Pitting and Corrosion on Inner Bottom of Hull

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JStauf
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Pitting and Corrosion on Inner Bottom of Hull

#1

Post by JStauf »

Looking for some help and/or advice.
Recently took my Harbercraft (Jetcraft) 18 ft welded aluminum boat to a friends aluminum fab shop to remove the marine plywood floor and replace it with aluminum diamond plating.
This was the first time some of this flooring had been up since I bought the boat new in 2002. The boat was used EXTENSIVELY in salt water the first half of the time I've owned it. Large amounts of blood and fish slime washed into the bilge and washed out with multiple 5 gal pails of saltwater, pumped out by bilge pump.

Upon pulling up the boards the first thing I saw was the most SEVERE pitting and corrosion anywhere the green treated plywood floors had been in contact with aluminum. I then started prying out all the blown in urethane. Some of it was already loose and some was still stuck to the aluminum. The aluminum on the inside bottom of the boat was covered in years of moist dark slime.
We then hot water pressure washed the floor we found even more deep pitting on the inside, bottom of the hull. It was disturbing enough that I drained and pulled my in-floor plastic fuel tank to inspect and clean underneath it. Pitting and corrosion were under the tank also.

Some pits appeared to be 2/3 the depth of the thickness of the bottom of the boat. I spent days scrubbing and Zep-A-Lume Acid washing the corroded pits. I was having huge problems getting the corrosion out of the bottom of the pits with brass brushes and Scotch Brite. Ultimately out of frustration I had to resort to a heavy duty Stainless Steel brush mounted to a grinder. Even dishing out some of the pits I still couldn't get EVERY bit of corrosion.

With most of the corrosion removed, but with some spots not to my satisfaction, I decided NOT to seal everything up with epoxy primer like I had planned at first. Instead I coated the bottom of the boat with a product I have used previously on aircraft called CorrosionX.
The shop then went to installing my new aluminum floor. They attached rubber strips everywhere the new floor touched the boat structure. And.........just like they always have they installed the rubber strips with SILICONE!! instead of 5200. I have since read on this website of silicone corrosion problems.

External Zincs look IMMACULATE!
NO electrical items have EVER been installed to be grounded thru the hull!
The ONLY place there is corrosion on the boat are the places detailed above.
Interested on any thoughts on how my pride and joy boat could have ended up this pitted!
Welding shop said only way to fill the pits in by welding would be to drill out the pits through the hull and fill them in because of the contamination of the corrosion.
I spread a thickened mixture of epoxy to fill the pits in the area below the fuel tank before reinstallation.

Thanks for any ideas!
John


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Below the fuel tank. This is after MULTIPLE acid washes. Notice all the black spotted pits.
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Close up of below the tank.
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Last edited by JStauf on Tue Dec 30, 2014 12:22 am, edited 1 time in total.
JStauf
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Re: Pitting and Corrosion on Inner Bottom of Hull

#2

Post by JStauf »

A few more pictures of the project above.

Pitting in the floor, where urethane had been blown in.
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This is corrosion where the treated Marine plywood was in contact.
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All back together and hidden!
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Chaps
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Re: Pitting and Corrosion on Inner Bottom of Hull

#3

Post by Chaps »

That is unfortunate, boat had a bad combination of treated plywood decks that leached moist chemicals (likely including copper) into the bilge where it had no chance to flush out because of the foam flotation. The innards of the hull has been marinating in an acidic, copper infused broth. Really poor design and build. It would not surprise me if the alloy they used for the hull was also not a grade ideally suitable for marine immersion use. Good luck, you did what you could, don't venture far from shore and wear life jackets.
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goatram
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Re: Pitting and Corrosion on Inner Bottom of Hull

#4

Post by goatram »

Hopefully you used screws to hold your new floor in. When a hole develops fix it and use it some more. Now that you cleaned it and did not reinstall the foam your boat will be fine. Keep it drained and throw some soap in the bilge every once in awhile to neutralize the acid content in it.

Your friend can weld up the holes after he drills it out.

