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Old 05-09-2021, 10:45 PM   #1
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Aluminum boat corrosion

My brother has an aluminum fishing boat that he uses for crabbing in Puget Sound. It's not that old, but it's developing some corrosion at the seams and holes in the transom. It sits in the water at a marina during the Summer. There are no anodes on it, except for on the outboard. It's a well known name brand. Any ideas what's causing this and how to fix it?
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Old 05-10-2021, 12:31 AM   #2
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I would suspect the lack of anodes may be the issue. I would have a certified marine electrician that works with corrosion issues and have the boat inspected. It will just keep corroding away.
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Old 05-10-2021, 06:21 AM   #3
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You (or he) may also want to post this on aluminumalloyboats.com. There are a ton of threads on galvanic and electrolytic corrosion, and many aluminum boat experts (builders/welders) who will offer advice. (I used to own a 26' welded aluminum fishing boat.)
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Old 05-10-2021, 11:50 AM   #4
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I would suspect the lack of anodes may be the issue. I would have a certified marine electrician that works with corrosion issues and have the boat inspected. It will just keep corroding away.
There are no electronics on the boat except for a handheld chartplotter. Not even a battery charger, so there's nothing for an electrician to look at. Since it's not connected to shore power can it even be affected by a "hot" marina?
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Old 05-10-2021, 11:51 AM   #5
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You (or he) may also want to post this on aluminumalloyboats.com. There are a ton of threads on galvanic and electrolytic corrosion, and many aluminum boat experts (builders/welders) who will offer advice. (I used to own a 26' welded aluminum fishing boat.)
Thanks, we'll give that a try.
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Old 05-10-2021, 03:40 PM   #6
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Aluminum vessels need anode protection. Both aluminum and SSs can suffer from a type of corrosion where one spot will become active or anodic to another spot nearby. There is a term for it which I have forgotten and after a quick look through my two books I still cannot find.

Keep in mind also that the welding filler may not be the exact same alloy as the skin plus the welding temp may alter the alloy of both filler and sheet.
Rivets the same thing may not be the exact same alloy.

Without an anode corrosion can still develop in any water that is not pure water which ocean water certainly is not.

It does NOT take any internal or external electrical influence to start it.

I think you will find the boat needs some zinc anodes.
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Old 05-10-2021, 06:04 PM   #7
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Is the boat bottom painted?
I looked into putting bottom paint on an alum fishing boat but any I found required a solid barrier coat to prevent interaction with the alum. I was concerned that any void in the barrier would result in concentrated corrosion?
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Old 05-10-2021, 06:24 PM   #8
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Wonít even attempt to try to explain the various corrosion paths with alum plate. However your photos show evidence of what is probably stray current attack. Without a few reference cells hanging off the hull itís not possible to confirm but my gut says yes. Somebody is leaking DC current into the water where this boat is tied up. DC current, as in a battery charger constavolt. It never AC. Move the boat
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Old 05-10-2021, 06:26 PM   #9
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Some of that corrosion, like that near the screw and hardware, might be galvanic with steel.

But, it really looks like pitting, the result if a small nick in the oxide layer letting salt water with chloride contact the metal and sit there. Different allous have different susceptibility. Once it gets started at a point, it gets hard to stop, at least in my experience.

I think you might need to drill or grind clean the pits, weld, grind smooth and paint it to stop it where pitted. Or try to grind or drill clean and fill.

I don't think an anode will help, unless there is steel on the other side of it that we aren't seeing or that was there earlier.
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Old 05-10-2021, 10:20 PM   #10
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Quote:
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There are no electronics on the boat except for a handheld chartplotter. Not even a battery charger, so there's nothing for an electrician to look at. Since it's not connected to shore power can it even be affected by a "hot" marina?
I had not thought of power leakage from another boat. If that is the case then an experienced marine electrician or corrosion tech. may be able to help determine if that is the case.

If leakage into the water comes from another nearby boat it will seek ground through the path of least resistance. THat may be by using the aluminum hull as part of the path.
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Old 05-11-2021, 12:42 AM   #11
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Thanks for the replies. My brother has someone coming down to take a look. We'll see what he comes up with.
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Old 05-12-2021, 08:50 AM   #12
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Please post info on what you find.

Don't put too much stock in my opinion here - but I wonder if there are several different things going on? The second and third photo seem to indicate poultice corrosion where standing water at the seam gets trapped, is deoxygenated, and leads to corrosion. If that is paint that I'm seeing (I can't really tell), then water leads to corrosion under the paint, this lifts the paint and allows more water in which further corrodes. That sets in a never ending cycle. The answer is to keep the area dry, which may be impossible with that carpeting in place. Absent that, I've also been told a couple gallons of baking soda in warm water sprayed into the area changes the Ph balance and helps stop further corrosion. But I've only heard that from one guy (granted, he knew a lot about aluminum workboats) and have never tried it myself. I kept all interior paint off my aluminum for just that reason.

The corrosion around the screws could be galvanic or poultice, I can't tell. If galvanic, then aluminum anodes would be called for. The easy way to check that is with a silver /silver chloride half cell. I assume your corrosion dude could test that with a clip on anode to see if it works before committing to bolting on anodes. But I've never used one of those clip ons before so I can't give first hand advice on it. Also, if removing the screws to make a repair, I believe best practice would be to reinstall the screws with a coating of Tef Gel.

