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What Creates Aluminum Hull Mushroom like Crystallization?

What Creates Aluminum Hull Mushroom like Crystallization?

by Skoiern IV » Sat Jul 03, 2010 9:04 pm

In the damp areas in my boat for years, I have had spots of Mushroom like Crystallization.
It is around areas that are closed off areas of Salt water Dampness & condensation with a lack of air circulation in areas where the hull is dry there is little or no growth.

I have two sizable zinc anodes wired to the hulls toe rail on a stainless steel lead2.
When the boat is on its mooring, I lower them over the side.
For years, the Zinc anodes remain in tacked showing very little or no electrolysis. .

I have received a number of suggestions that the mushroom like crystallization is created by dampness because of a lack of air circulation or alternatively some say, is it Electrolysis? In seeking a clear answer I find I get conflicting points of view.

I need some expert advice as to how the arrest this problem and what created it?
Here is the Flicker URL http://www.flickr.com/photos/aluminium_mushrooms/ link of photos I took this weekend illustrating the problem.

Boy was I surprised to discover this crop of growth as I have not inspected under the berths for almost a year do to other extensive up grades I have been completing.

I would be most grateful for Any expert input to finding a solution to arresting this problem and what created it?

John
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Re: What Creates Aluminum Hull Mushroom like Crystallization?

by colemj » Sun Jul 04, 2010 6:40 am

That looks like aluminum hydroxide from poultice corrosion, which is cause by dirt, etc trapping moisture against the aluminum. It is probably exacerbated by salt from the air being dissolved in the moisture.

Zincs protect against galvanic corrosion and will provide no benefit for poultice, chemical or electrolysis corrosion.

To mitigate poultice corrosion, you have to keep the metal clean and dry. You might be able to etch and epoxy coat the areas, but that would have to be done very expertly or it will make the problem much worse in time.

Mark
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Re: What Creates Aluminum Hull Mushroom like Crystallization?

by Skoiern IV » Sun Jul 04, 2010 9:23 pm

Mark,

I never heard of Poultice corrosion, I spend the morning looking it up on the net and found an article by Steve D'Antonio. I cut and pasted the key identifier that confirms your correct as it matches Steve D'Antonio marine consultant description eg..

"byproduct of poultice corrosion, aluminum hydroxide. If you've never seen it, aluminum hydroxide looks something like freezer-burned vanilla ice cream, it's often gooey until the blister's surface breaks, at which point it will dry out and revert more or less to a powdery state."

This is exactly what I'm experiencing on the hull. Now I know what it Poultice corrosion is the question is how to repair it?

Many Thanks for your input
John
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Re: What Creates Aluminum Hull Mushroom like Crystallization?

by osiris » Wed Jul 07, 2010 8:34 pm

There are "pencil" sandblasters that can get into tight areas. Use one to remove the bulk of the corrosion and then inspect to see how deep the corrosion goes into the hull. Assuming a hull plate, if the depth is anything other than superficial you are looking as cleaning and welding a cover over the area. If a plate to stringer problem probably just re-welding after a thorough cleaning will suffice. Final cleaning is done with Aluminum cleaner that you can purchase at boat and auto paint stores. It is important to get every microscopic particle of the corrosion out and get to "virgin" metal. After repair then Alodine the area and paint it.
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Re: What Creates Aluminum Hull Mushroom like Crystallization?

by Skoiern IV » Wed Jul 07, 2010 8:45 pm

Osiris,
Thank you for your reply and solution. I never knew there are "pencil" sandblasters sounds like a nice solution will see if I can hunt one up. The problem looks ugly but it is also fixable.

What do you think of using Devcon wet surface putty to do the repair once treated on the water? Getting a welder in there seems like it would be difficult under the cockpit as that is a tight area.

All the best
John
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Re: What Creates Aluminum Hull Mushroom like Crystallization?

by CaptCharles » Thu Jul 08, 2010 7:28 am

I built a pencil blaster with an airbrush- I used a plastic funnel at the top (where the paint goes in) and removed the airbrush needle- viola!

Does a nice job with a small air compressor or dive tank.
-Charles

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Re: What Creates Aluminum Hull Mushroom like Crystallization?

by Skoiern IV » Tue Sep 07, 2010 6:11 pm

Thanks you everyone for your suggestions.

It turns out that there is a weeping of sea water from using Sikaflex which expands with time if submerged on the flange joints to transfer water from port to starboard tanks. This dampness and enclosed area looked dramatic in the photos. However on cleaning it up it only scared the hull plate and not pitted it as I first suspected.

The question is what to use on connecting flanges if Sikaflex is not up to the job?

John

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