Aluminum Fuel Tanks

[j-hood]

I've read many of the existing threads on fuel tanks, but they all seem to be geared toward specific issues that are not mine. I am in the process of trying to purchase a 1985 Southern Cross 35 which has (per a previous survey) aluminum fuel tankage of 35 gallons. I have read about weld-seam issues with these, and I am wondering if there is a consensus (ha!) on the viability of aluminum tanks? Responses are appreciated. I am not losing sleep over this, but I am wondering how prepared I should be for the need of replacement or repair. The previous survey is from January of 2011 and lists the tank as "appears recently installed." Thanks in advance.

[delmarrey]

Professionally built tanks should have been pressure tested. Look on the tank to see if there is a builders tag. If no tag then question the quality.

Being a Machinist I find nothing wrong with alum fuel tanks as long as they're built to specs.

When installed they should have a bit of and air space around them for gradual temperature change so they don't sweat inside.

On powerboats where there maybe more exposure to the elements try to keep the temp outside the tank warmer then the inside with heaters or lamps while in storage. This will keep the sweat factor down.

And then annually drain inspect the tank. Myself in the spring I scrub the diesel into containers then clean the bottom of the tank of any liquid left over. A good tank will either have an inspection port or a large sending unit hole to work thru.

Go here for more info>>> New Boatbuilders Home Page* - 33 CFR Subpart J Fuel Systems

[j-hood]

Good stuff. Thanks. =)

[rivonia]

We have two fuel tanks and they are Aluminium. Both fine and they were installed in 2000.

Ours do have a inspection lids so we could if required screw of the lid to see inside. Never have. We always leave the tanks full.

Peter

[noelex 77]

The problem with aluminium fuel tanks is that its very difficult to avoid crevice corrosion of the exterior skin under the supports that the fuel tank is resting on.

This is the area to look for problems.

If the area under the supports can be kept dry, or the aluminium can get oxygen under the supports the tank will probably last forever.
However this is very difficult to do. Since crevice corrosion can be very rapid it's difficult to predict the life. A range of 4-5 years, to forever, just about covers it. .
Looking at the installation and application will give you the best guide.

Also look for a stencil on the aluminium indicating what type of alumnium was used. Non marine grades are sometimes used, which are bound to fail in a reasonably short time. A proper marine grade would give me far more confidence.

[delmarrey]

Originally Posted by noelex 77

The problem with aluminium fuel tanks is that its very difficult to avoid crevice corrosion of the exterior skin under the supports that the fuel tank is resting on.

This is the area to look for problems.

If the area under the supports can be kept dry, or the aluminium can get oxygen under the supports the tank will probably last forever.
However this is very difficult to do. Since crevice corrosion can be very rapid it's difficult to predict the life. A range of 4-5 years, to forever, just about covers it. .
Looking at the installation and application will give you the best guide.

Also look for a stencil on the aluminium indicating what type of alumnium was used. Non marine grades are sometimes used, which are bound to fail in a reasonably short time. A proper marine grade would give me far more confidence.

You mean 5086 alloy!
Just another reason to have that air space around the tank. But if someone has water around the bottom of their tank then they have other issues. Unless it's a planked wood hull, then it needs to be mounted up off it's base with tabs welded to it's sides. It all comes down to construction.

[Cheechako]

where are the tanks located? If not in the bilge no problem. If in the bilge (under the floorboards eseentially) then you take your chances. SS is no better though. Also, some tanks are thin-ish sheet metal. These are usually standard tank sizes, production made. Custom tanks are often thicker.

[noelex 77]

Originally Posted by delmarrey

You mean 5086 alloy!
Just another reason to have that air space around the tank. But if someone has water around the bottom of their tank then they have other issues. Unless it's a planked wood hull, then it needs to be mounted up off it's base with tabs welded to it's sides. It all comes down to construction.

