Lead in aluminium keel – fill void with oil after repairs?

[hans]

We have an aluminium sailing boat (Koopmans 39) built in the Netherlands 1995 – 1996. Last year we discovered a faulty weld in the bottom of the keel, water slowly dripping from the keel when hauled. The boat has spent the winter in a shed for some major surgery to the keel. On pressure testing the keel we discovered 3 faulty welds in the bilge area. Apparently this is not that uncommon in older aluminium boats and is by some felt as the main reason for our keel problems, with water leaking into the keel from the bilge.

We found some very useful information about this on this thread on a Canadian website:

http://www.morganscloud.com/2012/07/01/one-tough-old-aluminum-boat/


The keel section is filled with lead. We have no details about how this is placed in the keel. We have now made access holes to the keel so we can see top of what we believe is the lead, but cannot really see whether the lead is moulded in place or set as ingots in epoxy. Ultrasound measurement of the keel shows no areas with less than 6 mm (the plate thickness), although this is of course no guarantee against there being pin-hole electrolysis. None is visible from outside, and bottom 1' of keel was sand blasted before the repairs.

We have spent a lot of effort in repairing the keel and searching out and repairing the leaks in the bilge area. We plan to treat the bilge with epoxy. Although the keel now is presumably dry after several weeks of vacuum applied, there is bound to be some salt in the voids between aluminium and lead. The combination of aluminium, lead and salt water should constitute a potent battery, and we are told we should fill the keel cavity with oil to neutralize this effect. I understand this will make later welding repairs difficult, but our welder states that with the new access holes any oil can later be rinsed out before any further welding.

My question to the forum is this:
Does anyone have any suggestion on what kind of oil would be best to use for this purpose? Some suggest vegetable oil, some suggest diesel, some old engine oil, or could we use anti-freeze made for aluminum engines? The oil/liquid needs to be thin to creep into the presumably small voids. Epoxy does not seem to be an option as there is no way to prep the surface for adhesion.

Any thoughts/suggestions much appreciated!

 

[sarabande]

vegetable oil might be best in case of another leak, as it would not cause an environmental issue. Check the pH of the veg oil, as some can be quite acidic.

You may have to warm the keel and voids to enable an easy percolation into the voids.


The freezing point of the oil may be an issue (roughly between -10C and -20C), but I can find no reference to the anomalous expansion of vegetable oil when it freezes. Any water already inside the keel should be removed to prevent expansion when approaching freezing point (0C).

 

[Ric]

How about diesel? Cheap and low viscosity so will penetrate well.

 

[alahol2]

My first thought would be toward mineral oil. I think it may have less smell and less possibility of going 'rancid'.
http://mistralni.co.uk/products/white-mineral-oil-grg?gclid=CODrhe_klsQCFSOWtAodE28Avw

 

[vyv_cox]

I suggest that the theory is incorrect. A galvanic couple is created when two metals in contact are immersed in a conducting liquid. In the case of your keel the two metals are already in contact, so no added oil or similar non-conducting liquid is going to create an insulating barrier. On the assumption that the oil is intended to prevent the ingress of water I suggest that this would be a very hopeful suggestion, doomed to failure.

A setting material such as epoxy might be a better bet but I suspect its viscosity might be too much to alow it to flow down into all the crevices. Drilling and tapping injection points at the lowest point of the keel might be a better bet, plugging them after injection.

 

[KellysEye]

>How about diesel? Cheap and low viscosity so will penetrate well.

Diesel will go through a microscopic hole and can be used to check welds by putting talcum powder on the outside of the weld.

 

[charles_reed]

Aluminium and lead - an interesting galvanic couple.
I'd go along with Vyv, suggesting I'd rather not start from here.
If filling the voids with oil makes you feel better, do it, but don't expect that action to correct the original design misdemenour.
The most-likely-to-stave-off-the-evil day of your ballast falling out is epoxy, poured in hot weather and a low viscosity variant from West.
At least that might help to keep it stuck in.

 

[Beamishken]

I wouldn't use diesel it contains moisture & can cause localised corrosion
Personally I'd lean towards the antifreeze as it contains corrosion inhibitors designed for exactly what you want it to do
Oil relies on excluding the water but any water that finds it's way in will sit at the bottom ready to do its worst (although with reduced oxygen the effect is lessened)
Antifreeze will also drain easily if you do need to weld in future & won't leave a flammable residue
I'm no expert on keels but this would be my thoughts on the subject
I'd bo more research in the antifreeze direction

 

[hans]

Thank you for the feedback: some very useful thoughts here. My conclusion for the time being is to do nothing; primum non nocere or whatever the saying is. Again: Thank you!

 

[noelex]

For galvanic corrosion to occur there has to be dissimilar metals in the presence of an electrolyte.

All three elements are necessary. Lead in contact with aluminium will not produce corrosion unless an electrolyte is also present.

When lead is added to an aluminium keel the area is completely sealed and pressure tested. This means the only possible electrolyte is condensation from the water in the atmospheric air contained in the sealed chamber. No new moisture can enter, like in say a fuel tank, because the area the sealed.

Additional steps can be taken to isolate the lead from the aluminium with a barrier system (like imbedding lead ingots in epoxy) or reducing the any condensation of the water vapour trapped in the sealed keel space by filling the space with oil.

In practice the condensation is minimal as there is no air exchange. So the amount condensation and therefore electrolyte is very limited. The additional steps of isolation or oil immersion are not needed. The exception is if the keel is punctured or has a defective weld.

The former is unlikely. Aluminium will deform a great deal before it punctures and the keel can be built very strongly without any great concern about weight. The latter depends on a good yard with expert welders.

If seawater has entered the keel space this is a serious problem. The water needs to be removed. Remove the electrolyte and there will be no corrosion.

I saw an aluminium yacht that had its keel simply cut off for transportation. It was welded back on at the destination. In the worst case a new keel could be fabricated reusing the existing lead ballast. However, in practice the keel will be very thick material and any internal corrosion will be slow even if not all the water (electrolyte) is completely removed.