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Old 15-11-2021, 19:26   #1
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How do you deal with Pitted Aluminium?

Whilst this is yet another post about aluminium corrosion, I havenít yet been able to find out much about de-passification. So I thought Iíd float it by the Forum.

Some years ago before I bought her, my yacht suffered electrolytic corrosion which, according to the vendor, was caused by a defective shore power connection, which lead to shallow pitting throughout a wide, shallow bilge. That was comprehensively addressed and the boat was rewired with high quality marine cable and all AC connections/cables removed. I do regular reference checks and have engaged a specialist in the area of aluminium corrosion so am confident that there is no stray current activity.

So this post is not asking about the cause/s of the pits but more about their treatment (if there are any treatments). I had been comfortable in the belief that without stray current or galvanic activity, I was safe (or the boat was). Thatís when I came across the possible consequences of the aluminium being de-passified via the pitting.

If I understand this correctly and in my simple terms, de-passification is when the natural oxidisation protection of the aluminium surface is breached and the aluminium is not allowed to protect itself. That breach (so I have read) can occur when aluminium is pitted, perhaps through another corrosive activity Ė in my case stray current damage. The pit exposes internal metal with slightly different galvanic properties to the surface. In the presence of an electrolyte, a tiny galvanic cell is created between the surface and internal metal which causes corrosion within the pit. As an aside, it can be extremely humid where I am. Last weeks high humidity resulted in damp salt crystals throughout the bilge, which I guess would be a good electrolyte. Iíve since flushed and scrubbed the bilge with fresh water.

This is rather a long way of getting round to asking if this ďde-passificationĒ in a pitted aluminium hull is cause for my concern and what, if anything, could/should be done to re-passify or protect the aluminium? There are far too many pits to spot weld (Iíve had a welder look at it). The previous owner had coated it with a lanolin product, which created a horrible, sticky coating that attracted dust and other crud.

Iíd really appreciate hearing from those with good knowledge in this area and/or who own aluminium yachts. Iím learning as I go here and am quite happy to be corrected on my rudimentary understanding of this topic.

Cheers
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Old 16-11-2021, 06:19   #2
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Re: How do you deal with Pitted Aluminium?

Do you know for certain what series aluminum you have...were samples sent to a lab during construction? You are talking about the interior of the hull...correct? Is the pitting on the hull plates or near or under structure welded to the plate.
Is the damaged area the entire surface where salt water was present?
What is going on inside the tanks.
What was this lanolin product and why.
No AC , not even an inverter?
Is the exterior of the hull epoxied? How many zincs.
Who built it...when.
Photos or reports please.
Regards, the manatee crew.
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Old 16-11-2021, 06:57   #3
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Re: How do you deal with Pitted Aluminium?

When exposed to the oxygen in air, aluminum naturally forms a hard, relatively inert surface layer, aluminium oxide.
Unfortunately, not every aluminum alloy does a good job of forming this oxide layer, meaning that extra protection might be needed.
If you do not want to plate, paint, or coat your aluminum, there are two main options for creating a passivation layer.
The first is known as chromate conversion coating [Alodine or Irridite]. Such coatings, which can vary in thickness from 0.00001–0.00004 inches, are amorphous in structure, and have a gel-like composition when hydrated with water.
A second option is known as anodizing. This is when the aluminum undergoes an electrolytic process in order to thicken the oxide layer. The anodic coating will create a layer of hydrated aluminum oxide that is more resistant than the natural passivation layer to corrosion.
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Old 16-11-2021, 07:51   #4
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Re: How do you deal with Pitted Aluminium?

We either replaced the plate anywhere there was numerous pits deeper than 25% of plate thickness, or we spot welded if there were only one or two in a large area. Anywhere less than 25% got wire brushed and cleaned with a needlegun to get out any gunk that can causes corrosion.

I left them to oxidize thinking I was going to keep a dry bilge and have no further problems... foolish! Less than a month into cruising the saltwater intake of my watermaker burst and filled the bilge with saltwater. Any and all construction dust flowed into these holes and packed them again even after a good freshwater cleaning. It was a constant headache trying to keep these clean.

In retrospect, I should have filled the pits with epoxy after getting everything cleaned out - I don't like a painted aluminum bilge, so this would be a spot treatment.

I would clean the area with a strong degreaser; wire brush the pits; use a small diegrinder (Dremel) or drill bit (with depth collar) to make sure they are cleaned out, then paint in epoxy until filled. This gives a flat and cleanable surface so any water/gunk can't get in these pits and cause more issues.

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Old 16-11-2021, 23:37   #5
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Re: How do you deal with Pitted Aluminium?

