Aluminum bilge diesel tank

[Mercator30]

How concerned should I be?
Have a 60 gallon Aluminum diesel tank under the floorboards. There was standing freshwater "brackish" that was in the bilge when I bought the boat that rose approx 1/2 way up the tank sidewalls. I pulled the water out (this part of the bilge would not normally have water in it) and inspected the exposed rear side of the tank, no corrosion. The top of the tank has no corrosion, I can see the rear pad, looks like hard rubber that sits under the tank.
The boat although a 78 was not put into the water until 2005. It was under a tarp on the hard, and dry prior to that. It did have diesel in the tank.
Although the diesel has been recently polished I plan to pull all the fuel and burn it in my old Mercedes, then clean the tank the best I can on the inside.

My concern is could the tank of corroded on the bottom out of sight? I know that moist areas can corrode, like where a aluminum tank sits on a water absorbing mount. But what if the tank is submerged in water for some time? Is this different than being "wet" from a below tank soaked mount?

The tank is very heavy plate, with the floor board pulled up I can stand on it without any deflection anywhere on the large top surface. This may be a positive.

Is my risk high enough to warrant propping the tank up, pulling mount out and with a bore inspection camera, inspect the underside? Would I be able to see pinholes?

No leakage at this time.

[SV DestinyAscen]

Any signs of pitting on the tank? If it's freshwater and exposure was long duration, and the wrong type of alloy, pitting should be visible if corrosion occurred.

[Mercator30]

No corrosion visible at all on the exposed parts of the tank. 90% of the top of the tank is exposed and one side is exposed that was 1/2 way submerged.

I know that the original builder had the tanks custom made but cant find in his files what aluminum was used. The aluminum is thick.


Sounds like I would see corrosion on the tank side if it had occurred. That confirms what I was hoping to hear.

[SV DestinyAscen]

Do you know the source of the fresh water? Condensation?

[Mercator30]

A large engine hatch cover in the floor of the cockpit that dripped (I have added a internal gutter system to catch any errant future drips). The drips added up over time to a bilge water level that went over the top of a bilge (bilge pump not working) "bulkhead" and partially filled the center area of the boat right above the ballast partially submerging the tank. I'm not sure though what made the water so foul, all that is in that area is fiberglass and the tank but the water was foul, smelled as I hand pumped it out, about 4, 1 gallon buckets.

It wasn't sea water, no sea water has entered the boat. And I think this happened to the prior owner about a year ago.

[jpstevens]

I have been told that , in the days when there was more sulfur in diesel, the combination with water produced acid that damaged AL tanks. Can you install an inspection hatch and see if you have interior corrosion? That also facilitates occasional cleaning.

[out42]

I had a problem in a 1979 Nicholson 345 which had a built-in aluminium diesel tank. In 1996 it let go and started to leak diesel into the bilge. It was not usually in contact with water as it was under one of the side-berths.
On removing it (what a job!) and cutting it open, it was clear that the corrosion was from the inside out and mostly on the seams. The actual walls were OK. Something to do with the welding perhaps? I replaced it with a flexible plastic/rubber tank.

I am pretty sure it was sulphur in the diesel as water didn't seem to be involved. Whether low-sulphur diesel nowadays makes that a non-issue, I don't know. I doubt I would try and repair an aluminium tank for diesel given that you can't be sure about marine diesel supplies.

[MCOS]

Suggestion,

Cut a suitable inspection hatch/opening in the top of the tank. Take the cut out piece to a metal fabrication shop and have a cover plate fabricated of similar aluminum or stainless steel so that it overlaps the cut out hole with significant surface for a secure seal against the existing tank. If the tanks is not of sufficient thickness to afford a rigid surface after cutting the inspection hole you will have to have the fabricator make a ring of stainless steel to fit inside of the tank. This might be the easier way to go because you could also have the inner ring fitted with a number of studs or bolts, drill corresponding holes in the tank top, add at least two holes drilled and tapped to allow securing the inner ring to the inside of the tank to hold it in place when the inspection hatch is not bolted on.The two fasteners for these should be with chamfered machine screws with a chamfer cut in the existing tank top to allow them to remain flush or some other means of holding the inner ring in place when the hatch isn't in place.

This will allow access to clean and inspect the tank. Should you determine that the bottom, or any other portion of the tank leaking or appers that it may be, there is a product sold by Casswell Plating, Lyons, New York that can be used to seal the tank. This product is designed to seal leaking gas tanks. I have used it several times to salvage tanks that are difficult to replace, mainly on vintage motorcycles but also have used on a boat tank. The instruction for the use of the product direct a through cleaning, flushing, rinsing with solvent and then sealing all openings and pouring the mixed two part epoxy like material into the tank and rotating the tank in all directions to coat all of the interior with the honey consistency goop. You can call Casswell or email them and get their opinion on the use of this material using a paint brush to spread it inside your tank. I am fairly sure this will hold up to diesel fuel and will give you peace of mind that your tank will not leak - ever.

I have also used this stuff off label to coat a leaky oil pan - inside and out. Perfect solution as far as I was concerned.

Casswell plating website https://www.caswellplating.com

Good luck and safe sailing