Pitting in Aluminum Water Tank - Major or Minor Concern ?

[Konsensus]

I inspected an Ovni 395 aluminum boat from 2005 today that I am considering buying. The only thing that i found that could be of concern was pitting in the water tanks, in the form of several pinhead pit marks in the size 2x2mm /0.08x0.08 inches.

I have attached photos for description.

Is this something that I should be concerned about?

Otherwise, the aluminum construction was sound in the bilges and below the waterline on the outside.

Is it something that needs to be addressed, treated with chemicals or mechanically?

Hope someone can enlighten me regarding the situation.

Kind regards,

Andreas

[noelex 77]

You do not say how deep they are. Is that 2mm deep or 2x2mm and unknown depth?
If not too deep they would not concern me. Its likely some small flake of a dissimilar metal (like a stainless steel screw) got into the water tank and sat in the same place for a long time.
It would be worth looking carefully for the offending piece of metal which may have moved.
Some people treat their water tank with high levels of chlorine which is not good for aluminium, but this is a very small area of damage.
If the thickness of the bottom plate is compromised the corroded piece needs to be cut out and a new piece welded in. This is not difficult for a good aluminium welder, but the bottom thickness should be about 8 or 12mm (check with Ovni) so there is a lot of reserve if its 2mm deep.
Its always worth having a boat surveyed

[Konsensus]

Originally Posted by noelex 77

You do not say how deep they are. Is that 2mm deep or 2x2mm and unknown depth?
If not too deep they would not concern me. Its likely some small flake of a dissimilar metal (like a stainless steel screw) got into the water tank and sat in the same place for a long time.
It would be worth looking carefully for the offending piece of metal which may have moved.
Some people treat their water tank with high levels of chlorine which is not good for aluminium, but this is a very small area of damage.
If the thickness of the bottom plate is compromised the corroded piece needs to be cut out and a new piece welded in. This is not difficult for a good aluminium welder, but the bottom thickness should be about 8 or 12mm (check with Ovni) so there is a lot of reserve if its 2mm deep.
Its always worth having a boat surveyed

Thank you for the reply. They are about 1-2mm deep. They were quite numerous. Every spot in the picture is a pit. It is probably the use of chlorine as you have indicated.

But to conclude, you wouldn't worry too much even though they are quite numerous?

Is there anything that can be done to prevent it from increasing in size/to stop the decay?

Will it stop if you use unchlorinated water?


Kind regards,

Andreas

[noelex 77]

1 to 2mm deep should not be a great concern. The low level of chlorine in town water are not a great problem, but sometimes the water tank is treated with high levels of chlorine or bleach to sterilize the tank over winter.
Ovni paint their bilges and water tanks .I do not know the Ovni painting schedule, but typically for aluminum boats it is 5 layers of interprotect followed by a couple of layers of paint. This results in a considerable thickness and some or all of the pitting may just be this coating breaking down. It would be worth considering replying the coating over the effected areas.

[ronh732]

I would never want an aluminum water tank or a steel one. Water chemistry changes from place to place and is typically corrosive to some degree even without chlorine. And it's a real pain adjust the ph to 7 all the time. Also Any metal water tank whether aluminum or steel should have a nontoxic multi-layer coating on the interior to prevent corrosion.

[noelex 77]

Aluminium water tanks on say a fiberglass boat is a poor application of the material. Crevice corrosion between the bottom and the supports is a common problem. Aluminium needs oxygen to stay corrosion free and its unlikely to get it where the tank rests on suports.
Compounding this problem, the water tanks are often made of very thin inferior grades of alumium and welded by people that do not know what they are doing.
Integral tanks in an alumium boats suffer none of these problems. They have a lot of advantages such as no wasted space and provide a double hull
over that part of the boat.

[Hud3]

That sort of corrosion comes from allowing chlorine in the tank.