Thanks for your comments guys to which I'll try to respond.
1. The boat was reportedly built of 5083 alloy. I've not even thought about having it tested.
2. Interior hull - yes.
3. I do not know what salt water ingress may have occurred - all before my time. BUT, the pitting is only below the waterline. I can only speculate that she had salt water in her bilge at some stage to catalyse the electrolytic process.
4. Good pick-up Manateeman about the connection to the tank. The hole that caused the leak was blatantly obvious but I don't know the cause of the hole for sure. But I will say that there was 1" of crud inside the tank which from my limited knowledge could well lead to corrosion.
5. Lanolin - I have no info about why it was applied. It was transparent - so the hull could be seen. The vendor said it was to protect the pitted hull. I've since talked to a couple of more knowledgeable people about this and they are similarly in the dark as I am about what it was intended to do.
6. No AC whatsover although there was extensive AC throughout the boat before my purchase (hence the vendor saying it was the source of corrosion). Beyond that I don't know for sure. I have an inverter but not connected yet pending my sorting all this out.
7. The hull was (apparently) coated with 5 coats of International Interseal epoxy. I think this was applied after the electrolytic problem.
8. Boat built by Astrolabe Marine in Vancouver, Canada in 1995. I believe that company no longer exists although I haven't looked very far.

I've been wondering if zinc chromate is the way to deal with this but a good surveyor who I saw today recommends epoxy on the deeper pits alone (once thoroughly cleansed).

I very much appreciate your views and time responding. But I'm still in the dark whether corrosion continues in even shallow pits through the depassification process I tried to describe above. Or am I being too concerned about a theoretical/lengthy concept and should just get on with a bit of epoxy?

Cheers
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Old 17-11-2021, 01:25   #6
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Re: How do you deal with Pitted Aluminium?

the galvanic cells inside the pits will not go away: at the bottom of the pits oxygen-starved aluminium will react with the galvanically nobler aluminium higher up & around the pit & continue to eat away & deepen the pit, eventually holing the plate. We saw an aluminium boat on the hard where the owner had drilled through all pits & put toothpicks into each hole for the welder not to miss one. Looked decidedly funny, like an inverted hedgehog.
On our own boat there were only few & shallow pit & I opened them up with a big drill taking care only going deep enough (!).
Imho chromate conversion will not reach the bottom of a typical pit unless you "opened it up". It also is an extremely thin & vulnerable layer.
Trying to weld the pits shut from the inside without drilling them out beforehand will cause the inevitably present moisture at the bottom of the pit to evaporate "explosively" & blow the welding material away, resulting in cavities (& maybe burns of surroundings & operator).
(Another prob of aluminium boats is a keel envelope corroding through from the inside. Fortunately (for us) our boat only revealed this after we had sold it, but further investigation showed that this was not at all uncommon, 20 years of age being about the time where one should watch out. Centerboarders are not immune from this: in 2019 we met a french boat who was chasing thin spots/holes in his ballast compartment (Via 38).
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Old 17-11-2021, 02:13   #7
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Re: How do you deal with Pitted Aluminium?

Unless you use an isolating device you will ALWAYS get corrosion from a connection to shore power. I bought an isolating transformer. There are other kinds of devices which transfer power without any actual physical connection.

If you do not use an isolator, your aluminium hull sitting in a saltwater electrolyte is going to be about the best earth plate the mains system has for miles around. Lotsa luck with that.
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Old 17-11-2021, 07:55   #8
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Re: How do you deal with Pitted Aluminium?

A complete discussion of pitting in aluminum : aluminum-guide.com
The other 12 types of aluminum corrosion: Fractory.com
Aluminum aircraft donít fall out of the sky because of pits every day.
The aluminum beer bottles the manatee crew toss to each other do not explode.
TIG welders donít run away from aluminum repair jobs.
The manatees donít avoid swimming under the lead keels of aluminum boats.
Obviously some aluminum boats are not well built or not maintained.
Some owners have had very bad experiences with aluminum.
Look at new commercial vessels and you will see just how popular aluminum has become for vessel construction.
Fishing, oil field, research...even the Coast Guard.
Aluminum. Treat it with care. Keep it happy and youíll be happy too.
Itís the reason manatees drink so much beer.
Happy trails to you.
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Old 18-11-2021, 00:38   #9
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Re: How do you deal with Pitted Aluminium?

Quote:
Originally Posted by double u View Post
the galvanic cells inside the pits will not go away: at the bottom of the pits oxygen-starved aluminium will react with the galvanically nobler aluminium higher up & around the pit & continue to eat away & deepen the pit, eventually holing the plate.
So much for my naive belief in the infalibility of aluminium! I avoided steel because of the rust, yet aluminium if pitted seems to present it's own on-par challenges. Or so it seems as I am learning.