There are a number of marine aluminium alloys
5083
5086
5251
5254
5383
5454

These all have different strengths and properties, but for internal fuel tanks corrosion resistance is the most important factor.

A lot of the 6 series aluminium alloys are also suitable and much more suitable to extrusions like

6061

In addition there are some alloys that are given trade names like
Sealium

There are a lot more, but also many that are entirely unsuitable. The unsuitable alloys are generally cheaper and more readily available so there is temptation to use them.

Crevice corrosion generally occurs where the tank rests on its supports. A fuel tank will be heavy and must sit on some structures to support the weight (unless its an integral tank in an aluminium boat). If the aluminium under the support is deprived of oxygen (common) and moist ( try to avoid this combination) this is the environment where crevice corrosion occurs.

[delmarrey]

I just happened across this web site, which talks about the proper install of aluminum fuel tanks, so thought I'd share. >>> Boats, Yacht: How to Install an Aluminum Fuel Tank <<<

[Stumble]

I am still convinced that when you compare the cost of installation, including ripping out the sole and internal cabinetry to get to tanks, the best way to go is titanium tanks. But then I sell them so take it with a grain of salt.


As mentioned the biggest failure with aluminium tanks is thru poltice corrosion, where water is held against the tank anywhere. It could be submerged, but it can also be at the supports, and even the internal sump if it has any water can be damaging.

[Vasco]

Well here's my experience. You decide. I have a 1988 CS36M with an aluminum tank. I have taken the tank out once (very easily done) to be cleaned. Got some bad fuel and took it to a rad shop. I've had the boat since new and she has done 8 trips from Toronto south so she's been in salt water. Absolutely no problems with the tank.

My buddy has the same boat, actually 2 years newer. Boat has been in the Great Lakes all the time, only fresh water. Last summer his tank developed a massive leak. The whole bottom was rotten. He took it out and had the bottom four inches replaced. I think they're about 36 gallon tanks.

The tank sits on top of a shelf in the cockpit locker, not down in the bilge at all. It does not sit in water at all. Go figure. I suspect his tank had water in it.

[Andrew Troup]

All metal tanks, to provide long trouble free service, have to be made by someone very well acquainted with the unique challenges of the application: ie sailboat fuel tanks.

This is particularly true for aluminium tanks.

It's really quite a challenge to come up with a truly durable seam and stiffener design for a pressure vessel subject to slamming and the resulting internal hydraulic shock, let alone weld it without ANY secret crevices which might later become leakage paths, under the influence of flex induced minor cracking, or corrosion.

Most of the best designs for aluminium result in cylindrical tanks with domed ends because corners are problematic in several ways.

However these do not make efficient use of the space in a small sailing vessel, which is always at a premium.

[konakoma]

Andrew's point about quality of construction is on point. Our Morgan 44 has 2 x 50 gal aluminum tanks, which are drained and pressure tested every 2 years. The welds are the problem, and not just the corners.

We used to run floatplane aircraft with aluminum fuel tanks (Beavers) and pressure testing was done yearly. We had them removed many times for repair of welds, but sometimes hairline cracks too. In a plane, like a yacht, the constant pounding they take wear on the welds and material, and ongoing monitoring/maintenance is mandatory.

Also, the cap fittings on aluminum tanks made from brass or other metals create non similar metal corrosion, which many users found unmoveable after a time.

If we had to replace the alum tanks, would go with weld free hard poly tanks, built to size.

[delmarrey]

The nice thing about aluminum tanks over plastic is they can be made into some pretty odd shapes at a reasonable expense. >>> Custom marine fuel tanks for recreational, commercial crafts, U.S. Coast Guard, aluminum, steel, stainless steel, Jersey Shore, NJ, New Jersey It's really hard to find anyone willing to do that with plastic. And plastic or SS can leak just as much as aluminum if not welded properly.

