Re-Welding Stainless Tanks?

[Strait Shooter]

Ten year old stainless steel tank.



I replaced them with aluminum. I'll get back to you in ten years and will let you know how they're holding up.

[hellosailor]

a64-
"My aluminum tanks are 27 yrs old,"
Naw. That's why there's pilots, and there's crew chiefs, and you are only allowed to BORROW their aircraft not perform the work on it.
You have aluminum alloy tanks, not aluminum. All the "aluminum" is different alloys and there are stories, perhaps littoral legends, of early aluminum hulls where someone dropped a penny in the bilge and the resulting electrolysis ate through the hull!
Different alloys, different qualities, strength and corrosion resistance among them. I used a small aluminum project box to house some electronics, mainly because I needed a small case and that's what I could get. Two weeks after it was installed on a boat that was kept in brackish water, the entire box was discolored with a white oxide film. Ditto for a custom engine pulley made of a presumably different alloy from a different source.

Salt, moisture, zero maintenance, forgetting to ask Poseidon's blessings...all these things can eat any metal on your boat. Except of course, if you like a really light and fast cruising boat and you've robbed some big banks to pay for the titanium (alloy!) or composite custom fuel and water tanks. And mast.


Rust on stainless welds? Could easily be from the wrong welding stick being used. Welding is easy, which is why oil pipeline companies won't hire anyone with less than a full ten years of prior experience. And even then, they have an incredible amount of rework to do after the welds are inspected.

[Cheechako]

Originally Posted by a64pilot

So,
What corrodes water tanks on a boat? Is it primarily from the outside, or from the inside?
What about fuel tanks?

My aluminum tanks are 27 yrs old, look good from the outside, can't tell about the inside
Will they fail tomorrow? Which one is most likely to go, the water tank?
I've got a 1946 airplane with very thin aluminum fuel tanks, they are in great shape, why should a boat aluminum fuel tank not last as long if it's not corroding from the outside?

From what I've seen tanks usually corrode from the outside. Often in the area where they sit on a supporting structure/wood etc. Dampness or water gets underneath the tank where the support is holding the tank and stays there in a oxygen depleted environment causing corrosion.

Tanks may corrode where the weld is, and SS ones may rust where the weld or heat affected zone is. If I remember right, the short story is this is because when the SS is welded it precipitates out carbides in the grain structure and leaves the iron available to readily rust. This can only be avoided if you solution heat treat after welding... which boat builders are unwilling to spend the money on.

[a64pilot]

I know they are an alloy, since they are welded most probably 6061 and not 2024, I'm also sure they are not Alclad, nor have they been Alodined. I guess I could Alodine them but essentially there is no external corrosion and I believe they are hung, not sitting on anything, but I'll look into that this weekend.

I had planned on replacing them with 316L, maybe not now, I guess Monel is the best?

[model 10]

Originally Posted by Cheechako

From what I've seen tanks usually corrode from the outside. Often in the area where they sit on a supporting structure/wood etc. Dampness or water gets underneath the tank where the support is holding the tank and stays there in a oxygen depleted environment causing corrosion.

Tanks may corrode where the weld is, and SS ones may rust where the weld or heat affected zone is. If I remember right, the short story is this is because when the SS is welded it precipitates out carbides in the grain structure and leaves the iron available to readily rust. This can only be avoided if you solution heat treat after welding... which boat builders are unwilling to spend the money on.

Key word there is "may". If the weld area gets to hot bad things do happen. Properly done the weld area is only a different color but still has the oxide coating. Further passivating the surfaces takes care of the rest.
The skanky water collecting in the sump of an AL fuel tank will cause pitting. So how do you get the water out? Easy in an airplane but not in so easy in a boat.

[CarinaPDX]

If you really want to pit an aluminum fuel tank, add a biocide when you get algae in your fuel. The dead critters form a corrosive layer between the water at the bottom and the fuel above. I have a neat little ring around the bottom of my day tank...

And one of my SS water tanks has been weeping for years - give me plastic!

Greg

[Terra Nova]

Originally Posted by Guy

...how do you get the water out?...

Best fuel tank installations have access to a small drain sump fitting with valve for draining the water and sediment. The tank's bottom is intentionally pitched slightly to this sump, which can be a large diameter threaded nipple to which a bell reducer and shutoff valve are added. Also sight gauges with valves. Also large clean-outs for each baffled section.

For tanks with only top access, it would seem wise to periodically remove water by using a dip tube, pumped through a clean-out or sender opening. As part of any major refit, tanks should be emptied and scrubbed.

[Canibul]

I well realize that the chemistry is completely different, but I recently removed the 30 gallon holding tank from this same boat. I believe that tank was probably made exactly from the same materials and shop that made the water tanks. The welds around the inlets and outlets were letting go on the holding tank. The vent pipe weld at the upper edge of one end plate broke open when I lurched against it with my shoulder. I'll spare you the rest of that sequence for now, but it was ugly. I found out all the welds were very brittle, and pitted. we no longer use a holding tank, so that one is solved. It's still my intention to eventually replace these four stainless water tanks with plastic.

The idea about lining the tank with a plastic sheet would work, except for the inlet at the lower edge of the end plate. No way to use a plastic sheet to seal a tub with a two inch hole in the bottom of it that needs to remain open. This is a low pressure storage issue, with weld corrosion being the main problem I think. I'm hoping the wet cure expoxy stick stuff will hold it together if i glop it on thick enough. I just need six more months.

If it looks feasible once I get the tank out of the boat and can really inspect it, I might have it re-welded as a temporary fix. But I have four of these in total and I don't want to have to keep going through this so total replacement is the real fix.

