Aluminum Fasteners

[honkonbobo]

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?

 

[ntsqd]

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.

 

[DavidGraves]

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

 

[Zunami1]

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.

 

[rubberlegs]

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.

 

[honkonbobo]

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).

 

[smlobx]

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

 

[ckent323]

The following is paraphrased from this site:

https://www.albanycountyfasteners.com/blog/2017/09/15/stainless-steel-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

 

[camper rich]

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.

 

[oldhotrod]

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...

 

[SunMan]

A friend and member here (TT) replaced his with stainless several years ago, zero issues and screws look like new. When my bucket of white tip steel screws runs out I will probably go that route.

 

[ClimberRob]

Use stainless and dip each fastener in a quality sealant, such as 3M or Silka, and you won't have any issues.

 

[ckent323]

<Use stainless and dip each fastener in a quality sealant, such as 3M or Silka, and you won't have any issues.>

As long as the sealant is between the head of the fastener and the Aluminum as well as between the threads and the Aluminum - I concur.

Regards,

Craig

 

[rubberlegs]

Yup, "wet" sealant is the way they would fasten at Boeing. The new carbon wings (787, 777X) have aluminum ribs against the carbon fiber reinforced plastic (CFRP) wing skins. They use sealant between since there is a galvanic battery between those. Occasionally they use "wipe on/wipe off primer" to fill voids with primer. Many of the fasteners have special (i.e., expensive) coatings to help out. Anyway, it sounds like good advice to coat the screws before installing.

 

[ntsqd]

Except that the mechanics of how the screw goes into the joint is different than it is with a rivet. In the case of the screw there is a continuous wiping action on the load-bearing side of both the male and the female thread and between the underside of the screw-head and the top panel for the duration of the screw being twisted into place. That wiping action may or may not be removing all of the sealant/isolation film, but it is sure to remove some of it thus completing the galvanic corrosion circuit. Which means that the assembly's seal against moisture becomes critical to preventing corrosion around the fastener. Once that seal is breached the process will start because there is no back-up.

If the fasteners chosen are, say, galvanized, then the metal in contact with the aluminum is Zinc. If you look up Zinc's position in a galvanic potential table you'll find it right next to aluminum. Which means that the galvanic potential is very, very low. Said differently it makes an extremely weak battery. Add the sealant to that assembly and you have a two stage protection scheme.

 

[ckent323]

The Galvanic process requires an electrolyte on both surfaces. If the sealant prevents the electrolyte from wetting both materials galvanic corrosion can't occur. For that reason I advised making sure there is sealant under the head and on the threads. The sealant between the head of the fastener and the skin of the camper should actually be sufficient to keep the threads dry. The sealant on the threads helps lock the fastener in place and adds a little insurance.

Therefore, I think the issue of the sealant being completely removed from the threads is not worth worrying about - keep in mind only a thin film is actually needed on the surface of the material to keep the electrolyte from causing a problem anyway and it is unlikely that all of the sealant will be wiped off.

All that written the potential difference between Zinc and Aluminum as well as Cadmium and Aluminum is small. One could use Zinc or Cadmium plated fasteners and probably not have an issue except perhaps in salty coastal environments.

However, the same concern raised about e sealant during installation applies to plated fasteners. What happens when the fastener is threaded into the part? One needs to be sure that the plating is robust so it doesn't flake off and there probably will be some plating removed during the threading process.

It seems to me that a belts and suspenders approach would be to use Zinc or Cadmium plated fasteners and a sealant or non-metallic washer under the fastener head and perhaps even putting sealant on the threads as well.

I hope this is helpful.

Craig

 

[ntsqd]

My poorly made point was that the action of screwing a threaded fastener into a hole will wipe at least some of the sealant off and result in some metal to metal contact or exposure within the threads. It won't wipe all of it off, but it doesn't need to in order to set up the start of a problem. Same with the under-head area, though less so. It might seal initially, but with time and even a tiny bit of relative motion the seal could quite easily be broken. The quality, flexibility, and durability of the sealant chosen will be crucial.

In my experience the biggest problem with plated fasteners is the quality of the plating. Which is generally pretty poor on hardware store sourced fasteners. The idea of buying mil-spec, the only way that I know of to get quality plating, seems abhorrent and exceedingly expensive given the application. Not sure what the solution for this is.

 

[Beach]

I have been replacing the screws in my camper with stainless when needed or adding mods,used sealant to prevent water intrusion. After 5 years there is no visible corrosion. It would be wishful thinking to coat the threads on a self tapping screw and think there will be any sealant remaining where the threads bite into the aluminum frame, sealant will be wiped as the screw threads into place..

 

[JaSAn]

I think you guys are trying to solve a problem that doesn't exist. I replaced all the roof screws on my 1977 Grandby a couple of years ago. All the exposed screw heads were rusty, some even starting to deteriorate. None of the screw bodies were rusty. And replacement screws of the same size tightened up nicely. The few screws I pulled on the side windows were in the same condition.

jim

 

[ntsqd]

It may not exist in Minn., but its a big deal here on the Coastal Desert where we frequently have a marine layer condensing on anything metallic. We've discussed this regional variability in the past.