aluminium corrosion in keel

[SV Mighty Quinn]

I am chasing some minor corrosion issues in the keel of my 43ft aluminium mono hull.

Ballast is led balls incapsulated in some sort of epoxy. From the little I can see, it seems the epoxy has shrunk a little bit over the years, or there was some voids left when it was manufactured. Either way, I believe there is some water between the ballast and the metal.

I am considering taping a couple holes in bottom of the keel, one at the leading edge, and one at the trailing. Then attempting to pump some antifreeze into the void, and plugging the holes with machined aluminium bolts.

Thoughts? Experiences?

[SailingHarmonie]

The approach sounds like treating the symptoms, and not the actual problem.

Certainly, epoxy does not shrink over time, so your basic diagnosis is not correct.

But be that as it may be... I do know that not all aluminum alloys are galvanically compatible. If the bolts you install and hull are made of alloys that do not play well together you could be making a much bigger problem.

[SV Mighty Quinn]

I would make sure the metal is compatible.

Short of removing and rebuilding the keel, I think I may be stuck with 'treating the symptoms'.

The corrosion is not that severe, I am only trying to minimize any further issues.

[boatpoker]

If the dielectric coating of the lead balls has been breached, the keel is likely suffering from galvanic corrosion on the inside surface of the aluminum keel. What you see on the outside is quite possibly much worse on the inside.

I can't see how anti-freeze would have any effect on this type of corrosion.

All I can think to do is cut a sample of the worst area and look at the inside surface. Ignoring this issue could be a costly mistake.

[SV Mighty Quinn]

I am for sure not ignoring the issue!

I had a good look when I found the bit of corrosion at this latest haul out. From what I could see, it didn't look immediately concerning. However a small amount of water did came out of the hole we cut before patching.
Weather this was from after the pin hole was formed, or from another place, is impossible to tell.

So I was thinking if I could get something with some anti corrosion properties to fill any voids, it couldn't hurt.

What to use, and how to do it are what I am pondering....

[boatpoker]

Here is a galvanically corroded, pitted stainless steel prop shaft that broke due to corrosion and an x-ray showing one of the pits.

The outside shows very small pits while the inside looks completely corroded. Galvanically corroded aluminum is probably much worse

[Rucksta]

Originally Posted by SailingHarmonie

Certainly, epoxy does not shrink over time, so your basic diagnosis is not correct.

However

Epoxy does shrink as it attains full cure.
Even more so if it has fillers e.g. Qcells, Aerosil.
Lower density fillers display more shrinkage.

Casting resins e.g. Luciclear are formulated with low endothermic properties resulting in bubble free slow set to resist this shrinkage.

[barnakiel]

Originally Posted by SailingHarmonie

Certainly, epoxy does not shrink over time, (...)

Oh yes, it does.


https://incurelab.com/wp/how-to-reduce-epoxy-shrinkage



Not to say it did in this particular case.


b.

[barnakiel]

ps I would invite a certified alloy welder and cut small inspections areas (starting at the lowest part of the keel), then depending on what is found inside I would decide what to do


if the damage is minimal, I would thoroughly DRY the keel 'void', weld over the inspection area, drill small holes way up to the floor - then I would pour epoxy into the first (lowest) hole ... then into the second lowest hole ... and so on)


pouring epoxy like this, not too fast, in many stages, would re-isolate any broken resin isolation on the balls (lead balls, I mean),


it does not need to be west system, you can use any commercial grade technical epoxy (e.g. the stuff bought in kegs by pool builders)


it will not be too much epoxy that you need - the keel is already nearly full, any voids are a small % of the keel's volume



b.

[JPA Cate]

Some of the old Vias had the epoxy shrinkage problem, but not lead pellets, rather lead bars. The folks we knew with them removed the lead bars, and re-epoxied the sides and bottom of the keel with thickened slow cure epoxy, then replaced the bars, and epoxied over the whole lot.

Maybe you could do something similar? it will depend on how much access you can get to your keel. I would want to speak with a specialist in aluminum boat repairs, because there are so many aspects to consider, and ponder getting a 2nd opinion, too. ymmv.

Ann

[NorthCoastJoe]

I agree with Ann. Our aluminum boat is constructed similar to how she describes a repair.
We have an epoxy coated bay that the coated lead rests in.
All the lead is removable by unbolting a cover over the keel in the keel.

[SV Mighty Quinn]

Unfortunately I need to remove the engine to access the top of the keel.....
I think next haul out ill drill or cut some inspection holes in the bottom of the keel and see what runs out, probably install a couple more anodes. The bits we cut out to effect the repairs this year looked fairly good on the inside surfaces, I don't think the keel is going to fall off or anything, just trying to stay ahead of it.

[Matt Johnson]

I had the same issue of the lead ballast attacking the aluminum plate in my keel. We had one small area of concern, but I hired a surveyor experienced in aluminum boats to ultrasound the entire keel. We mapped out the corrosion, and when cut open, what we saw was the same as what the thickness gauge showed.

That one small hole in the keel blossomed into replacing about a 4'x8' area... about 60% of the keel plates.

BTW, it wasn't big areas of internal corrosion, but more hundreds of deep pits. So not really something obvious if looking from the top (our keel cavity was welded from the top and I didn't have access).

[dlj]

Originally Posted by Matt Johnson

I had the same issue of the lead ballast attacking the aluminum plate in my keel. We had one small area of concern, but I hired a surveyor experienced in aluminum boats to ultrasound the entire keel. We mapped out the corrosion, and when cut open, what we saw was the same as what the thickness gauge showed.

That one small hole in the keel blossomed into replacing about a 4'x8' area... about 60% of the keel plates.

BTW, it wasn't big areas of internal corrosion, but more hundreds of deep pits. So not really something obvious if looking from the top (our keel cavity was welded from the top and I didn't have access).

Doing an ultra sound inspection is a very good thing to do. Excellent technology for what you need to determine.

dj

[KelseyB]

Agreed on the usefulness of UT. If you don’t want to shell out for the full survey, or at least not at first, you can buy an ultrasonic tester on Amazon for $100-200, and they’re more accurate than you might think, although obviously not as good as the really fancy ones surveyors use. If you have some spare metal lying around you can compare the UT’s reading with calipers to get a sense of its error. They’re good for getting a general sense of thickness, but not for detecting voids deeper inside.

It’s interesting that no one has suggested concrete. If you’re going to dig out all the epoxy and repour, I’ve heard that concrete is the more common capper, although maybe that’s for steel boats - that’s what we did over the lead pellets in our steel hull. Heavier, obviously, but maybe that’s a good thing, depending on the situation.

If you’re thinking of pouring something anti-corrosive in there just to stop it from progressing, I’d think you’d want something a bit thicker than antifreeze sloshing around in there. I’m not sure what that is - something thin enough that it could seep into any voids, but also thick enough that it isn’t sloshing around, but also doesn’t cure so rigid that it can crack and separate. Almost a gel, maybe…