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Author Topic: differant types of metal fastened together  (Read 2426 times)
robertglines1
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« on: April 18, 2011, 08:06:22 AM »

Decided since we have so many new bus nuts we need to revisit basics. Back in Jan we covered basic elect and plumbing. How about a discussion on possible corrosion caused by attaching alum directly to mild steel or stainless? Should it be separated by a mechanical barrier or will paint do? What kind of fasteners?
« Last Edit: April 18, 2011, 08:13:16 AM by robertglines1 » Logged

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« Reply #1 on: April 18, 2011, 08:08:50 AM »

I know on my '77 AMGeneral, great pains were taken to separate the steel side and roof beams from the aluminum skin.  Hence I have no cancer on the outer skin (there are a couple of very small spots that are really not worth worrying about).  Good Luck, TomC
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« Reply #2 on: April 18, 2011, 08:28:12 AM »

Here is one of the easier to understand explanations on the issue of galvanic corrosion.   http://www.ssina.com/corrosion/galvanic.html

you'll note that stainless steel and aluminium are far apart on the chart, and aluminium is more anodic - and the anode corrodes while the cathode does not.  So un-coated, un-anodized aluminium in direct contact with stainless steel is bad.  Mild steel, on the other hand, is very close to aluminium on the chart.  That means that while the aluminium will still corrode if in direct contact, you need a pretty good electrolyte to make it bad.  Like salt water, road salt in a vehicle, etc.

so coating the more anodic material is a good thing, anodizing aluminium is good, paint, plastic, tape, adhesive bonding, etc, all work.  On the other hand, pop-riveting aluminium to stainless steel as a structure on the underside of the bus and then driving it in the winter with road salt is a sure-fire way to be doing that repair again soon...

Brian
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« Reply #3 on: April 18, 2011, 08:46:17 AM »

Timely thread! Thank you!
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« Reply #4 on: April 18, 2011, 10:39:34 AM »

  On the other hand, pop-riveting aluminium to stainless steel as a structure on the underside of the bus and then driving it in the winter with road salt is a sure-fire way to be doing that repair again soon...

Brian

  I keep going out and admiring my old new to me MC5. I thought I had read MC5's were stainless skin up to the tops of the cargo bays, but the skin up to the windows looks stainless too. Huh

  The stainless/aluminum thing, if its so bad, why do all the outboard boat engines use stainless steel bolts?? Same thing with aircraft, stainless steel screw kits to replace all the rusty iron/steel screws are quite common.
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« Reply #5 on: April 18, 2011, 10:59:37 AM »

My 5C is aluminium on mild steel framing  above the belt line (top of cargo bays) but earlier MC-5's do indeed appear to have the stainless siding up to the windows.  My 5C is pretty much stainless framing below the belt line, with stainless siding.

Brian
« Last Edit: April 18, 2011, 12:44:07 PM by bevans6 » Logged

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Sean
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« Reply #6 on: April 18, 2011, 12:20:42 PM »

... possible corrosion caused by attaching alum directly to mild steel or stainless? Should it be separated by a mechanical barrier or will paint do? What kind of fasteners?

I would not put aluminum and steel together in an outdoor application.

We had our steel roof replaced with an aluminum one early on in the conversion process.  We used butyl rubber tape on all the trusses before the skin went down, and butyl-sealed rivets throughout so there is no metal-to-metal contact.

The stainless/aluminum thing, if its so bad, why do all the outboard boat engines use stainless steel bolts??

Somewhere on that outboard engine you will also find a hunk of zinc, known as a "sacrifical anode."  Since zinc is even less noble than aluminum, the zinc will act as the anode for the entire system, gradually being eaten away until its gone.

"Zincs," as they are called, need to be inspected and replaced on a regular schedule.  If you forget to replace the zinc, then, yes, the aluminum parts of the outboard will start to corrode from being in contact with the stainless parts.

-Sean
http;//OurOdyssey.BlogSpot.com
« Last Edit: April 18, 2011, 12:22:31 PM by Sean » Logged

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« Reply #7 on: April 18, 2011, 04:49:00 PM »

Zinc Chromate primer is an excellent barrier between Al and anything else. Has always worked for me.

Be  advise it is toxic so wear a mask.
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« Reply #8 on: April 18, 2011, 06:12:25 PM »

Zink Chromate is not worth crap any longer,,, EPA mandated the ZINK removed, so now is junk.>>>Dan
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« Reply #9 on: April 18, 2011, 06:38:26 PM »

Somewhere on that outboard engine you will also find a hunk of zinc, known as a "sacrifical anode."  

-Sean

  I know about the anodes. I also know what plain steel and iron bolts will do in an aluminum casting. Ive taken apart outboard motors and other aluminum engines that were 30 or 40 years old and never had much trouble until I would run into an iron bolt someone had mistakenly replaced with an original. It really doesnt take long to bond the two together so hard your forced to drill it out and retap. Ive never had to drill out a stainless bolt.

