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Old 09-16-2006, 08:34 PM   #1
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Alum steel contact ?

After a cleanup and POR the frame I was thinking about putting something between the belly pan/banana skin and the steel, not that it would last but like duct tape.

Anyone try that?
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Old 09-16-2006, 09:27 PM   #2
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Well, good question to think about!

We did not do anything in particular in insulating the two metals from each other... but on the outriggers my hubby suggested maybe gluing some kind of rubber strips...

Another problem still to look at is that the alum rivits that hold the belly pan are still going to go through to the frame, still have some bimetal contact.

Be interesting to see what others suggest!

Mrs. NorCal Bambi (traveling in S Tardis)
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Old 09-16-2006, 10:15 PM   #3
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If you use POR over all of the steel areas (100% of the steel frame), then you shouldn't need anything. The POR, properly applied, should be enough.

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Old 09-16-2006, 10:21 PM   #4
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But with road vibration the two parts have to rub each other over time, most of my outriggers have small perforations thru the alum.

I know the POR is tough but it may not be enough??
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Old 09-16-2006, 10:28 PM   #5
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Well, there's a couple of different questions in there.

First, the outriggers have holes in them on the top where they touch the aluminum, or, the aluminum channels that rest on the outriggers have holes in them?

There should not be a lot of movement (i.e., none) between the two once everything is put back together. The channel holds the wooden floor which is all bolted to the outrigger. AT least that's how it is in mine.

Also, once the POR cures, you can hit it with a hammer and it will barely scar. It's hard stuff when applied correctly.

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Old 09-17-2006, 08:14 AM   #6
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Perhaps this is a question for the metalurgists out there? If dissimilar metals are a source of corrosion perhaps we could take a tip from the marine industry and provide an alternative for the galvanic corrosion by providing a sacrificial anode. These are used on boats and replaced as they degrade over time.
Those with a better understanding could chime in here please....and thanks.
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Old 09-17-2006, 08:48 AM   #7
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Quote:
Originally Posted by GlenCoombe
Perhaps this is a question for the metalurgists out there? If dissimilar metals are a source of corrosion perhaps we could take a tip from the marine industry and provide an alternative for the galvanic corrosion by providing a sacrificial anode. These are used on boats and replaced as they degrade over time.
Those with a better understanding could chime in here please....and thanks.
Glen,

Interesting thought! I have had exterior corrosion on several aluminum pieces on my '06 CCD that has required factory replacements.....specifically on the rear tail light housings. These are big, solid alum. castings but started to show a significant amount of white corrosion under the clear coat. The factory sent replacements.

My office shares a building with a marine repair service, and the owners thought that a sacrificial zinc anode would solve the problem. They added that it should be a fresh water type. I will be installing one soon to see if it solves the problem. Be sure to have it make contact with both the steel frame AND the alum. skin to be effective.

BTW, the factory had NO suggestions for a fix!
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Old 09-17-2006, 09:05 AM   #8
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Lew, the more I think about it the more I remember. If you install sacrificial anode(s) more is better. I'd be tempted to hit each corner with a small anode rather than try to use one larger one instead. Not sure why I remember some of this stuff but it's in there..... Smaller multiple units vs. single large one.
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Old 09-17-2006, 10:05 AM   #9
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I plan on using thin teflon washers anywhere aluminum meets steel. The first line of defense would be to Etch, Alodide and then Epoxy prime ALL mating surfaces including Aluminum. Common water will start corrosion. This is what we do in the avaition commuity. No two mating surfaces should have direct bare metal to metal contact.
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Old 09-17-2006, 10:15 AM   #10
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Sacrificial anode

I knew that a boat's sacrifical anode operated by actually being immersed in the electrolyte. Also seemed to recall a very good post on the subject, so I searched on the term. You'd find multiple other discussions on the topic too, but especially this post by ABRACADABRA.

On the frame I certainly put my faith in POR-15. With the galvanized steel end caps, bimetallic reaction is everyday bread and butter to an Argosy owner. Letting it get out of hand is where you see the real damage. Pull an Airstream apart and you will not see much degradation if moisture is not present. Even still I have used stainless screws to put the Argosy back together -- and don't understand why, but some people say SS and aluminum react even stronger.

We have a fresh thread started here on an old subject. Being fresh it gets more attention than re-posting to a thread that asked a different question. And that's why I'm not going to chase my own tail to attempt to limit duplicate threads...
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Old 09-17-2006, 11:54 AM   #11
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hi lipets

a large volume has been written here on this topic and every variation....

i found the exact answer to your question in at least 3 threads.

search 'galvanic corrosion' and of the 25 or so at least 10 are useful

serach 'dissimilar metal' and 40 appear.....again 10-12 very helpful

search more narrowly 'dissimilar metal corrosion' or 'aluminum steel contact'

and more great info....narrowed for you.

the zebra post only addresses the sacrifical anode subtopic....that is why it's a zebra for the greater issue, so its useful to read when someone suggests using a sac anode on an airstream...

the threads on metal2metal contain great stuff.

cheers
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Old 09-17-2006, 01:07 PM   #12
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I don't see how the zinc's would work, you would need to put them at every contact point. ps they are as heavy as lead.

Assumming aruendo, when they were fully sacraficed you're back to metal 2 metal.
I think Arowood is on the right track,
we something to place in between like teflon, but washers are to small you need something like 2" wide stripes that can be cut to length, maybe kept in place with vukem
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Old 09-17-2006, 01:16 PM   #13
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From BMW auto's

When bare aluminum joins with bare steel, contact or galvanic corrosion can result. Where aluminum meets steel in the 5 Series main structure, the joints are made using adhesives and rivets, a process known as rivet-bonding. Everywhere aluminum and steel come together, the adhesive protrudes at least one millimeter outside the joints. This helps ensure that direct contact between bare steel and aluminum is completely avoided.
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Old 09-17-2006, 01:51 PM   #14
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I have been doing auto body repair for well over 30 yrs. Anytime you have two pieces of metal held together with a bolt, screw, or rivet in a trailer, car or truck it is going to wear through whatever you put between it. It dosen't seem like there would be that much movement but there is. Most Airstream and Argosy's I have seen the belly pan last around 30 yrs. If you put it back the way it was original, it should last another 30 yrs. Trying to do more than that is kinda like putting seat covers on a new truck seat, you are just keeping it nice for the next owner. If you want it to last 40 yrs. just put a little thicker metal in it.
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