Remove the silicone and DO NOT use Brass Brushes. Use Stainless Steel ones instead.
John Risser aka goatram
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JStauf
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Re: Pitting and Corrosion on Inner Bottom of Hull

#5

Post by JStauf »

Thanks for the response guys! Yes screws were used to hold down the new floor. Any thoughts on the CorrosionX bath I gave the floor. Works great on airplanes, but I understand they don't usually get near as wet. I was surprised how much of the corrosion couldn't quite get out of those pits all the way, even using a VERY abrasive stainless wire wheel, I would still get a little dark color when I reacidized many of them.

I'm going to get a closer look at which brand of silicone we used, but I'm afraid it had the vinegar smell, removing all of that will be quite the job. Pretty sure it was Silicone II

So........is that the consensus then, the green marine copper treated wood and the urethane blown in holding the moisture in place???

Appreciate it!
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goatram
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Re: Pitting and Corrosion on Inner Bottom of Hull

#6

Post by goatram »

Yes the treated wood and the foam helped hold the water next to the plate. The corrosionX might not stop the Corrosion much might even accelerate it due to the encapsulation of the corrosion. Lack of Oxygen. Aircraft use LPS 3 or Dinatrol in their bilges. A wax type of product that seals water out. Next time your in there clean it out and leave it bare with frequent rinses along with a annual scrub to get the fish blood and debris out. :deadhorse:
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kmorin
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Re: Pitting and Corrosion on Inner Bottom of Hull

#7

Post by kmorin »

JStauf, here's a link to one of the threads about pits where I tried to illustrate the problems with pitting. The pits aren't conical so they can't all be drilled out with a cone, OR with a drilled cylindrical hole. They're sort of like icebergs a little, there can be a whole lot more under the surface.

viewtopic.php?f=4&t=3900&hilit=+corrosi ... 982#p31813

this is why sealing the cells can be a problem and they continue to react with new acid washes since the acid will often neutralize or react before the entire cell's lower galley is cleaned fully.

I agree the combination of copper treated plywood and the foam to hold the newly galvanic solution of water and copper to the hull in the bilge is the main cause. I'm not sure of the depth, but having been involved with this type of pitting before I think it would be best to have all the coatings off so you could periodically wash.

One thing is for sure, neutralizing any acid corrosion with baking soda water or even house hold ammonia in water will do more to arrest the acid cells than having them sealed with some acidic material left in the pits. Also the copper solution probably just started the problem off fast, meaning that the rain and rinsing of the free copper ions to make the water a galvanic bath probably stopped in a few months when the free copper was washed off into the bilge. But the crevice corrosion sites have continued from the foam keeping thin films of water (acidic water) from rinsing the bilges with floods of fresh water (the best except for dry bilge).

Unfortunately the boat's entire lower panels are at risk since its almost impossible to know which pit is how far through? So the boat may be fine for years or being leaking next time she's afloat. I don't think it will become a torrent or flood so if you just watch the bilge then its "manageable". If you'll be welding up the pits? I'd suggest that the welds be TIG only and you rig a laying down shelf for the welder to be able to lay on his stomach with his arms supported at the elbow otherwise you're expecting someone to work below their knees with a hand torch for hours and that's not going to be very pretty.

Cheers,
Kevin Morin
Kenai, AK
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bfetcho
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Re: Pitting and Corrosion on Inner Bottom of Hull

#8

Post by bfetcho »

This is a great post, I found a few answers I was looking for. I have a similar problem with my wood floorboards sitting on top of foam filled spaces. I too took all of the foam out, wire wheeled (Stainless steel) all the metal, and finally acid washed the bilge and also those areas containing foam. I definitely had some corrosion going on down there but maybe not quite to extent that JStauf had to deal with.

A couple issues I noticed after doing this:

1) The areas that held the foam will also hold water, as in it will not drain. The water has no where to go if it gets in this space...no wondering it was corroding the metal.