BTW- 46 CFR section 58.50 +

58.50-10
Diesel fuel tanks.
(a) Construction. (1) Tanks may be of either cylindrical or rectangular form.
(2) The materials used and the minimum thickness allowed in the construction of independent fuel tanks shall be as indicated in Table 58.50-10(a), except that consideration will be given to other materials which provide equivalent safety as indicated in § 58.50-15.
(3) Tanks with flanged-up top edges, that may trap and hold moisture, shall not be used.
Table 58.50-10(a)
Material ASTM specification (all incorporated by reference; see 46 CFR 58.03-1) Thickness in inches and gage numbers 1 vs. tank capacities for—
1- through 80-gallon tanks More than 80- and not more than 150-gallon tanks Over 150-gallon tanks 2
Aluminum 5 B 209, Alloy 5086 6 0.250 (USSG 3) 0.250 (USSG 3) 0.250 (USSG 3).
Nickel-copper B 127, Hot rolled sheet or plate 0.037 (USSG 20).3 0.050 (USSG 18) 0.107 (USSG 12).
Steel or iron 4 0.0747 (MfgStd 14) 0.1046 (MfgStd 12) 0.179 (MfgStd 7).
1 Gauges used are U.S. standard “USSG” for aluminum and nickel-copper and “MfgStd” for steel or iron.
2 Tanks over 400 gallons shall be designed with a factor of safety of four on the ultimate strength of the material used with design head of not less than 4 feet of liquid above the top of the tank.
3 Nickel-copper not less than 0.031 inch (USSG 22) may be used for tanks up to 30-gallon capacity.
4 For diesel tanks the steel or iron shall not be galvanized on the interior.
5 Anodic to most common metals. Avoid dissimilar metal contact with tank body.
6 And other alloys acceptable to the Commandant.
(4) Openings for fill and vent pipes must be on the topmost surface of a tank. There must be no openings in the bottom, sides, or ends of a tank except as follows:
(i) The opening for the fuel supply piping is not restricted to the top of the tank.
(ii) An opening fitted with threaded plug or cap may be used on the bottom of the tank for tank cleaning purposes.
(iii) Liquid level gages must penetrate at a point that is more than 2 inches from the bottom of the tank.
(5) All tank joints shall be welded.
(6) Nozzles, flanges, or other fittings for pipe connections shall be welded or brazed to the tank. The tank opening in way of pipe connections shall be properly reinforced where necessary. Where liquid level indicating devices are attached to the tank, they shall be of heat resistant materials adequately protected from mechanical damage and provided at the tank connections with devices which will automatically close in the event of rupture of the gage or gage lines.
(7) All tanks exceeding 30 inches in any horizontal dimension shall be fitted with vertical baffle plates where necessary for strength or for control of excessive surge. In general, baffle plates installed at intervals not exceeding 30 inches will be considered as meeting this requirement.
(8) Baffle plates, where required, shall be of the same material and not less than the minimum thickness required in the tank walls and shall be connected to the tank walls by welding or brazing. Limber holes at the bottom and air holes at the top of all baffle plates shall be provided.
(9) Iron or steel tanks shall not be galvanized on the interior. Galvanizing paint or other suitable coating shall be used to protect the outside of iron and steel tanks.
(b) Installation. (1) Tanks containing fuel for emergency lighting units shall be located on an open deck or in an adequately ventilated metal compartment. No tank shall be located in a compartment where the temperature may exceed 150 °F.
(2) When cylindrical tanks are installed, longitudinal seams shall be located as near the top of the tank as possible. Fuel tanks shall be located in, or as close as practicable, to the machinery space which is served.
(3) Fuel tanks shall be so installed as to permit examination, testing, or removal for cleaning.
(4) Fuel tanks shall be adequately supported and braced to prevent movement. Portable tanks are not permitted.
(5) All fuel tanks shall be electrically bonded to the common ground.
(c) Tests. (1) Prior to installation, tanks vented to the atmosphere shall be tested to and must withstand a pressure of 5 pounds per square inch or 1 1/2 times the maximum head to which they may be subjected in service, whichever is greater. A standpipe of 11 1/2 feet in height attached to the tank may be filled with water to accomplish the 5 pounds per square inch test. Permanent deformation of the tank will not be cause for rejection unless accompanied by leakage.
(2) After installation of the fuel tank on a vessel the complete installation shall be tested in the presence of a marine inspector to a head not less than that to which the tank may be subjected in service. Fuel may be used as a testing medium.
(3) All tanks not vented to atmosphere shall be constructed and tested in accordance with part 54 of this subchapter.
[CGFR 68-82, 33 FR 18878, Dec. 18, 1968, as amended by CGFR 69-127, 35 FR 9980, June 17, 1970; CGFR 72-59R, 37 FR 6190, Mar. 25, 1972; USCG-1999-5151, 64 FR 67180, Dec. 1, 1999; USCG-2003-16630, 73 FR 65188, Oct. 31, 2008]