I had a lot more pitting than what you see there before I figured out I needed to remove the chlorine from city water with a charcoal filter. I also worked out a way to lay up the boat for the winter and then sterilize and flush the water system when re-commissioning without putting any chlorine in the tank itself. All was good then.

[cburger]

By the pictures posted its a little difficult to judge the extent of problems with the aluminum. As a previous poster stated of real concern with metal tanks is the bottoms where they come in contact with a shelf or support and will usually start to corrode in this area rather than the tops of the tanks. Usually the welds start to go before the plate. The picture looks like you are looking through an inspection or access port. Can the tanks be completely inpected inside and out, top to bottom or is this impractical due to the installation? This all being said unless there is a very significant money cost involved with replacing the tanks in the event of future problems and you like the boat, issues in this area shouldn't be a deal breaker.

[Billy Higgins]

There is a two-part tank coating material called "Amerlock" that is popularly used in municipal water tanks, and which is impervious to chlorinated water. It is unfortunately not for aluminum tanks, but for pretty much every other material. Check out the web site for www.genesischemical.com

[estarzinger]

http://www.tnemec.com/product/view/S...40-PotaPoxPlus

Originally Posted by Konsensus

pinhead pit marks in the size 2x2mm /0.08x0.08 inches.

Is this something that I should be concerned about?

Is it something that needs to be addressed, treated with chemicals or mechanically?

The inside of aluminum water (but NOT diesel) tanks are usually coated with an FDA approved (or equivalant EU drinking water standard) epoxy paint (here and here are examples). Doing this would stop the pitting and prevent furture developments. But you need good access to the tank insides because you need to clean all the surface extremely well prior to the painting (if you don't the epoxy will just flake off).

[Billy Higgins]

. . . and wear a really good respirator and have the boat ventilated extremely well - both inlet and outlet fans going full blast. Two-part epoxy is not to be applied casually in enclosed areas.

[Cheechako]

2mm is .080 inches. That would be a concern to me, unless those are real thick tanks. They could be .125 thick... which only leaves about .04" left. I guess it comes down to how hard is it to replace the tanks ... just in case. Is it only on the bottom plate? I have heard of just welding a new bottom plate on tanks that are otherwise good. If the tanks are as thick as that baffle I see, I wouldnt worry I guess.

[sdowney717]

Yeah, coat it before it is destroyed.
Plus you likely getting aluminum in the water which is not good, so dont drink the water.
You eat acidic foods and also should reconsider aluminum pots and pans for the same reason. I always wondered about aluminum coke cans loaded with phosphoric acid.

Just FYI,
I use a lot of PL premium polyurethane construction adhesive which I have found to be 100% waterproof and you can paint on thin coats with a brush. After it is cured I believe it is ok, a harmless polyurethane. And it will bubble up but after it is cured for a day, the surface is tough enough.
I also noticed it holds up to heat well.
To me that would be better than raw aluminum. And a cheap fix.

[Billy Higgins]

Evans - Are your water tanks aluminum and integral to the hull on the Hawk?

And what are your opinions on integral sea-water ballast tanks for performance-oriented cruising with a metal hull? Is the trade-off of complexity and loss of storage space worth gaining some stability for performance and comfort, plus having the "double bottom" benefit?

I'd be interested to hear what you have to say considering how many boats in commission and actually out cruising you and Beth must have come across.

[Billy Higgins]

sdowney - A lot of that fear of aluminum cooking pots came from popularized medical research in the 80s that noted that some of the compounds in the dentrites in the brains of Alztheimers corpses contained aluminum salts. However, the connection of aluminum cookware to Altzheimers has since been disproved.

Also, I would really think twice about using polyurethane construction adhesive for a potable water tank coating. It's the "construction adhesive" part that alarms me. Polyurethane is not bad per se, but make sure what you are using is ANSI NSF Standard 61 compliant. That is the ANSI potable water standard, and if the label does not state it, I would certainly not use that coating.