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Alloy Boats and Water Tanks

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I was asked a question I could not answer by a friend rebuilding an alloy boat. It has alloy water tanks. He's been given all sorts of conflicting advise about using the alloy tanks for drinking water. So, do any of you industrial chemist types know the real story with alloy tanks?

 

  • Interior left alloy or should be painted? If painted, with what?
  • Is there any reason Chlorine cannot be used for water sterilisation?
  • Best way to clean out a used tank with very limited access.

What do you all think, and does anyone actually know?

 

Thanks

Matt

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I'm no chemist, nor do I know much about about alloy water tanks, however I do own an alloy keeler with two 90 litre alloy water tanks that are at least 25 years old. Here's what i know. . . .

  • They arent painted.
  • Ive only ever used the standard water purification tablets in them from the chemist and never had a problem, I don't know if they contained any chlorine
  • I filled mine up with a concentrated bleach solution, left them for a few days then flushed.

So far I've never had a problem with them.

 

There does seem to be a lot of slightly informed, opinionated 'experts' on all things alloy around and little of their advice has been worth listening to. 

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Ditto Tuffy Luffy, I have 2 x 200l tanks, 35 years old,  I did open them up 10 years ago, but only because I wanted to inspect them, I replaced both lids as there was some minor pitting on the underside, not sure what had caused it. We have used clorinated Auckland / Whangarei water to stream water from the barrier, and whatever we could get up in the Islands. No issues.

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I've been researching this a bit today. Seems that long term, chlorine in alloy tanks is not a good idea. Probably what caused the pitting in your lids KM.

Some users report white powder/crystals in the water, sometimes clogging the pumps/strainers. This is oxidization of the alloy. Apparently the chlorine accelerates the oxidation. The recommendation is to use peroxide to sterilize the water and clean alloy tanks.

This is all 2nd (or 3rd) hand though, I'd like to hear from someone who really knows this stuff!!    

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The chemical path is 2AL + 3CI form's ALCL3 and is the white chalky substance they will be talking about. You can see an advanced reaction rate if you heat foil and dip it in a heavy chlorine solution. Without heat this reaction is fairly slow and I doubt you'd live long enough to see it eat a hole in a tank.

 

Having said that ALCL is a nurotoxin with heavy links to motor function based illness's and whatnot. If you cant avoid the use of the material then i would seal it with a food safe sealer, Take your pick of epoxies or whatnot - even 291's approved for water

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About 6 or so years ago, I built two 650ltr tanks out of Ally. One for fuel and one for water. I was nearly finished when I had the client come to me in a panic. He had another contractor tell him he couldn't use Ally for fresh water holding. Thinking that was the stupidest thing I had ever heard and that I was using a Marine grade Ally that has a high degree of corrosion resistance, but at the same time knowing the Contractor and knowing he was not an idiot, I went and had a talk to him about it. The story was that the big Mussel Boats working out of Havelock were having big trouble with the fresh water becoming tainted and undrinkable. They sent samples to the Cawthorn institute for testing and the result came back that something (He couldn't remember what it was) in the fresh water in Havelock, was reacting with the Ally and producing a chemical that was causing the problem. So I ended up coating the inside of the fresh water tank, to stop contact with the water. I don't remember Havelock having Chlorine added, but it may have perhaps.
Beccara is correct that Aluminium and Chlorine react to produce Aluminium Chloride. ALCL3 is a white powdery stuff and is often seen around a little pitted hole. However, Aluminium Oxide is also the exact same in appearance and affect and so the only way to distinguish between the two is to have it chemically tested. But once ALCL3 is created, it doesn't always stop there. ALCL3 can go on to react with other trace elements that are common to find in fresh water. Such as, Iron, Calcium and Magnesium. One trace element found in ground water in some areas around NZ is Arsenic. I don't know what that would if it could create, but it is highly likely it will make something nasty.

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OK, so then the consensus is that it is probably best not to use chlorine in alloy tanks. What about peroxide for sterilisation then?

 

To paint the inside of these tanks would require major surgery (very limited access - they would have to have hatches cut, lids made etc), and replacing them is not really an option...

 

Anyone used peroxide before? Is it chemically safe with alloy, can any taste be removed with a carbon filter etc...

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Yeah it's pretty safe, Handbook of corrosion say's Aliminium is used in distilling and transport making reference to Alloys 1060,5052 and 6063. You may accelerate or build current oxide deposits in the tank due to the strone oxidizing nature of peroxide but you'd kill any bugs. A 100 micron filter would pick up most of the free floating oxide deposits without clogged up every week

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OK, so if peroxide will create oxidisation of the alloy, and chlorine will create the ALCL3, then it seems to me it does not matter which you use as long as you filter before drinking?? Which one of the two would cause less deposits then ??

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Oxide isn't too bad in that it's fairly safe to humans, AlCL3 is not safe and is a know nurotoxin.

 

There's two things here, One is the water it's self - Taking city supply to fill your tanks will create ALCL3 in your tank that will hang around in your water supply so either do rain collection or filter the city water in fresch charcoal filters. The next thing is tank cleaning which your specifically asking about - Using either peroxide or chlorine should always result in a very good rinsing of the tank with pure water only to prolong the tank life and avoid pinhole leaks. It's safer on the user to use peroxide to clean the tank for the user and the tank.

 

The creation of deposits at what rate is rather hard to say, temp, agitation rate, concentration of the cleaner and the level of biological contamination will all change it hugely. Best advice I can give is filter water coming into the tank (Depending on source 50/25/5 micron and charcoal) and coming out of the tank aswell,100/50 micron to protect the pump and avoid blocking taps etc, Charcoal i would put also past the pump if the water in the tank is not being cycled often. 3% peroxide to clean anything biological and the ice cubes + bumpy ride to "scour" the deposits on the bottom of the tank

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