Aluminum jon boats?

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gandrfab
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Aluminum jon boats?

#1

Post by gandrfab »

In Florida we have thousands if not hundreds of thousands aluminum jon boats sitting on carpeted trailer bunks.
And pontoon boats.
I have had a few dozen or so people looking to repair pontoons for pitting and galvanic corrosion, but only one or 2 jon boats.

What makes these boats lives so long?
I know it's not the special care they receive.
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Re: Aluminum jon boats?

#2

Post by welderbob »

Every Jon boat I've seen had a green Zinc finish. Guess its enough to protect the metal.
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gandrfab
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Re: Aluminum jon boats?

#3

Post by gandrfab »

welderbob wrote:Every Jon boat I've seen had a green Zinc finish. Guess its enough to protect the metal.
That green wears off.
I have welded rivets, cracks, and replaced the rotten wood transoms with aluminum.
Some of these sheet metal hulls spend decades neglected, abused, and used.

I have seen corrosion in the transom from SS screws, but almost no corrosion in the hull bottom.
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Corrosion of Aluminum jon boats Bottoms

#4

Post by kmorin »

G&R, poultice corrosion, crevice corrosion, and other water induced cells happen in de-aerated water that becomes acidic. Free flowing rain, salt water or fresh and other free flowing water conditions have plenty of oxygen so that aluminum alloys can 'defend themselves'.

A drop of water that is left to bead up has one surface area but if flattened out- say between two pieces of hard flat surfaces may expand the surface area of the same volume 100X. So the water between to tight places will expose its entrained oxygen at much higher rate to the surroundings and therefore become acidic/de-aerated faster and more completely in less time.

A jon boat is very light hull compared to a plate boat, the plate boat's loading on bunks is higher so the compression of carpeted bunks is greater and the water droplets are therefore at MUCH high surface to volume ratios. Let's just look at one example.

A drop of water between a piece of tin foil, and table top will compress (let's say) from 0.125" sphere or 1/8 tall bead of water to a 0.060 tall spot because the loading per area of the tin foil is some what small. Therefore the area may gain 2x or 3x and the rate of loosing oxygen is increased by 2x or 3x over an undistrubed droplet.

But what if we put some 1/4" plate on that droplet- this loading will mean the single drop may be 6" in diameter or 220" sq'd (top and bottom added) of area!!! from a 1/8" circle to a 200" area???? that's some surface area change.

Therefore I'm guessing that the lighter jon boat is not creating the tighter, narrower, thinner volume to make higher surface area crevices that is presented by the much higher loaded bunks under a plate or pontoon boat? The closeness of the gap in which a poultice or crevice corrosion cell is created is a critical part of understanding this type of aluminum corrosion.

Also capillary action of water does not happen with http://en.wikipedia.org/wiki/Capillary_action bigger gaps as readily as it does with narrow gaps of flow paths. This action will fill a narrow, wetted carpet acting like a wick, between the bunk and hull much more effectively when the gap is narrow from a heavier boat than one that's lighter on proportionally the same bunks.

If you leave an aluminum bolt on fixture on the deck, say you have a rod holder that has a flat base and you bolt it to your skiff's guard deck? The plate is bolted directly to another bare plate of 5086, where the water can form a very narrow film between the two pieces of flat metal with no holes drilled to allow the area to drain so it has to dry from the edges inward to the center of the 3" x 3" base plate.

This will corrode without a boundary film. But if the same plate were installed with a film of plastic (non reactive) or was on 'stand offs' or legs of 1/64" washers on the four corner bolts, so the water is free to flow in and flow out and dry without limit, this area will not corrode like the plate to plate installation will.

So I'm suggesting your observation is the result of the gap or crevice gap in the matter of wet carpet bunks on zinc coated light wt jon boats, compared to plate metal pontoons or hulls where the lighter boat has a bigger gap and the water is free of capillary forces to retain it, and the rate of de-aeration is also affected due the surface relative surface areas.

my 2 cents.

Cheers,
Kevin Morin
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gandrfab
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Re: Aluminum jon boats?

#5

Post by gandrfab »

Thank you.
That makes sense.
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Re: Aluminum jon boats?

#6

Post by JETTYWOLF »

gandrfab wrote:In Florida we have thousands if not hundreds of thousands aluminum jon boats sitting on carpeted trailer bunks.
And pontoon boats.
I have had a few dozen or so people looking to repair pontoons for pitting and galvanic corrosion, but only one or 2 jon boats.

What makes these boats lives so long?
I know it's not the special care they receive.

SWEETWATER :roflmao: :roflmao:
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