Aluminum fuel tank information

I found this information on aluminum fuel tanks, and I thought it would be of interest here, if some of us were contemplating new tanks. The article deals with aluminum and stainless steel tanks. I found it very informative and hope others do too.

Ernie





According to the responses from boat owners who participated in the UL survey, a great amount of confusion exists in the boating world as to what constitutes an acceptable and reliable fix for a leaking aluminum fuel tank.
Some owners settled for temporary repairs, while some owners and manufacturers replaced failed tanks with OEM specification tanks using original installation methods. Some boat owners also went to great expense to try to modify the original installation, although the effectiveness of the fixes was also questionable.

According to the UL report, one of the most convenient courses of action for both new vessel construction and for repair/replacement of damaged aluminum fuel tanks is to use thicker aluminum sheets for tank construction. The report states that pitting for aluminum has been shown to be proportional to the cube root of time. Therefore, while it can be shown that by merely doubling the thickness of the material, the time required for perforation due to pitting is theoretically increased by a factor of eight.

However, in the absence of more practical experience with tanks constructed of thicker aluminum sheeting subjected to the conditions in the bilge of a boat, the UL report notes that there is no way to determine a suitable thickness, which would guarantee an acceptable service life in relation to the service life of the boat itself.

Some of the people who were interviewed during the course of the UL study stated that the "industry fix" for corrosion problems with 0.090" wall thickness aluminum fuel tanks was to switch to a thicker 0.125" sheet aluminum for any replacement products. The 0.125" sheet aluminum not only reduced the susceptibility of the tanks to corrosion, but also reduced the number of fuel tank failures due to fatigue at the baffle welds. However, some of the failed tanks which were examined were constructed of 0.125" thick aluminum, meaning they had service lives which were similar to the thinner 0.090" material. Other considerations for assessing the limitations of a thicker aluminum sheeting are the extra cost, the weight penalty, and the increased difficulty in manufacturing the tank.

The UL report indicates that some boat owners chose stainless steel as the material for replacement fuel tanks, since this material is obviously "stronger" than aluminum; however, it is also susceptible to pitting and crevice corrosion in the marine environment, although at a different rate than aluminum. Stainless steel is also susceptible to stress-corrosion cracking and is even more prone to that type of failure at weld areas.

Only the 316L stainless steel alloy with a specified minimum wall thickness of 0.031 inches is considered suitable for use in the construction of marine fuel tanks. American Boat and Yacht Council standards ANSI/ABYC H-24, ABYC H-33, and ANSI/NFPA 302 all require stainless steel fuel tanks to be less than 20 gallons in capacity and cylindrical with domed heads to limit the wall stresses experienced in service.

While the uninformed boat owner who happens to construct a small capacity rectangular tank may be lucky enough to avoid a failure due to stress corrosion cracking; some boat owners who participated in the UL study spent considerable amounts of money to have replacement tanks constructed to capacities as high as 150 gallons. Many other boat owners who were surveyed mentioned that, on a cost independent basis, they would have preferred to have used stainless steel for their replacement tanks.