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Old 21-12-2006, 22:09   #16
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Styrofoam and contact cement...

The previous owner of my steel boat glued 20mm styrofoam sheet to the upper inside of the hull with what appears to be contact cement.
I have my misgivings but it seems to work ok.
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Old 22-12-2006, 04:09   #17
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Could I suggest (if you are one that can find copious rejects to use) empty 2 gallon wine bags - from inside cardboard wine casks?
These can be washed out - inserted via quite a small hole into underseat or bow / stern voids - and blown up by mouth to create low cost boyancy bags.
Merry Xmas
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Old 22-12-2006, 04:17   #18
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Spraying Aluminium / Bags as an alternate

Quote:
Originally Posted by unbusted67
How does anyone suggest securing the styrofoam to the hull of the boat? I know it is easy to stuff a bunch of blocks into the lockers etc. but what if you have places that are exposed to foot traffic or weather.
I know someone who had a 13 ft aluminum work boat. He needed all of the space he could afford so rather than putting big chunks in the cabin he sprayed foam into the many nooks and crannies of the boat. How he could know that this would afford him buoyancy in a time of crisis is a mystery to me but the thing actually sank on he mooring during a blow and well, didn't sink - if that makes any sense. Anyway the point of this diatribe was to see if anyone had any thoughts of spray foam vs. styro chunks.
I've been told that if anyone coats seals an aluminum surface just take care its been anodized first.
If not, the subsequent lack of oxygen to the surface will prevent oxidation - and without another coating I understand that oxidation is needed to prevent further corrosion to the metal.
The chap who advised me on this watched his aluminium work boat sink as they had voids 100% enclosed and starved of oxygen - it simply corroded over 4 years from the inside out.

A low cost alternate to foam which has been mentioned before on this forum is the good old fashioned plastic bladder one finds inside carboard wine casks. Washed out and inflated by mouth - one can use multiple bladders to fill a void and provide enough boyancy to keep a swamped dinghy afloat - and they would be a lot lighter than blown foam.

Merry Xmas

JOHN
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Old 22-12-2006, 11:19   #19
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No that is not possible. Oxidation is a reaction with oxygen. It doesn't matter what the alli is coated in, if it is a barrier to oxygen, it won't corrode. This is assuming it is normal oxidation. The issue you friend could have had may not of been oxidation. It may have been due to a acid build up under what ever the alli was coated with. Or there may have been a nother metal in contact. One of the big killers that many don't think of, is having treated timber bolted to the alli. The Arsnic and other metals in the timber preservative is in a high enough concentration to cause elctrolysis and eat away the alli.
I am not sure what you meant by "inside out". If you are talking about the alli internally, then I suggest he had a compleatly different problem again. Either the wrong type of alloy i.e. not marine grade, or faulty sheets. But that would be vary rare and highly unlikely.
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Old 22-12-2006, 14:39   #20
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Quote:
Originally Posted by Alan Wheeler
It doesn't matter what the alli is coated in, if it is a barrier to oxygen, it won't corrode.

I am confused by this sentence. Are you saying that oxidation won't occur and therefore the metal won't corrode? Or are you agreeing that in order for alai the not corrode some oxidation must occur. Please clarify this a little for me.
Also, Are these people talking about the bags that come in the really crappy boxes of wine? Or is this something totally different. Do you guys have boxed wine down there? If not you are lucky. It is the spam of alcohol.
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Old 22-12-2006, 16:06   #21
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Sorry. Aluminium will ONLY corrode if the
"pure" unprotected alloy is exposed to oxygen. This can be an issue if Ali is violently exposed to heat and shear exposing the "pure" unoxidised metal. It will spontaniousely combust like magnesium does and can be very difficult to stop once burning.
We all know Ali is a passivating metal. Meaning that it spontaneousely produces a surface oxide that becomes a barrier to oxygen and thus stops the oxidation process from continuing. "Coating or sealing the surface" as you originaly said, even if it meant the removal of the oxide somehow, would still sheild the Ali from oxygen and prevent oxidation just as the natural fim does. However, if the "alien" film is scratched, the imdeiadte surface below will instantly oxidise and be protected.
Notice I am using the term oxidation and not corrosion. Although the two are the same, I consider (rightly or wrongly) that corrosion is more of an electrolytic errosion issue than oxidation. And this is the most likely culprit of the orginal situation.
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