2) There are two boxed stringers running on either side of the bilge (Hewescraft design) that are filled with foam. These beams have holes drilled in the top of them I would assume as a port to insert the foam nozzle during production. The beams do not have continuous weld against the hull as there is foam that expanded out of several spots along the hull in the bilge. What really worries me is that there must be water int there too right? How to I get enough oxygen to this space to slow down the corrosion because it looks like there is no way to pull the foam out? The other foam areas were not corroding the hull directly but I believe this is a definate possibility inside those beams.

It looks like the best plan for the aluminum structure is to keep it bare. What about the marine grade floorboards? They have vinyl on one side but they are getting saturated from the bottom up. Would it be a bad idea to water proof the underside? Paint? Poly?
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kmorin
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Re: Pitting and Corrosion on Inner Bottom of Hull

#9

Post by kmorin »

brett, sorry not to notice this post, as you can imagine, being the AAB.com's resident corrosion 'gnat-see' and mill scale ranter, I have opinions about the boat's condition and what may be some help in contemplating cures or corrections?

I have argued in the past, on the site, that if a boat has a welded void, that void should always be air tested and that means foam of any type put in that void is a contradiction in logic. But let's all agree, I'm not the owner/designer/engineer/lawyer/planner for any major boat building firms! There are many sides to the foam in metal boats discussion and I'm only voicing my point of view- I don't do it except above the waterline as sound and heat insulation almost exclusively in the cabin areas.

#1 I agree, kind of a foregone conclusion that wet foam against an aluminum surface will promote corrosion sites' origins.

I prefer to either weld engine beds/major folded longitudinals solid OR leave them fully limbered where water can rinse (even if this is done as maintenance) through those areas to flush out any corrosion enhancing bilge materials. (Your #2 Q?) Yes, if your hull has poured in foam, in welded in captive volume that is not sealed and pressure tested; there is a very good change that the foam has promoted a crevice/poultice corrosion cell site where the metal has trapped stagnate water film between the bottom of the foam the inside of the hull.

To see if the foam is promoting corrosion you may hole saw a side wall opening or drill and excavate some foam? Then using a bore scope (not expensive today's tool market or tool rentals) see if you can look directly at the surface: Alternative method- get some litmus paper and wet the upper hull side of the engine stringers/folded hull longs see see any moisture seeps out the lower (toward keel) edge? IF so? (then) soak litmus paper (ph test) with this bilge water and IF it shows an acidic color on the strip ? = you got corrosion going on.

What to do? well is you're not taking the foam out (not sure how best to do that?) then fill the bilge to a 1' or so with an alkaline water solution? Plug the boat, mix some household ammonia in a bucket and mix in with a flooded bilge. This will at least work toward neutralizing the acids? Test again with ph papers when drained and dried. Not saying this is a cure, but it will help slow down any crevice corrosion cell sites' action.
bfetcho wrote:What about the marine grade floorboards?
use as patterns first then: Land fill.

If you're going back with cellulose floors/decks/soles? buy the material and epoxy encapsulate - penetrating or soaking epoxy being the sealer designed for this (the Rot Doctor's products) and when soaked a few coats; top coat with a thicker/coating/paint version of (WEST/ System 3 / major brand name) epoxy until the cellulose is a plastic product; then glue on vinyl or covering w/paint. When installing use some barrier like vinyl carpet runner as ribbed 'gasket' or other bedding that will not promote a stagnate film of water on the framing elements' top surfaces.

As mentioned above, another method to coat the foam (if you leave it) to aluminum surface under the long's you've mentioned are filled- would be flood the area with one of the various penetrants goatram mentions- they'd help seal the corrosion sites so that more acidic compounds would excluded slowing any active corrosion and preventing other sites from forming- but its a fall back measure compared to removing the foam if possible. Are there any foam solvents? I know that styrene foams will dissolve in acetone... so are there any urethane foam solvents? Might work to pour some solvent in, rig a vac tank capture vessel and dissolve the foam and extract the residue?

Hope this helps some in your planning,

Cheers,
Kevin Morin
Kenai, AK
kmorin
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