[Zai]

Originally Posted by Canibul

I well realize that the chemistry is completely different, but I recently removed the 30 gallon holding tank from this same boat. I believe that tank was probably made exactly from the same materials and shop that made the water tanks. The welds around the inlets and outlets were letting go on the holding tank. The vent pipe weld at the upper edge of one end plate broke open when I lurched against it with my shoulder. I'll spare you the rest of that sequence for now, but it was ugly. I found out all the welds were very brittle, and pitted. we no longer use a holding tank, so that one is solved. It's still my intention to eventually replace these four stainless water tanks with plastic.

The idea about lining the tank with a plastic sheet would work, except for the inlet at the lower edge of the end plate. No way to use a plastic sheet to seal a tub with a two inch hole in the bottom of it that needs to remain open. This is a low pressure storage issue, with weld corrosion being the main problem I think. I'm hoping the wet cure expoxy stick stuff will hold it together if i glop it on thick enough. I just need six more months.

If it looks feasible once I get the tank out of the boat and can really inspect it, I might have it re-welded as a temporary fix. But I have four of these in total and I don't want to have to keep going through this so total replacement is the real fix.

There is a liquid rubber compound, which is used to seal aircraft tanks that are riveted. You have to clean and dry the tanks and roll them around to make sure all the surfaces and corners are coated.

[model 10]

Originally Posted by Zai

There is a liquid rubber compound, which is used to seal aircraft tanks that are riveted. You have to clean and dry the tanks and roll them around to make sure all the surfaces and corners are coated.

On another recent post, a ceramic coating from the petro industry is mentioned and it too works well as a tank coating. The sealant that Zai mentions if done correctly with the right materials is almost bullet proof. It sometimes is a multi- step process with several materials and it is pretty expensive by boat standards. It is amazingly smelly, messy, toxic and tough.

[Canibul]

well, if you keep in mind where I live, we can pretty much forget about exotic aircraft quality compounds with elaborate prep, application, and curing requirements......

HOWEVER...

this suggestion made me instantly think of the hard plastic truck bed stuff I bought here in a gallon can. It's a Canadian made black polymer compound that dries quick and is really, really tough. And resists liquids, chemicals, etc as you would expect for a truck bed.

I'm thinking of pulling the leaky tank out and putting a few coats of this stuff on the outside of the tank, which I can get to with sandpaper and acetone easily.

This stuff is really tough. If I can get it to stick to the tank material, it won't split or leak.

[Strait Shooter]

I would personally like to hear of the results of your experimentation, please do keep us posted.

[Canibul]

The boat is still on the hard, I spent most of yesterday getting her ready to relaunch. Should be a couple of days yet, or longer if we decide to have the bottom painted on this haulout. I'll find out tomorrow what the boatyard is going to say about lead time on a paint job. If it's less than two weeks we'll probably go for it.

But in ANY case I can't wait to try this out on the tank. I have some stainless here, the holding tank I recovered from a wrecked catamaran on West Caicos. I am going to go down this morning, rough that up, and put some of this on it. Will be a good test. So I should have some results for you very shortly.

I'll hit it with 60 grit abrasive and a solvent and see how well it bonds. We might be on to something here.

[Canibul]

Okay, I just boogied on down to the garage, took the stainless tank, sanded a patch of it to shiny steel with 100 grit and put various thicknesses of this rubber compound on it. Since it's about 80 degrees and sunny this morning, I think I will have some preliminary results within an hour or so. If this stuff sticks to the roughened stainless plate the way it sticks to other stuff, I will be happy.

If it peels off too easily, I'll try again but prep the steel with a solvent this time. I'll try some acetone and maybe a lacquer thinner, since that has a collection of mean-ass solvents in it.

the Canadian product I have on hand is called Fast Dry, billed as a 'paintable, black rubberized undercoating". http://dominionsureseal.com/index.ph...ts-a-coatings-

It's their "BUF5" product, which uses acetone and toluene as evaporative solvents. This is good. I can get these here, for thinning, cleaning, etc.

[Canibul]

Meanwhile, back at the ranch.... after letting it sit for an hour, in the HOT sun, I took it inside the garage, cooled it off with water, and started scraping the coating off the steel. It sticks better than I hoped. I had to use a razor knife to get it off, and (importantly) it doesn't peel off. Each flake has to be individually carved out and prised away, on a very small scale. I tried fingernails, small flat blade screwdriver, a nail, and the aforementioned razor knife. Standard utility blade.

I put a single coating on one area, and glopped it on thick in another. After some investigation, I believe the answer might be an initial thinned coat to get into any little weld crevices, etc. as it's pretty thick out of the can and dried very quickly here. Would be easy to trap some air with that. I'd Follow that up with an unthinned coat or two, to build up some thickness. I found that subsequent coats adhere well to the previous coat. I believe that removing the tank from the boat is critical, as this stuff is highly flammable. or inflammable, your choice, since those words seem to mean exactly the same thing.

One surprising outcome of my first experiment is that it seems to stick to the un-cleaned metal as well as it does to the sanded metal. Maybe better. So far it doesn't appear to me that the steel requires any major prep. Just clean any obvious slime or loose scale off and this stuff sticks like glue.

I've been taking photos of the process. Will probably stick them in a blog post.

The manufacturer does not advertise this product for this purpose, but it seems to work. They might well have something they would recommend over this for sealing tanks, but I'm looking at what's available. Importing t'ings with volatile explosive solvents seems to be an annoyance here.