58.50-15 Alternate material for construction of independent fuel tanks.
Effective: Tuesday, November 01, 2011
(a) Materials other than those specifically listed in 46 CFR 58.50–5, Table 58.50–5(a) and in 46 CFR 58.50–10, Table 58.50–10(a) may be used for fuel tank construction only if the tank as constructed meets material and testing requirements approved by the Commandant (CG–521). Approved testing may be accomplished by any acceptable laboratory, such as the Marine Department, Underwriters' Laboratories, Inc., or may be done by the fabricator if witnessed by a marine inspector.
(b) [Reserved]
[USCG–2003–16630, 73 FR 65188, Oct. 31, 2008]

[Andrew Troup]

I don't think it's a big surprise that all materials can leak after welding.

What's different about aluminium from most other metals is that it has almost the worst possible combination of factors to produce complete fusion to the root of the weld at every section along a seam with no porosity, inclusions or other defects which may result in leakage (not necessarily evident until it's been in service for some time.)

These difficulties are compounded wherever several welds meet at a corner.

The characteristics which make it difficult include:

an oxide layer whose melting point is so much higher than the parent metal that you'd just about have to vaporise the metal to melt the oxide

a crucial inability of the parent metal to keep a compact weld pool, because the rate of heat conduction is so high, especially in comparison with stainless. The devilish dilemma arises because of a deficiency in 'hot strength': a pool which is even a smidgen too big cannot hold itself in place against the pull of gravity - and melting happens with no colour change, so it's a proper bugger to predict.

One characteristic pushes you in the direction of lots of heat (which would in any case cause unacceptable distortion) and the other pushes you towards flirting with insufficient heat for full fusion to the root.

It's also "hot short", ie very weak during the phase when it's hot but not molten. This makes it extremely prone to cracking as it cools.

It's also a metaphorical 'magnet', in the molten condition, for gases, to the extent that even an inert gas shield is no guarantee against bubbles of porosity in the weld.

These difficulties, while very serious, can be overcome, but virtually none of them apply in the case of stainless.

It has its own difficulties, but these don't make it difficult to weld, provided you understand what parent metals to use, what electrode and filler rod to select, and have sufficient flow of suitable shielding gas.

The actual welding is a delight; the metal flows like water where it's molten, levelling almost like paint, but holds together really solidly in the immediate vicinity.

A good welder with good eyesight and fine motor control can be taught to TIG weld stainless to a high standard in a few hours.

The same person would need to practice for months to get results half as reliable in aluminium TIG welding, for leak-free seams.

Modern synergic pulse MIG can be used in thin material, but it's not a rescue package for a welder who doesn't already have lots of chops.


I'm not saying it can't be done, but you DO have to select a fabricator carefully.

And it's wise to specify a thickness which is on the generous side. By the time you get up to the sorts of thicknesses that hulls are plated from, the difficulties are considerably reduced ... which is just as well, otherwise aluminium boats would not be viable.

And to be extremely vigilant for the possibility of poultice corrosion where it's mounted, as others have pointed out.