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Thread: Connecting dissimilar metals

  1. #1
    Mike Switzer's Avatar
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    Connecting dissimilar metals

    I need to attach a forged aluminum part (landing gear mounting bracket) to a bulkhead made of 4130 steel, probably with bolts, but I may decide to rivet it.

    What are the standard practices used in the aviation industry to prevent corrosion in this situation? Any type of non conductive material put between the 2 parts would be subject to some fatigue, and there will still be conductivity at the bolt/rivet holes.

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    If you look under the inspection covers in the floor of a Cessna, you will see steel landing gear bolted to what look like aluminum forgings that transfer the loads into the fuselage structure. All I see are AN bolts and painted spring steel.

    I know that stainless steel and aluminum create dissimilar metals corrosion, but based on my observations of the construction of Piper/Cessna/Mooney aircraft, I do not see lots of concern about more common steels being in contact with aluminum, held together with cad plated bolts.

    A good 2-part (epoxy) primer on the forging will probably be a good thing.

    I will suggest that bolting is preferable to riveting so that if you ever see a hint of corrosion you can jack the airplane and take it apart to check.

    Best of luck,

    Wes
    N78PS

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    RV's have quite a few aluminum to steel joints. A good dose of primer seems to prevent problems - I haven't seen any corrosion in those areas on 11+ years. Like the previous poster said, I'd bolt, rather than rivet, just because landing gear get bent occasionally and it is nice to remove them with a few bolts rather than tens of rivets.

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    Mike Switzer's Avatar
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    I was considering some sort pf primer - I wonder if POR-15 makes something that sticks as well to aluminum as it does to steel? Of course if I spray the fuselage frame with POR-15 primer & Chassis Black I probably don't have to worry about anything else, that stuff is pretty much indestructible.

    BTW, I am using Cessna gear, so Cessna aluminum forgings. I have had some bad experiences with corrosion in race & street cars where aluminum & steel came together so I wanted to be sure I was thinking this thru properly. And I do think bolting is best.

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    At the airlines we used A 1/2 but even B sealant will work. http://www.aircraftspruce.com/menus/...ksealants.html .. I just spent the afternoon removing super glue some einstein used to secure a bagage door seal on a C-182. I should have been a half hour job.. I find amatures use alot of stuff from home depot that was not clearly made for use on aircraft.. I even found liquid nailes to hold the carpet in a PA-140. That was a two day job removing that with a plastic scraper... I even found self expanding house insulation. Do yourself a favor and stick to materials in the aircraft spruce catalog.
    Last edited by RV8505; 05-31-2012 at 01:19 AM.

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    I will lobby for the use of white primer in enclosed spaces and on parts that will need inspection. Black blends in the dirt and hides cracks, corrosion, and other issues.

    Best of luck,

    Wes
    N78PS

  7. #7
    Mike Switzer's Avatar
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    Quote Originally Posted by WLIU View Post
    I will lobby for the use of white primer in enclosed spaces and on parts that will need inspection. Black blends in the dirt and hides cracks, corrosion, and other issues.
    That is a good thought. I wonder why all our motor mounts are black? A lighter color would be easier to inspect.

    The POR-15 primer is silver & they did used to have a white topcoat.

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    Mike Switzer's Avatar
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    Quote Originally Posted by RV8505 View Post
    At the airlines we used A 1/2 but even B sealant will work. http://www.aircraftspruce.com/menus/...ksealants.html ..
    Everything on that page appears to be for sealing fuel tanks???

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    I've installed trailer hitches made of steel that are powder coated that mate with aluminum structure on the towing vehicle, the hitch manufacturer provided what they called "galvanic isolators" to go in between the dissimilar mating parts. The isolators were a clear material with adhesive on one side, like packing tape but roughly 2-3 times thicker.

  10. #10
    Mike Switzer's Avatar
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    Quote Originally Posted by DaveH View Post
    The isolators were a clear material with adhesive on one side, like packing tape but roughly 2-3 times thicker.
    We used to use something like that in industrial applications ~20 years ago. It worked fairly well, at least in the short term. I didn't usually stick around in a job long enough to know about the long term.

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