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Old 08-06-2012, 03:05 PM   #1
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Galvanized steel as an alternative to aluminum belly skins

I was wondering what you folks think of using Galvanized Steel for the flat belly skins as opposed to aluminum. The reason I am bringing this up is that aluminum does not seem to be a good material for the belly skins. There is always the problem with two different metals and galvanic corrosion. My holding tank pans look perfect with no rust or signs of corrosion. The aluminum looks terrible and every place there is a rivet there is a big hole where the aluminum lost to the steel frame in the corrosion war. I am seriously thinking of using galvanized steel to replace my ugly hole filled belly skins in the front section that I have not gotten to yet.

If you add salt to the above problems the aluminum really suffers as we have seen on trailers less than 10yrs old that already need new belly skins after being exposed to northern salt.

I would use aluminum on the side wraps and put them outside the galvanized belly skins to protect them from the steel.

Perry
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Old 08-06-2012, 04:23 PM   #2
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Hi Perry,

I'd have no issue with a galvanized belly skin. In theory you could use a lighter gauge of material than with aluminum which would help offset any weight difference. Belly skin...similar to an automobile windshield, should for all practical purposes be considered as a consumable item. It is meant to protect from debris and weather and may be sacraficed in order to protect other potentually more vital/expensive components from damage. It may not be as light, but I see no reason why it isn't viable as long as steps are taken to minimize galvanic reactions between aluminum materials.

Just my $0.02.

Kevin
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Old 08-06-2012, 04:54 PM   #3
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Perry, I have a 1980 Excella that is coming home from the lake in the fall and my intention is to drop the belly pans and clean up the frame and paint with POR 15. I have the same situation with the war between dissimular metals. The belly pan is still intact but if I am going to drop the belly pan I would rather replace the pans with a procuct that I only have to do once. If you find a solution, I would appreciate it. Thanks
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Old 08-06-2012, 06:25 PM   #4
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Since a PO replaced part of my belly skin with galvanized I feel qualified to tell you what happened. Any place the galvanized overlayed the aluminum the corrosion was very noticeable. The zinc galvanizing held up well but the aluminum was destroyed! If you are thinking of going this route you need to spend considerable thought on ways to isolate the dissimilar metals to avoid any galvanic reaction. The easiest way to avoid this is simple-DON'T DO IT!
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Old 08-06-2012, 11:02 PM   #5
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Has anyone thought of using fiberglass or a composite material? There wouldn't be any electrolysis.
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Old 08-06-2012, 11:15 PM   #6
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They make a rubber roofing tape that comes in 4" wide rolls, that might be sandwiched between two layers of different kids of metals to reduce the corrosion.

Also you could always undercoat the belly pan after it is installed. I wonder how por 15 would hold up on thin flexible metal like a belly pan.... If I used Gal sheet, I would prime and paint. The zic coating only lasts so long..
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Old 08-07-2012, 09:40 AM   #7
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Ok guys wait a minute. Zinc should be anodic to both steel and aluminum. The aluminum should not suffer. You should be able to put the zinc plated steel next to the frame and then the aluminum side wraps on top of that. The question is what type of fastener should be used. My thought is stainless steel. Aluminum is going to corrode I think but the zinc may protect it.

Salt on bare aluminum is going to eat it up no matter what you do unless you anodize it.

POR15 is going to help but when you drill a hole to put in a screw or rivet you loose the electrical insulation. You could wet install the fasteners with POR15 but it would be messy. Wet install means you dip the fastener in the paint and install it wet. This is done on mil spec stuff and stuff for NASA. I think we used Koropon on the space station stuff.

http://www.ppg.com/coatings/aerospac...or_Primers.pdf

Perry
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Old 08-07-2012, 10:32 AM   #8
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Cool we are getting advice from a real rocket scientist!

Good to know about the gal/ zinc interaction, I assumed it would be the same as steel. Why does stainless steel not have the same effect as normal steel? Is it the nickel and boran that is added to make stainless?

Thank you Perry great info!
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Old 08-07-2012, 11:34 AM   #9
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Stainless steel has a tough oxide layer that protects it from the elements. Out of the three metals Zinc, Steel, and Aluminum the stainless will be the last to go. Some will argue that the stainless will cause corrosion on the other metals but I don't think that is the case unless you are submerged in salt water.

Perry
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Old 08-07-2012, 12:17 PM   #10
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Without getting into specifics - I offer my two cents worth.
Depends on the grade SS, type 304 and 308 are low grade and would be subject to oxidation much faster than type 316 with a higher nickel content. Regardless, SS would likely be too heavy and add too much weight, hence the reason for aluminum.
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Old 08-07-2012, 01:32 PM   #11
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I was thinking stainless for the rivets through the aluminum skin; zinc plated steel belly pan and steel frame. Weight is a consideration but is say .030 steel going to be excessively heavy?

You are welcome to check my numbers. Steel has a density of .284 lb/cu in. The front section of my trailer is about 20ft x 5 ft which is 14400 sq in. Multiply that by .030in and you get 432 cu in. Multiply that by the density and you get 123lbs. Aluminum of the same thickness would be about 1/3 of that. I am sure you could go with thinner material than .030 since it is steel. However, the thickness is more important to it not sagging than tensile strength.

I don’t think that 100lb weight penalty would be that bad of a trade off if you don’t have to worry about rust through etc.

Actually now that I think about it, I would not use rivets at all but stainless steel screws with zinc plated washers to attach all the stuff together.

Perry
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