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Old 05-19-2005, 03:13 AM
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galvanic corrosion, aluminum on steel?

I will be building an aluminum floor for my '54 Chevy and am wondering how to treat the join with the steel at the rockers and everywhere else it comes into contact with steel. I remember studying some basic metallurgy in my aerospace engineering days and seem to remember that if dissimilar metals are in contact with each other in a corrosive environment, I'm going to get some accelerated corrosion at the point of contact.

Should I build some kind of a lip all the way around where the aluminum floorpan can sit on a rubber seal and be bolted down? What are your thoughts?

Rich

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Old 05-19-2005, 05:05 AM
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Panel adhesive may be an option, it would provide the seal between the two metals and also offer increased strength to the bolted in assembly. Check with Lord Fusor.
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Old 05-19-2005, 07:50 AM
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you would only have a corrosion problem if there was no ground. if the 2 dissimilar metals are in contact and they are grounded to the battery, which they should be then you wont have any more of a problem than you would with any other joint. the adhesive just mentioned would obviously insulate the conntection but the seam would be sealed so that would work fine but to be sure bed the pan with the glue then tack weld one corner or put a sheetmetal screw in one or each of the corners to make the connection. normally in a dry enviorment i wouldn't worry about any of this but if i remember right your near the water and salt air.
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Old 05-19-2005, 09:21 AM
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Use steel for the floors. You want to weld them in to be a integral part of the chassis. Its a safety item. Pop riviting floors in or mechanically fastening them in will allow to much flex in the case of an accident. Remember, your seatbelts bolt through that floor !!

You will get a glavanic reaction no matter what, unless you isolate the materials. Even glue will allow contact in places. Go with steel...
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Old 05-19-2005, 11:28 AM
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I would suggest the steel route also..........cheaper, easier to work with, and stronger.
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Old 05-19-2005, 11:46 AM
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Quote:
Originally Posted by Beenaway2long
Use steel for the floors. You want to weld them in to be a integral part of the chassis. Its a safety item. Pop riviting floors in or mechanically fastening them in will allow to much flex in the case of an accident. Remember, your seatbelts bolt through that floor !!

You will get a glavanic reaction no matter what, unless you isolate the materials. Even glue will allow contact in places. Go with steel...
Thanks Beenaway2long, but I'm using aluminum to loose weight, I'm using it in a lot of places. The floor will be braced very well for strength and I want to keep it isolated from the steel rockers and wherever it joins the body but still have a strong bond. That's why I posted the question.

The frame is getting a large 'X' member anyway because of the retractable roof, so the whole chassis should be plenty rigid. Good point on safety though, I'll be sure to have steel under the floor seatbelt mounts.

You answered my question about galvanic corossion though, thats what I thought, thanks.

I'll keep thinking about this one.

Thanks,

Rich
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Old 05-19-2005, 11:48 AM
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You can strengthen it up with some square & round tubing if your set on the Alum. flooring.
Or, You could just "cover" the existing floorpan with Alum. by some angle pieces tacked to the existing floor,tunnel.
I'm no scientist but I believe you'll be OK with the flooring on the corrosion issue.Like Jim said,As long as there is a common ground. Which IMO is what causes this anyway as the current is using the spots where contact is made to conduct it's "circuit".
Electricity of any sort will take the path of least resistance,so a good "connector" between the 2 should keep it at a min. if not completly.
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Old 05-19-2005, 12:34 PM
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corrosion

The galvanic corrosion will happen when the 2 dissimilar metals contact each other. It has nothing to do with grounds or current traveling between them. It it because of the chemical make up of the 2 metals. There is a compound called penatrox. It is used by electricians to stop this typ of corrosion. I am not sure if it would work in your application or not. It is kind of messy and i am not sure how it would react to being out it the weather. I would think if you used some typ of poly sheet between the 2 it would stop it from happening......

keith
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Old 05-19-2005, 02:03 PM
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paint

I believe if you just put a good coat of epoxy primer on the contact areas of the aluminum and body and used stainless screws or rivets one could sleep well..

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Old 05-19-2005, 09:24 PM
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Sorry,but galvanic corrosion IS all about an electrical CURRENT between the two metals.
And the Penatrox,Lynox,ect is used on Aluminum to Copper connection for this purpose. It prevents #3 below.

OK, I did some checking on this and here is the deal.

Three things need to occur for GC to occur.

1.Two electrochemically dissimilar metals
2. An electrically conductive path between the two
3.A conductive path for metal ions to move from the more anodic metal to the more cathodic metal.

If ANY of these 3 conditions does NOT exist,galvanic corrosion will not occur.

Basically, isolate the two, somehow. AND,
Moisture or equivelent has to be present to provide the path for #3.

Heck, get a sacrifical anode from a plumbing supplyer and mount it under the frame if your that concerned.
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Old 05-19-2005, 09:25 PM
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I don't think electricity has too much to do with the corrosion problem, seen it to many times with non electrical parts, AC lines are one example, any fastener with steel and alluminum components for that matter. Anything would work to isolate the two materials but panel bond would offer extra strength. An epoxy or urethane seam sealer would also work good. Roofing tar
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Old 05-19-2005, 09:42 PM
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OK, Think battery.
This is exactly what happens inside a battery only on a much larger scale.
There is an electrical current happening, granted it's VERY SMALL but that's what causes the breakdown of the metals. One is consuming the other for it benefit and the only way this occurs is thru ions moving (current) between the two.
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Old 05-19-2005, 09:49 PM
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I suppose all cunductive materials will have some small amount of electrical current traveling through them in an automobile? I'm probably wrong.
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Old 05-19-2005, 10:12 PM
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LOL...
It dosen't have to be metal.
Check this out.
NASA
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Old 05-20-2005, 03:05 AM
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According to that site, this is the definition of galvanic corrosion:

Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. It occurs when dissimilar metals are in contact.

That's exactly what I'm trying to avoid. Our moist, salt air is more than enough of an electrolyte to be a problem. Most cars don't last more than a few years without rust coming through fresh paint. Cape Town has a reputation for it, just like L.A. has a rep for it's traffic and smog.

This is also why I'm trying to get rid of every last speck of rust on the body, and I'll do the same when it comes to building the chassis. Inside of lapped seams and really innaccessible cavities, I will have to open them up as best I can, clean as much as possible and stop the rust in there from getting worse by sealing it off from oxygen using a cavity wax or even the dreaded POR-15. I've had to take off my drip rails completely to make sure I get rid of the rust under them.

It will all get a heavy coat of epoxy primer, including any panels I make from aluminum so that I give this car the best defense I can and a chance to survive for a while.

I'm just not wanting to overlook things like galvanic corrossion since I am intending to use two dissimilar metals together in rebuilding the body. Most people don't even know what it is let alone what steps can be done to prevent it. The only reason I knew about it was because I learned about it in the first half of a BSc in aerospace engineering (that I never finished ).

Anyway, thanks for your help, I think this has ended up becoming quite an interesting discussion.

Rich
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