Recommended Aluminum Alloy --Fuel Tanks

[China Doll]

I am having a set of aluminum diesel fuel tanks built for which I spec'd to the fabricator to be built in Aluminum Alloy 5056-H32, 1/4 thickness. The fabricator is apparently building the tanks out of what's labeled on the sheet aluminum as:

'ALUM LAM H 1100F'

I am not sure if this meets my original specifications as from what I have researched, this is basically a pure aluminum sheet that meets the 'F' specifications for hardness. The material is 6.045 mm thickness which is close to the 1/4 plate that I had asked for in my original specs.

Any ideas on the use of this aluminum alloy for marine diesel fuel tanks --these are fairly flat 50-gallon tanks with some bends and welds sized around 32 x 44 inches, max depth around 14 inches. The corrosion resistance is a main issue as the tanks sit in the bilge of the boat (on some narrow closed cell foam strips to keep them air dry) with some exposure to salt air, inside (fuel moisture contamination) and out.

I noted elsewhere that 1100F alloy is used in the manufacturing of tank cars (railroad?) --but I am not sure of this usage and the corrosion resistance and 'softness' of this mostly pure alloy. I had planned on painting the outside of the tanks with a vinyl wash etch (Interlux) followed by a two part epoxy harded paint (Interlux Bottom/Bilge-Kote).

Other notes on the sheet aluminum are as follows:

Casa Sommer SA
#ECO: 7337.0
#O.C.: 217714.0
#Pda: 1.000

6.045 1219x3048 [these are the product sheet size in mm as ~1/4 inch 4 feet x 10 feet sheet]

61.80 kg[?] / peso [feet?]

[noelex 77]

There are big differences in the corrosion resistance between the manine grade aluminiums (mostly 5 series) and others.
I would insist on 5052 (or 5083 / 5086 which are beter, but more expensive and overkill for a tank)
Consider carfully how the tank is going to be supported its difficult to avoid crevice corrosion in this area. With an all aluminium boat its not an issue, but if its sitting on wood or fiberglass it can be a problem.
Thick welded aluminium feet or bearers are probably the best solution.
A drain down the bottom is good on any fuel tank and consider inlets and outlets for fuel polishing which you may want to add later.

[China Doll]

I went back to the 'Taller' (fabricator) for these tanks and he stated he'd had indeed ordered 5063-H116 (which I did see in his shop aluminum being used to make a tank for a 'local' captain on a 80-foot mega yacht --a single ~200 gallon tank-- which was laser inscribed with 5063-H116).

For my tanks (I had asked for 5052-H32), apparently the supplier sent him unmarked aluminum sheets. When this was questioned, the supplier insisted the sheets were the correct grade. However, the SKU label sticker placed on the sheets by the supplier absolutely showed the sheets to be a rolled aluminum laminate of alloy 1100F (which is really just a low impurity aluminum and not an 'alloy').

As what I have learned off the net, 1100F can only be 'rolled' (mechanically) hardened (this is done by the press during the lamination process --the amount of pressure is the 'F' hardness [? I believe]). That the welds are harder than the 1100F tank itself, and on larger tanks, cracks can form between the harden (O-hardness depending on the weld rod alloy) weld and the soft malleable pure aluminum (1100).

Corrosion, however, is the main issue --as you've mentioned-- since the metal is surfaced pressed hardened, once a pit starts (like a tooth cavity in one's teeth), the corrosion goes fairly fast even through 6.045 (~1/4 inch) thick plate (not because of the hardness but because it lacks the 'alloys'); that the pitting will more than likely happen inside the bottom of the tank even if the sump is drained often. Surface painting on the outside to control the corrosion helps, but anyplace the paint system may fail, like near a stringer due to mechanical reasons (abrasion caused by rubbing from even slight movements), the tank can quickly start to corrode, even if dry, just from being in a 'salt-air' (marine) environment.

Since I have gone back to the 'Taller' --today-- and have confirmed with him the tanks are not being made from the 'requested' and 'correct' alloy of aluminum, he has stated he will reorder new aluminum sheets from the supplier and he will ensure the correct aluminum sheets will be used. At least this was caught early-on as only the first of four tanks --labor-wise-- was almost completed.

As far as using the 1100F tanks, I have, with no shadow of a doubt, no desire to accept anything less than the 5052-H32 alloy. Especially since getting these tanks in and out of the boat was no small feat. So hopefully I have an honest tank fabricator who will do right by me and get the correct aluminum 'alloy' tanks made.

I will try to post any follow-ups --if anyone is interested-- as things progress (and not digress!). But as far as the talent welding the tanks, both mine with the incorrect materials and the 'local' guy's ~200 gal tank with 5063-H116, the work the shop has done is excellent, and the shop does have a way to properly pressure test the tanks.

So I am hopeful this issue will get resolved correctly.

-Rob.

[DeepFrz]

As 1100 has excellent resistance to corrosion it is widely used in the chemical and food processing industries also for giftware and applications where eye appeal is important.

Aluminum 1100

[China Doll]

Thanks for the reply, DeepFrz...

However, on that same website, under 'alloy' 5052:

Aluminum 5052 is one of the higher strength non-heat treatable alloys (Annealed it is stronger than 1100 and 3003). Alloy 5052 has excellent characteristics with a high fatigue strength it is used for structures which are subject to excessive vibrations. As 5052 also has excellent corrosion resistance, especially in marine atmospheres [my emphasis] and is therefore commonly used in boats, marine components, fuel and oil tubing.

This was not stated for 'alloy' 1100 --"marine environment". So I am going to have to stick with my guns on insisting on what I had originally spec'd which is 5052 alloy. My original tanks were 1/4 inch thick mild-steel encased in fiberglass and those tanks lasted nearly 20 years before any issues.



(And, actually, only two of the four had a failure, and only one of those two proved to be unfix able by method of epoxy repair, as the surface area literally turned into a 40 micron mesh --rusted-- and would 'lift' the epoxy right off the metal, as psi goes up exponentially by square inch, there was too many inches to reseal. The rust started near an inspection and was not caught in time as it form under the fiberglass encasing (hidden) due to small 'leaks' caused by the bend in the glass forming the inspection plate housing.)


But, since pulling the four tanks out was no walk in the park, I need to make sure anything that goes back in the boat will get at least 20 years of service life without any issues. From my due diligence, putting 1100F 'alloy' aluminum tanks into the bilge of a very actively used fiberglass 50-foot sailboat (read: flexed), I will be lucky to not have to pull those tanks back out in 5 years' time, due to corrosion problems and leaks. And I will be extremely lucky if the tanks didn't get a seam crack in one to two years of boat use.


While the 1100F tank looks gall offal pretty; looks like it's tough enough to do the job; I just don't think it the right application, even if it's used to make railroad (non-marine) tank cars!


-Thanks!