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Aluminum Fasteners

aluminum fastener screw

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#1 honkonbobo

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Posted 10 October 2018 - 12:33 PM

i am looking at replacing all the rusty exterior screws on my 2010 eagle to freshen up the appearance and reseal. 

 

i have read every thread i can find related to this including all the discussions on galvanic corrosion and what screws are recommended from stainless to zinc plated to coated etc.  i could only find one passing mention of using aluminum screws but no discussion around them, confirmation that anyone has really used them or feedback on how they held up.

 

i left messages and emailed FWC more times than i feel is acceptable with no response on that and a number of other issues (they must have been busy providing their "amazing customer support" to everyone else).

 

yesterday i finally got someone from FWC on the phone and asked why there is no talk of aluminum fasteners as that seems like it would be the easy solution to the galvanic corrosion issue.  his response was "not strong enough."

 

my question is in this application (exterior trim pieces, roof perimeter screws) would aluminum not be fine?  intuitively it seems to me that if i am screwing into aluminum, using a fastener made of a material stronger than aluminum adds no additional value beyond cost effectiveness.

 

can someone with knowledge in this field set me straight or confirm?


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#2 ntsqd

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Posted 10 October 2018 - 01:09 PM

One point of concern is install torque. If this is a self-tapping screw going into a new hole then the fastener needs to be much stronger than the material that it is being threaded into in order to not twist off while forming those threads. With a new fastener going into a 'used' hole if the thread pitch and minor diameter of the new screw aren't an exact match to the old screw the install torque will also be high, though perhaps not as high as with a new hole. It also would run the risk of seizing/galling in the new hole and twisting off.

 

In machine screw threads none of this applies, but the only ones of those in our camper are those that I've added. I suspect that this is the case with most-all campers.


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#3 DavidGraves

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Posted 10 October 2018 - 02:36 PM

What he said.

 

You will be using screws....not threaded bolts.

 

Drive a bunch of different screws into any type of metal to get a feel for this.

 

The real tricky part about replacing any screws that have already been driven (all of em on your camper) is that the next inserted screw has to be exactly a wee bit larger or the hole could easily strip out.

 

A stripped screw hole will not draw the two materials together...has no strength.

 

Aluminum screws are the weakest of all.

 

David Graves


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#4 Zunami1

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Posted 10 October 2018 - 03:50 PM

I have used aluminum screws.  Some success but if true holding strength is needed you have to be careful.  There are different quality alloys and the off the shelf at big box stores are likely to be weak.  You'll do fine on finish work but not structural components.  You'll also do fine on light materials or precisely previously drilled holes.  Anything but ideal situations will lead to snapped and stripped screws.  In a manufacturing setting it would be a no go for sure.


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#5 rubberlegs

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Posted 10 October 2018 - 04:15 PM

At Boeing, I only saw an aluminum fastener used once, for a case when we wanted it to break if someone overloaded it. Titanium is used a lot though.
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#6 honkonbobo

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Posted 10 October 2018 - 05:49 PM

One point of concern is install torque...

 

i had not considered this.   thanks for the comments.

 

I have used aluminum screws.  Some success but if true holding strength is needed you have to be careful.  There are different quality alloys and the off the shelf at big box stores are likely to be weak.  You'll do fine on finish work but not structural components.  You'll also do fine on light materials or precisely previously drilled holes.  Anything but ideal situations will lead to snapped and stripped screws.  In a manufacturing setting it would be a no go for sure.

 

i was considering the 7075 screws from a company that specializes in aluminum fasteners (as opposed to the readily available big box ones) and as i said in the OP using them only for the siding/trim and perimeter of the roof.... definitely nothing structural like jack brackets or strut mounts.  anything that doesn't bite with a #8 gets a #10 (as one would do with steel screws).


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#7 smlobx

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Posted 10 October 2018 - 06:19 PM

Just brain storming here but the post from Rubberlegs got me to wonder if you could use rivets?
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#8 ckent323

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Posted 10 October 2018 - 10:19 PM

The following is paraphrased from this site:

 

https://www.albanyco...l-and-aluminum/

 

Steel or stainless steel in contact with Aluminum in the presence of an electrolyte (e.g. water) will lead to an exchange of electrons = galvanic reaction = galvanic corrosion.  The duration that the electrolytic is present makes a big difference. 

 

If the steel/Aluminum interface only sees occasional water there will not be much corrosion.  If there is constant presence of an electrolytic such as in a rainy or foggy environment there will be more corrosion and it will happen faster.  However this requires both materials to be exposed to the electolytic. 

 

If only one of the materials is exposed then there will be no exchange of electrons (galvanic reaction).

 

There are a few steps you can take if you MUST use these materials together.

 

  1. Add an insulator between the two materials so they no longer connect. Without that connection, the transfer of electrons cannot occur. Well Nuts are a commonly used fastener to help separate materials that can suffer from galvanic corrosion.
  2. Use materials with the same potential. Metals with the same corrosion resistance are typically ok to use together.
  3. If you are in a situation where only one of the materials will come into contact with an electrolyte then transfer of electrons will not occur.
  4. If there is a coating on the cathode it can prevent the transfer through increased resistance.
  5. Consider your environment before installing. Choose materials that will work for your environment.
  6. Coat or paint your assembly (completely) so that the electrolyte cannot make contact with the materials
  7. Use neoprene EPDM or bonding washers as a barrier in between the metals.

 

Aluminum fasteners are weaker than steel fasteners of the same size.  

 

It seems to me that using a good EPDM washer under steel screw heads coupled with dipping the fastener threads in a good sealant before installation should minimize any corrosion issues even in a wet environment because the steel/Aluuminum contact interface will not get wetted by the electolyte (water).

 

Craig


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#9 camper rich

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Posted 11 October 2018 - 12:24 AM

Regarding the strength of aluminum, our fiberglass Casita Trailer has all the fittings done with aluminum pop rivets.  First, because screws will expand and break the fiberglass when they are installed, and second, because the pop rivets will break before damage is done to the fiberglass.  I usually replace a few pop rivets after long trips on bumpy roads.  


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#10 oldhotrod

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Posted 12 October 2018 - 04:46 AM

I do not recommend aluminum screws, they will work harden as they are tightened, become brittle and snap off under load. Perhaps use stainless and high quality sealant and be done with it. The screws in my camper are 16 years old and it has not been an issue...
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