouldn't worry too much about the inside of the tank.

Being mounting under a trailer, there must be some codes somewhere that dictate material and size/location.

Chris, CSWP
SolidWorks
ctophers home

Well it will depend on what you're doing with the sheet?

Are you forming the sheet into a rectangular tank with some sharp edges?

Aluminium is "softer" than Stl steel, I.e. has a lower yield, but doesn't work harden as much which is where you might get "cracking"

The radius of any bent shape will be crucial as well. Make it 10 to 15mm and you shouldn't have an issue with either.

Make it sharp edged or <5mm then you might.

So much depends on details you haven't told us....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Thank you very much for all the replies, much appreciated. The reason for making this post is for me to understand WHY st steel tanks will crack but alum tanks won't. I think there might be a gap in my materials science understanding here.

As requested, I have added a typical cross section of the tank, see link below. Bottom section is bent from 5052-h32 bendable marine sheet. Internal radius is 9 mm. End plates are tig welded from outside and mig welded from inside. Will have baffles every +- 400 mm. Top sheet is only welded form outside.


LittleInch, thank you very much for your feedback. I do have some questions regarding the following you said:
"Aluminium is "softer" than Stl steel, I.e. has a lower yield, but doesn't work harden as much which is where you might get "cracking""

I don't understand how this can be true if the ultimate strain of alum sheet is less than stainless sheet? This was actually the original reason why I made this post, because in my mind it doesn't add up why st steel would crack before alum. I think I might be missing something?
5052-h32 ultimate strain: roughly 10%
5052-0 ultimate strain: roughly 25%
316 st steel ultimate strain: roughly 40%
So doesn't this imply that 316 can be bent (strained) much more than al before cracking.

As TugboatEng and EdStainless have suggested, maybe this relates to corrosion cracking.

I also don't think fatigue could be the culprit? Does anyone have any insights?

I really do appreciate the feedback.

• https://files.engineering.com/getfile.aspx?folder=997eaff4-c6f1-4c62-8681-0

Quote:

The reason for making this post is for me to understand WHY st steel tanks will crack but alum tanks won't.

You are trying to understand why someone told you that. You need to ask them.

Every suitable metal will crack if used improperly. Fatigue cracks are independent of plastic strain capability as they occur in the elastic region.

Yes, who is the “someone”? And why do you believe them? If you do, ask them to explain their reasons in detail.

It is the fabrication manager. Unfortunately, he doesn't know why. Experience and observation has led him to this conclusion.

I believe that he is correct. However, I would like to understand WHY this is true. Because if I understand the underlying scientific principles I can apply it elsewhere.

Same thickness, same weight, same cost, same details, same fuels?

Plenty of companies make stainless steel fuel tanks. If your fabrication manager doesn't have confidence in stainless steel, don't use it.

In a brief search I see the US Coast Guard recommends 316 and care taken in making welds; a number of of comments like monel for boat tanks over stainless and aluminum.

OP asked a very valid question. I don't see the reason to dog pile. Stainless steels are challenging materials to work with that don't succeed in many environments. I don't think a lot of engineers know this and I see over-application of stainless steel everywhere. The issues associated are unusual and not always the result of design or quality control.

I think this is more about welding than forming cracks. Welding SS is not easy and needs careful work, especially on thin sections.

But you really to talk to a metallurgist or welding engineer.

There is also mention of SCC which may be more prevalent in bent sections.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Ask the fab manager where exactly he sees cracking?

In the weld?
At the bent sections?
Somewhere else?

That will give you something to go on at least.

3mm is not very thick so will heat up and cool down rapidly and my be harder to weld without cracking as a thin sheet than maybe pipe or thicker plate

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Quote:

he doesn't know why. Experience and observation has led him to this conclusion

So without understanding the why, he just declares stainless bad - never to be used.

It's commendable that you are trying to understand.

The SS will be stronger and much stiffer than Al.
But do you need those properties in a tank?
SS is easier to weld than Al in my book (I can't weld Al for crap).
It wouldn't surprise me if you couldn't fins SS tanks that were deep drawn and then the two halves welded together.
SS forms easily (think kitchen sinks).

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Talk to these folks:

https://www.tti.com.au/product-category/poly-diese...

Curious why not plastic or coated carbon steel. Less expensive, no corrosion issues.
Note that road tankers are made of carbon steel coated with oil-proof paint. Note that in the past wood was used extensively for this purpose.

I'm assuming this is a custom made tank and hence easier to fabricate from metal instead of a mould.

