aluminium corrosion

[alteredoutlook]

I have an aluminium boat with integral water tanks. Some time in its past it must have had some foreign objects within the water tank and the result is corrosion pitting at the bottom.

The plate is 6 mm thick and the pitting is up to 3 to 4 mm deep in places. Thus, another 2mm and sea water will enter. The problem is not structural.

What can I do to a) fill the pitting b) prevent re-occurrence.

Suggestions made so far include
a) replace the plates completely
b) apply another layer of plates to the outside of the boat
c) weld the pitts – a bit difficult as limited access – and can it be done with aluminium?
d) Apply epoxy to the bottom of the tanks. Does a non toxic one exist?. Would I have to paint over the epoxy to seal it? Will epoxy ‘stick’ to the alumiunium? Does a clear epoxy exist so that I can see if the problem is still happening.
e) Apply a layer of cement to the insides of the tank. Will it crack/flex off? What sort of cement do I use?

Any other ideas or suggestions on how to do the above will be appreciated.
Thanks


<hr width=100% size=1>

 

[Birdseye]

I remember finding one epoxy that was certified as potable, but I cannot remember wher I saw it. Vague memories of the seller being somewhere like Huddersfield. If you try the tech help line at SP resins, they are very helpful and might be able to guide you.

Personally, I think I would go for a bag tank as a liner. There was lots of talk about aluminium and altzheimers at one point.

<hr width=100% size=1>

 

[oldsaltoz]

G'day alteredoutlook,

I would not advise using an epoxy, the expansion rate of alloy and epoxy are such that the epoxy will crack, or at the very least leak.

Now might be a good time to replace or fully seal weld another plate in, give yourself a good access / inspection hatch / cover and etch then paint the internal to prevent rust and other more noble steels entering the tank and causing more problems, The rubberised paints designed for fresh water tanks seem to have less taste and stick ok.

I hope this helps.

Avagoodweekend........



<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[VirgoVoyager]

I would go for a flexible water tank fitted into the existing space - this seems like the most straightforward solution. Alternatively, some manufacturers do flexible paints specificially intended to waterproof exterior and interior metal surfaces. Canal narrowboats often have their water tanks treated in this way. Some research on the Web, or contact with one of the many canal boatyards should point you in the right direction.

Successful waterproofing!

Stuart

<hr width=100% size=1>

 

[MainlySteam]

In my view the correct action is to cut the plate out and replace it as the corrosion may not have stabilised. If you have access to a good aluminium boatyard this should be a very simple and relatively cheap process (cut plate out, weld new in, recoat the exterior bare plate and antifoul, plus the cost of the lift from the water) as long as there is access to the interior of the tanks allowing making good the welding of the new plating to any longitudinals or floors passing through them. I would suggest that it is not sensible to weld another plate on the outside as the only saving is likely to be the cheap and quick cutting out of the old plate with the cost of having an obvious patch on your boat.

If the problem is not widespread, the pits are small in diameter and not structural (as you say) then a quick but maybe temporary fix would be to use an epoxy undercoat paint on the bottom of the tank with filling of the pitting between coats in the normal way. Many of the common epoxy paints used on boats are certified for potable water storage and any of the large manufacturers should be able to direct you to a product. As Oldsaltoz says, do not use epoxy as used for frp construction, glue or for self levelling applications.

John

<hr width=100% size=1>

 

[richardandtracy]

I agree 100% with John.

One point though. The plate will be welded in the 'Up hand' position - which is very difficult with aluminium due to the low viscosity of the melt (it drips.. ow!). Check that the welder can do this BEFORE you cut the plate out.

Regards

Richard.


<hr width=100% size=1>

 

[alteredoutlook]

Many thanks for the info - but can you recommend a good yard that can deal with aluminium.

My boat is currently afloat at Glasson dock, lancaster - it needs to be lifted some time soon to paint the bottom - but am willing to do this anywhere(.....within reason!)

thanks, in anticipation

<hr width=100% size=1>

 

[extravert]

I used to have an aluminum boat (a 7m Extravert), and these people did excellent aluminum work for me...

Trinity Marine Ltd
Newry Beach,
Holyhead
LL65 1YA

Phone: 01407 763855

It's not too far to sail to from Glasson Dock, and the trains to get back home are also good.


<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep trying.

 

[alteredoutlook]

Thanks for the contact - Amazingly i had just discovered them on the internet 2 hours ago! I actually gave them a ring and they also suggested Holyhead Boatyard as they also build aluminium boats.

I know the Extravert - a friend had the similar E boat which unfortuantely/fortuantely got me into sailing. Fun but hard going beating into a chop. I didnt realise they were aluminium though.

<hr width=100% size=1>