  Not saying that chemically they belong together, just saying ive never seen it be  problem like iron is with aluminum.
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« Reply #10 on: April 18, 2011, 06:52:53 PM »

Zink Chromate is not worth crap any longer,,, EPA mandated the ZINK removed, so now is junk.>>>Dan

OK how can they call it Zinc Chromate with no Zinc?
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« Reply #11 on: April 19, 2011, 05:53:33 AM »

(snip)  How about a discussion on possible corrosion caused by attaching alum directly to mild steel or stainless? (snip)

Ah, yes, in Britain, they're changing the name of the National Anthem to "God Save the Galvanic Corrosion".   (Kinda like the Abraham Lincoln quote "God musta loved the common people because he sure made a lot of them".)
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Bruce H; Wallace (near Wilmington) NC
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« Reply #12 on: April 19, 2011, 07:59:18 AM »

Just an interesting side note. I have two 35hp Elgin outboards from 1958 that were positive ground.  Because of that it caused the electrical current to flow into the water away from the engines.  Hence, no sacrificial plates were needed and after over 50 years (I still have the engines) no rust or corrosion.  A lot can be said for positive grounding on a DC powered engine. Good Luck, TomC
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« Reply #13 on: April 19, 2011, 08:37:01 AM »

Just an interesting side note. I have two 35hp Elgin outboards from 1958 that were positive ground. 

  IIRC, Elgin was part of the Scott Atwater Mcullough Chrysler line?? Positive ground eh?

  I recall reading an article once about hooking up a battery or some electrical anti corrosion deal between the boat and motor and running current through to stop corrosion in seawater. This guy did it to his boat but got the wires reversed and in less than a week the aluminum part of the twin outboard motors below the waterline had completly disolved.

  I once had a matching pair of 4 cyl 35 HP Mercuries from the late 30's or 40's. They were white, pink and chrome and counter rotating. I always thought it would have been cool to put them on an old antique wood boat.
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Sean
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« Reply #14 on: April 19, 2011, 08:56:59 AM »

... I also know what plain steel and iron bolts will do in an aluminum casting. ...
  Not saying that chemically they belong together, just saying ive never seen it be  problem like iron is with aluminum.


Iron is also a problem with iron.  That's just plain corrosion, which is a different problem than the one we are discussing, which has to do with dissimilar metals.  Mild steel has problems in salt water even if it is not touching any other kind of metal.  So while you had more problems removing mild steel bolts from aluminum than stainless ones, the stainless ones caused more damage to the aluminum, especially if there was no zinc attached to the aluminum.

Incidentally, stainless steel also corrodes under water under certain conditions.  There is an anaerobic type of corrosion known as "crevice corrosion," which is why you don't want to have stainless parts sit in standing water and one reason why shaft journals are fed flowing water and cutless bearings are slotted.

-Sean
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« Reply #15 on: April 19, 2011, 12:32:42 PM »


I would not put aluminum and steel together in an outdoor application.

We had our steel roof replaced with an aluminum one early on in the conversion process.  We used butyl rubber tape on all the trusses before the skin went down, and butyl-sealed rivets throughout so there is no metal-to-metal contact.


-Sean
http;//OurOdyssey.BlogSpot.com


So Im assuming an indoor application would be okay? Also, what would be the lifespan of those rubber tape fasteners outside?

I have steel bulkheads and need to make bay floors. Right now Im exploring my options for doing the floors. I happen to have tons of al that I pulled out of the inside of the bus and would love to reuse.....
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« Reply #16 on: April 19, 2011, 12:49:29 PM »

If I recall an earlier conversation correctly your RTS is stainless below the belt line.  That is about what I was suggesting as a bad idea - riveting aluminium to stainless as a structure (a floor is a structure) on the bottom of a bus  Grin

I believe Sean used the rubber tape as an insulating layer, probably with some structural capability but the sealed rivets  would be the actual fasteners/loaded fasteners.  Some tractor trailers are put together with ultra strong double sided tape, so using tape to make things can actually work very well.

If I were making bay floors I would probably match the material to what was there previously.  But you can use aluminium if you do it right, the bay floors on my MCI are aluminium.  I never stopped to consider that until right now, to be honest, since the framing is steel on the interior bulkheads and stainless steel on the exterior tube frame structure down there.

Brian
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« Reply #17 on: April 19, 2011, 02:41:08 PM »

Brian, sadly mine was one of the ones made with mild steel at the bottom. Above floor level I have all stainless, but not below  Cry I got rust behind my wheels to prove it too  Cry
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« Reply #18 on: April 20, 2011, 09:46:30 AM »

So Im assuming an indoor application would be okay?


Well, let's say it is less of an issue.  This type of corrosion is facilitated by the dissimilar metals being in an electrolyte solution, and is further exacerbated by anything that "completes the circuit."  Salt water is an excellent electrolyte, which is why you hear about this problem constantly with coastal boats.  However pretty much any water, such as rain water or road spray, will pick up enough minerals to be at least a weak or mild electrolyte, and that's when dissimilar metal contact will begin to corrode.

Quote
Also, what would be the lifespan of those rubber tape fasteners outside?


The tape is not a fastener, and is only sticky on one side.  It is strictly a dielectric (insulator) to separate the dissimilar metals.  The fasteners are the rivets.  In order for the rivets themselves not to be the element that completes the circuit (and themselves subject to galvanic corrosion), a butyl-insulated rivet is used.  The butyl keeps the rivet head away from the skin and also seals the hole so that no water can ingress.

The tape should last as long as the joint is in place.  However if the panels ever need to be separated I would replace the tape with fresh.

-Sean
http://OurOdyssey.BlogSpot.com
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« Reply #19 on: April 20, 2011, 10:02:06 AM »

Thanks!
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