I had a decent surveyor round yesterday. Not the one who did my pre-purchase inspection! Yesterday's surveyor re-tested the plate thickness from the inside at 4.5mm which mortified me because it was sold as a 6mm plated yacht in which I want to sail remotely. The pre-purchase guy did it externally and found an average of 5.7mm which I could have lived with. That now really bugs me as I've got a 2.5mm pit which means I have only 2mm of plate left (or 45%) in that pitted area.

So if not zinc chromate, it leaves me with epoxy once I've cleaned out and acid etched as best I can. And even then the diddy little galvanic cells may still be doing there work. Is that a fair assumption? That or lifting her, cutting out whole plates and rewelding new, which seems like major and horribly expensive surgery which I can barely afford.

Manateeman - any other comments after I answered your questions (I hope)?

I very much appreciate your comments guys. Cheers
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Old 18-11-2021, 00:39   #10
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Re: How do you deal with Pitted Aluminium?

Quote:
Originally Posted by Mike Banks View Post
Unless you use an isolating device you will ALWAYS get corrosion from a connection to shore power. I bought an isolating transformer. There are other kinds of devices which transfer power without any actual physical connection.

If you do not use an isolator, your aluminium hull sitting in a saltwater electrolyte is going to be about the best earth plate the mains system has for miles around. Lotsa luck with that.
Hi Mike - I don't have shore power or any source of AC.
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Old 18-11-2021, 02:03   #11
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Re: How do you deal with Pitted Aluminium?

well made aluminium is the best material for a "blue-water-boat". Maybe Titanium would be even better, but for obvious reasons there are very few about.
"Infallability": NO boat material is infallible! If lots of stray current cannot get it - a reef can.
With aluminium construction "shortcuts" are tempting & "save" the builder quite some expense:
e.g. "insulating galvanically different materials from each other" means there is NO conductivity between them, & not just "plastic plates under winches " & "stanchions in plastic bushings", but a painstaking sleeving of all bolts, insulating washers, filling all cavities with sealant.
There may be NO voltage on the hull! (not so easy to totally insulate the engine), engine ground switched off when engine is not in use,...
All circuitbreaker two-pole,...
When visiting friends on their aluminium cruiser - the switchpanel was always open & the tester was lying around...I on ours was a little less nervous, I always closed it after the daily inspection...
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Old 18-11-2021, 06:30   #12
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Re: How do you deal with Pitted Aluminium?

[QUOTE=Wideocean7;3521942 That now really bugs me as I've got a 2.5mm pit which means I have only 2mm of plate left (or 45%) in that pitted area.



[/QUOTE]

That's typically time to weld. Either fix the spot if there are only a few, or replace the plate. But remember that this one deep pit - even if it was all the way through - Isnt enough area to sink the boat. It more of a nuisance and shows what may continue to happen if left untreated. Just do your best cleaning the spots; acid wash; fill with epoxy and go sailing.
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Old 18-11-2021, 06:48   #13
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Re: How do you deal with Pitted Aluminium?

Dear original poster. I went to see my physician once and asked what the brown spot was on my arm. ďCancerĒ. Then she laughed an said ďbut this type grows so slowly youíll be dead long before it can kill you.Ē
Please read up on the subject. Fix your boat. Keep things clean and dry.
Happy trails.
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Old 19-11-2021, 06:10   #14
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Re: How do you deal with Pitted Aluminium?

Some wise words there - thankyou. But if I'm sold a boat with 6mm plate and it only has 4.5mm, is that not a significant difference when the surveyor knew I intended high-lat sailing? Or am I getting hung-up on semantics?

As for practicalities mentioned by DoubleU, all my isolator switches were double pole when I bought her. Since then I've installed a main circuit breaker box with 6 double pole breakers. Of those, one supplies power to my main breaker panel. I switch this off whenever I'm off the boat. The stuff needing to be "always-on" has it's own double pole breaker to limit the leakage potential to just a few accessories.
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Old 19-11-2021, 06:17   #15
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Re: How do you deal with Pitted Aluminium?

a few more points:
1.once your pits have reached say 2-3mm it will NOT take forever to eat through
2.likely the pitting in the bilge has little to no connection with an electrical/wiring issue, but comes from wet gunk (lint, particles) in bilgewater
3. engine to hull should be open circuit or at least high-Ohms. Insulation between shaft & gearbox
4. check Voltage hull-batt+ & hull-batt-: should be zero in both cases
5.cooling water intake: siphonbreaker after throughhull/filter & before engine
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