Fuel Tank Electrolysis

I have a Motor Whale Boat (an old fiberglass hull Navy Lifeboat) that I'm replacing the aluminum fuel tanks (diesel). Both tanks are 30+ years old and had failed due to corrosion. One tank has a corrosion hole in the bottom the size of a marble (not near any fittings and not touching the fiberglass hull). The other tank corroded at a brass drain fitting. I would love to replace them with poly tanks but they are custom shaped and not an option for me. So I'm looking in getting aluminum tanks made. I intend to use rubber fuel lines but will need fittings to connect them to the tank. What would be the best choice of material for those fittings (plastic, brass, copper, other)? Would it be worth having an aluminum stud welded to the tank to attach a sacrificial anode?

 

Welcome to the forum HWP.

If you are going to use ally tanks again, and have some reservations, why not select a more appropriate grade of ally?
A low magnesium alloy such as 5454 or 5754 are less sensitive to intergranular corrosion. But not, these are lower strength alloys, thus you need to make allowances for that, compared to typical 5083 alloy.

 

Re how you would love to replace them with poly tanks - these folk build custom made 'plastic' tanks - however they are in England.
Custom Water, Waste and Fuel Tanks | Tek-Tanks https://www.tek-tanks.com/custom-tanks
If there is no similar type of Builder in the USA (surely there must be?) it might be feasible to import a couple of tanks?
I have a feeling though that they might be a fair bit more expensive than aluminium tanks - however it wouldn't hurt to ask them for a quotation.
They come well recommended on the YBW Forum in Britain - www.ybw.com/forums

 

You could use the original tanks as mold/liner. Have the rotted out fitting moved and welded into a healthier point and then just glass them over (and over) leaving nothing but the bungs exposed. Probably the cheapest, quickest solution.

 

This company makes custom plastic fuel tanks.
http://moellermarine.com/custom-con.../custom-fuel-tanks/custom-plastic-fuel-tanks/

 

Never brass fittings due to galvanic problems between the dissimilar metals in a water/salt environment
ABYC allows for stainless fittings 316 and you can also use aluminum fittings as well
The problem with aluminum fittings is that aluminum fittings when screwed into another aluminum
fitting can gall very easy meaning that often you cannot remove them or change the direction of the orientation easily of a fitting

 

Search for "HDPE tanks custom".

 

As Barry said, use 316 stainless fittings. Brass forms a galvanic couple and corrodes the aluminum. You can get custom made poly tanks, but they can be a bit expensive. However poly tanks come in many shapes and sizes to fit all kinds of odd compartments. Have you thoroughly research what is available?

 

What makes you think the tanks failed because of electrolysis, only ? The hole in the bottom may be quite unrelated, caused by contaminants. Depending on how expensive replacements would be, there are ways to repair old tanks, using various proprietary products, or even encapsulating them in glass and a resin that can stand up to fuel.

 

You don't happen to know if the boat was used for backcountry tours in Flamingo, Everglades NP, do you. There aren't a lot of those things left anymore.

 

The tanks should be 5000 series aluminum so bottom to side edges can be bent instead of welding every corner. Stainless only against aluminum. If you can't find a fitting look at McMaster, or have one machined. Brass/copper/bronze will eat up aluminum, and plastic fittings are not good in fuel systems. Also be cautious of chromed brass hardware around the tank, looks like aluminum but isn't.

Also make sure to use rubber to isolate the tank from any support it sits on. 5200 or similar the rubber TO THE TANK so moisture can't sit between the rubber and tank, instead of gluing the rubber to the fiberglass shelf and then setting the tank on the rubber.

ABYC has a good section on tanks, and there are some USCG rules out there also on baffles and such.

A hole in the bottom may have been a bronze nail or penny dropped in the fuel fill. A penny is the fastest way to sink an aluminum boat. Or it may have been an install issue which held water against the bottom of the tank in that spot (like a piece of rag or something holding water there).