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Old 11-29-2022, 10:06 PM   #1
Mini-Skoolie
 
Join Date: Aug 2022
Location: Lost
Posts: 48
Year: 2002
Coachwork: Bluebird
Chassis: All American
Engine: 8.3 Cummins
Aluminum vs Galvanized for Skins?

Sooooo, here's the deal, years ago I bought eight 4x10 sheets of 12 gauge aluminum for a trailer floor I was planning. That project never happened but at the time I paid $1.35 per pound for it. I also bought 12 sheets of 5x12 diamond plate aluminum and 400 feet of 2" aluminum angle, all at $1.35 per pound.



Anyway, 11-12 gauge aluminum seems pretty hard core versus buying 18 gauge galvanized but...I already have it. I don't know if my rivets would work with it but...I already have it. If I go to buy the Paintlock sheets I'm out $1,500 or something like that and, I already have the aluminum sooooo, what would you all do?


The only down sides I see to the aluminum are:


1) Galvanic corrosion
2) Potentially needing new rivets
3) Different expansion and contraction rates
4) Tolerance issues due to the thicker metal

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Old 11-29-2022, 11:03 PM   #2
Bus Crazy
 
Join Date: Oct 2020
Location: Near Flagstaff AZ
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Year: 1974
Coachwork: Crown
Chassis: "Atomic"
Engine: DD 8V71
I cannot help with the expansion question, but I would think whatever rivets you planned to use would be fine regardless of the material. We use 16GA steel on a roof raise and we use 1/4-inch stainless steel closed end rivets. Those, in the proper grip length, would work well for you.

As to the galvanic corrosion, that can only happen when you have two dissimilar metals in contact in the presence of an electrolyte. If you keep the joints sealed and prevent the intrusion of any electrolyte (water) then you won't get any galvanic effect.

Personally, I'd use what you have. I've owned a couple of Gillig buses with steel frames and aluminum skins...and they held up great.
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Old 11-29-2022, 11:45 PM   #3
Mini-Skoolie
 
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Posts: 48
Year: 2002
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Chassis: All American
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Quote:
Originally Posted by rossvtaylor View Post
I cannot help with the expansion question, but I would think whatever rivets you planned to use would be fine regardless of the material. We use 16GA steel on a roof raise and we use 1/4-inch stainless steel closed end rivets. Those, in the proper grip length, would work well for you.

As to the galvanic corrosion, that can only happen when you have two dissimilar metals in contact in the presence of an electrolyte. If you keep the joints sealed and prevent the intrusion of any electrolyte (water) then you won't get any galvanic effect.

Personally, I'd use what you have. I've owned a couple of Gillig buses with steel frames and aluminum skins...and they held up great.

Thank you for the info, I love forums and all the great comments and input like that!



I bought Avdel Interlock Structural Rivets with a 0.80" to 0.375" grip (part # SSPI-08-0) so they should work even when overlapping joins as 11 gauge would be 2x 0.125 plus the factory 0.50" (I think it's 18gauge on the sides and channel for a BBAA) which would put me at 0.350" but I may order some that are a little longer just to be safe for those situations.



I did order 14" channel from Skoolie to do a roof raise. I was super against it just based on time and energy restraints but based on the metal it is going to end up only being $1,000 additional versus just skinning the windows so I'm doing it. Plus, I have three people here to help which honestly makes it a no brainer. I also bought a rivet shaver so hopefully that makes quick work of the rivets as I was worried about those, the inside wasn't hard by any means but 500 rivets is nobodies idea of a fun day and the exterior rivets are even harder.
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Old 11-30-2022, 12:03 AM   #4
Bus Crazy
 
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Chassis: "Atomic"
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If it helps you with the process, I document a roof raise on our youtube channel at youtube.com/rollingliving and you might get some helpful layout and process tips in those. If you run into trouble or questions, give me a call at nine two eight 554-5219.
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Old 11-30-2022, 05:57 AM   #5
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Quote:
Originally Posted by TucsonAZ View Post
Galvanic corrosion
Corrosion of metal requires exposure to water and oxygen. Seam sealer and paint will prevent this exposure, so it shouldn't be a problem using dissimilar metals like this.

Quote:
Originally Posted by TucsonAZ View Post
Different expansion and contraction rates
A 10' long sheet of steel experiencing a 50°F increase in temperature will lengthen by about 1/24" while a same-size sheet of aluminum will lengthen by about 1/12", a difference of less than 1/16".
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Old 11-30-2022, 09:02 AM   #6
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Electrolyte

When bonding our 12vDC &/or 120vAC to the chassis, we have added the required Electrolyte (difference >0.75v).


Shown below are two factory installed Aluminum evaporator supports on an otherwise rust-free bus.

(Beneath a thick layer of paint and sealant. No apparent moisture or air flow, prior to disassembly.)


Options:
Interpose a non-absorbing, inert gasket material or washer between the dissimilar materials prior to connecting them.

Seal all faying edges to preclude the entrance of liquids.

Apply corrosion-inhibiting pastes or compounds under heads of screws or bolts inserted into dissimilar metal surfaces whether or not the fasteners had been previously plated or otherwise treated. In some instances, it may be feasible to apply an organic coating to the faying surfaces prior to assembly.

Where practicable, the external joint should be coated externally with an effective paint system.
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Old 11-30-2022, 08:21 PM   #7
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Dissimilar metals, even different alloys of the same base, will eventually result in galvanic corrosion. This is from experience (and training) in the aviation field. I've seen properly treated and sealed materials that completely disintegration from galvanic corrosion from dissimilar metal fasteners. Most usually this was an aluminum skin with a steel bolt.
However, that was in the aviation field on aircraft that had been in service for 20 plus years and often had been in an extreme salt environment (think aircraft carrier at sea).
I've been informed NOT to use stainless rivets in contact with zinc (galvanized such as on the steel of our buses) but I'm not aware of any particularly bad combinations with aluminum.
So, with a SKOOLIE build, aluminum with zinc plated steel rivets should be okay. Just be sure that mating surfaces are treated with paint. The rivet penetrations being the only potential contact points should be of minimal concern.


Just remember, EVERYONE, that everything your bus was built with was either mined or grown and every single part is trying to revert to it's natural state. The best we can do is slow down the process. And THAT is called "corrosion control".
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