But on the holes where there is no dissimilar metal or place for water to get trapped (some of the holes on first and fourth pic) would indicate stray current corrosion. I'd want to see what's on the other side of those holes to made a better guess.

Sorry I can't be more help, but I hope the guy you've got coming down is well versed in types of corrosion and gives you a better run down and brings a reference electrode to help test the boat in it's slip.

Please post results if you can. There is more mis-information on aluminum corrosion than there should be...even among experts.
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Old 05-12-2021, 09:36 AM   #13
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Okay, looks like your doing your homework itís interesting stuff huh ? Iíve spend many years around aluminum marine structures, fitting and hulls and poultice corrosion this is not. Poultice corrosion is a very weird type of corrosion often found where an alum alloy fitting or machine base is mounted atop a wood deck or a window frame ( examples ) and the substrate remains wet or damp. The resulting aluminum oxide or corrosion product resemble a white toothpaste like material. Itís unlike anything youíll normally ever find on boat but it happens. If the parts are separated and cleaned off you will clearly see wastage of the aluminum. Open boats of this type could suffer from this condition I guess if the conditions are met but interior or exterior hull plating like pictured is not the type of environment for this problem.

There probably is some localized galvanic corrosion in way of the stainless screws and itís more or less expected since there is a certain level of galvanic potential between SS and aluminum. However the screws are not enough mass to create aggressive corrosion as it would be if the screw was aluminum and the plate SS. Iím more concerned with the deep pitting as this appears to be stray current but without more info itís just a educated guess.
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Old 05-12-2021, 09:52 AM   #14
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Quote:
Originally Posted by garbler
Somebody is leaking DC current into the water where this boat is tied up. DC current, as in a battery charger constavolt.
Yes!
Your brother needs to find a qualified electrical sleuth, not just your nextdoor neighbour the house wirer, to trace the source and if the source is anouther vessel, the source AND the marina should be informed.

Could be as simple as a cord hanging in the water. How old is the marina?
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Old 05-12-2021, 11:04 AM   #15
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... and poultice corrosion this is not. Poultice corrosion is a very weird type of corrosion often found where an alum alloy fitting or machine base is mounted atop a wood deck or a window frame ( examples ) and the substrate remains wet or damp. The resulting aluminum oxide or corrosion product resemble a white toothpaste like material. Itís unlike anything youíll normally ever find on boat but it happens. If the parts are separated and cleaned off you will clearly see wastage of the aluminum. Open boats of this type could suffer from this condition I guess if the conditions are met but interior or exterior hull plating like pictured is not the type of environment for this problem.
That is an interesting observation and I hope the OP will update us so I can learn something. But looking at the second picture, I submit that carpeting in an open cockpit boat in the PNW is exactly the type of environment that could produce poultice corrosion...and I guess to be accurate, I should have said poultice or crevice corrosion depending on whether the source is the wet carpet or the crevice where the trim meets the area that is showing corrosion.

In your experience, will poultice corrosion show as a dry powder when the source goes through drying cycles - like in the case of carpet or plywood that gets wet and drys depending on the season? Or does it ALWAYS stay a paste as you describe?
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Old 05-12-2021, 12:13 PM   #16
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Does this boat have bottom paint?
If not then the entire hull could be acting & wasting away as an anode.

If it is outboard powered is the engine tilted clear of the water when
not in use?
Stainless propeller in salt water + unprotected alu hull = bad news.

When anode(s) get installed make sure they are aluminum alloy anodes
and not zinc anodes.


Will be interested in hearing the outcome.
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Old 05-12-2021, 01:08 PM   #17
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Bkay you are most likely correct about wet carpet. I didnít know this was part of the puzzle or missed it, but it could very well satisfy all the required conditions for poultice corrosion but Iíve personally never ran into it. You are also probably correct about powdery poultice when conditions dry out. Again not something Iíve ever seen but certainly possible and makes sense.

Crevice corrosion though possible with Alum is unlikely as there is enough oxygen present to maintain the oxide passive surface I would guess. Guessing is what this is all about without hands-on inspection. However one element that the photos show that makes me think this is stray current is the shiny or bright surface appearance around the pits. This is a characteristic of stray current
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Old 05-17-2021, 06:58 PM   #18
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I finally heard back from my brother. He lives on the other side of Puget Sound in Sequim. To answer a few questions: Yes, the boat has bottom paint; Yes, it's an open boat out in the weather; Yes, the outboard is raised when not in use. There was no DC current found in the surrounding water. It was determined to be poultice corrosion caused by the wet carpet. The corrosion around holes was most likely due to stainless screws. He will be adding an aluminum anode to the hull. Thanks to everyone for their thoughts and suggestions.
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Old 05-19-2021, 07:32 AM   #19
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Thanks for following up with the diagnosis. Best of luck to him. Aluminum is a wonderful material as long as you keep up with corrosion issues.
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Old 05-20-2021, 01:26 AM   #20
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Al anode on an Al boat, interesting.
I would have thought a different metal would be suggested like zinc. Learned something
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