But I would have thought some standard tank could be made to fit.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

How about something like this:

https://www.ebay.com/itm/394406762244?_trkparms=am...

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:

The secret of life is not finding someone to live with
It's finding someone you can't live without

If you look at plastic make sure that the manufacturer approves of it being hauled full of fluid.
Most plastic tanks are not intended for road travel when full.
Ask friends of yours with RVs.
Plastic tanks require a lot of support, like fully supported.
There are options out there in steel, Al, plastic, and SS.
Google

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I think I'd take a look at the fuel tanks on refrigerated semi-trailers and see if they can meet your needs or at least pick up some design details.

Big fan of fiberglass here. Garolite G-10 shapes with West System epoxy builds nice things. It wouldn't be pretty, do all of the bonding on the outside. That means lots of protrusions.

consider the worst-case environment as well. Most metals become brittle in extreme cold, but aluminum actually strengthens.

https://www.shapesbyhydro.com/en/expert-thoughts/i...

--
Mitch

Remember here that our one time poster is:

A) Based in Australia
B) So interested in the discussion that he or she hasn't bothered to look at any responses since 6th Sept.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Thanks everyone for the feedback. I do appreciate it and have been reading them.

PS my question was only regarding why st/st would crack and alum won't but I appreciate all the extra info regarding tanks, very informative.

You get salt deposits on the out side of the tank.
The salt dries.
You get a hot humid day.
The salt rehydrates and the SS surface temp reaches 140F.
You get pitting, chloride stress corrosion cracking, or both.

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P.E. Metallurgy, consulting work welcomed

Thank you EdStainless. Is this only applicable in countries where NaCl salt is used to melt ice on roads? Or would this happen in dirt roads in Australia with other salts encountered in the soil?

Aluminum also corrodes. See the bellies of aluminum fuselage aircraft. The susceptibility depends on alloy and temper.

There are many types of stainless steel. You really need to look at specific types individually to understand how each responses to environmental factors and manufacturing processes.

Any SS that you commonly find will usually 304 or 316.
A tank in a lean duplex SS (2101) would work great.
Very strong and more corrosion (esp SCC) resistant.

Not just where roads are salted.
Near a coast has chlorides, and many arid climates have lots of Cl (think salt flats).

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P.E. Metallurgy, consulting work welcomed

Ready made marine grade aluminum gasoline tanks

https://www.kencoracing.com.au/index.php/component...

Considering that tank has no brake or roll formed sides, 6061 would have been the preferred material choice. Why is everybody posting links to non-engineered 10 gallon tanks when OP specifically stated they needed a 50 gallon tank?

https://www.americanmobilepower.com/products/reefe...

we made fuel tanks for military vehicles,they were v shaped to conform to the bottom of the vehicle. they were rather large and thus were made from 6mm aluminium plate. why werent they made of stainless steel? maybe cause of weight savings, maybe because they were cheaper. this application was probably diesel though.

for ship use we also manufactured mounting brackets for above deck mounting of cupboards and panels etc.

Also for ship use we built these massive toolboxes with these lids that were bolted down and sealed against water penetration.


while i was with that company we also got truck fuel tanks in sometimes that had sprung a leak and then we did welding repairs on them. that was diesel as well though

the impression that i got was that aluminium is typically seen as a cheaper alternative to stainless steel, but it still gives reasonably good corrosion resistance. the fact that it doesnt spark when struck probably helps too.

Thanks all for the feedback. This has been most helpful.

@TugboatEng,

This is may be off topic but I was under the impression that ULSD (with water present) was actually more conducive to microbial growth than previous diesel with higher sulphur concentrations.

Or perhaps that’s only due to the addition of biofuel?

Link

The article doesn't make a whole lot of sense to me. There is talk about Ethanol causing water absorption but I don't agree with this line:

Quote:

It has been shown that mixing ULSD with small amounts of biofuel, such as ethanol, may accelerate tank corrosion. This is due to the microbes in the diesel fuel digesting trace amounts of ethanol, creating high-enough levels of acetic acid to cause significant corrosion of the surrounding tank.

Who mixes diesel with ethanol? Microbes don't digest ethanol, they produce it. Ethanol in excess of 10% is quite deadly to microbes which limits microbial growth.

Meanwhile, sulfate reducing bacteria are a well known cause of corrosion in tanks.

The article also goes on about lubricity issues. I've never seen any evidence that a reduction in lubricity has ever caused problems. Sandia National Labs analysed some pumps run on low lubricity fuel and none of the pumps failed due to fuel lubricity.

https://apps.dtic.mil/sti/pdfs/ADA234930.pdf