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I got some cheap aluminum windows for an outdoor room project, but I'm not sure what hardware to use to mount them. The windows have a nailing flange built in, but I don't want to use steel screws, or anything else thats going to cause corrosion super quickly. Can I just use aluminum screws to mount the windows to my wood framing? Is there a certain grades of stainless that will not cause corrosion with the aluminum?

Looking for suggestions.

Edit: My main concern is the aluminum (anode) and stainless steel (cathode) effect which from what I understand could severely weaken the window frame since its aluminum.

Thank you!

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  • 1
    For galvanic action to occur, don't you need an electrolyte of some sort?
    – MikeB
    Apr 17, 2023 at 9:40
  • They make aluminum screws. Though in my experience, you have to take care not to ruin the head (aluminum being fairly soft).
    – SteveSh
    Apr 17, 2023 at 14:20
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    @MikeB The electrolyte could be as simple as rainwater dripping down a dirty surface.
    – user4574
    Apr 18, 2023 at 0:39
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    Or even humid conditions. Water vapor in the air will condense on anything whose temperature is at or below the dew point.
    – SteveSh
    Apr 18, 2023 at 0:58
  • 2
    Galvanic corrosion in this case would take decades unless you leave the screws exposed to the elements. Even then it would take years and years unless you failed to use any caulking at all when you mounted the windows. The weakness of aluminum screws is a bigger issue. Use steel screws then caulk and cover the flange to protect it from the elements. (Wood rot is a bigger risk than galvanic corrosion of window frames.)
    – JBH
    Apr 18, 2023 at 3:37

9 Answers 9

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One of the major national manufacturers, suggest the following for aluminum windows:

1 3/4" galvanized roofing nails or #8 x 1 1/4" pan head or washer head (stainless steel recommended) screws. Fasteners must penetrate at least 1" into framing

I prefer screws as they hold better and can be removed without damaging the window.

I checked the installation specified and there was no suggestion for a dielectric break between the fasteners and the window. I would suggest using a house flashing tape product if you are really concerned, but the fasteners should not be exposed to the elements and this is not a significant concern in this particular case.

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  • Thanks for the response. I'm worried about the aluminum (anode) and stainless steel (cathode) effect which would weaken the window very quickly. I guess galvanized prevents this?
    – J. B.
    Apr 16, 2023 at 23:00
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Instructions for the ones I installed did not suggest there was any problem with using standard steel screws. Remember that galvanic reaction is mostly an issue when the metals are immersed in liquid, and most windows' mounting hardware stays dry most of the time.

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  • The implication here is put the fasteners on the inside, not the outside ?
    – Criggie
    Apr 17, 2023 at 0:03
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    In my experience, the fasteners go through the sides of the window frame, through shims, into the studs on either side. Screws next to the upper sash are not exposed to weather when the window is closed; any screws through the lower half might be, but vertical surfaces drain/dry pretty fast and the lower screws are less critical anyway. Unless you expect salt spray, or rainforest/waterfall levels of moisture, I'm just not convinced this is a significant issue. And as I say, the manufacturers don't seem to consider it one; the wood is likely to age faster.
    – keshlam
    Apr 17, 2023 at 1:42
  • "most windows' mounting hardware stays dry 99.7% of the time." You must live in the desert.
    – RonJohn
    Apr 17, 2023 at 14:17
  • Granted, overstating my case.
    – keshlam
    Apr 17, 2023 at 14:25
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Get rust free screws.

SS or zinc screws do not rust.

Galvanized is risky, since it is just iron covered with thin coating

Al screws do not hold (to soft)

If you are worried about galvanic currents, use a plastic washer as insulation between the screw and the Al frame.

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  • I should have been more specific, I'm worried about the aluminum (anode) and stainless steel (cathode) chemical reaction that would eat through my nailing flange if I'm not careful.
    – J. B.
    Apr 16, 2023 at 22:58
  • @J.B. for the galvanic currents see my updated post
    – Traveler
    Apr 16, 2023 at 23:20
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    it is a curiousity why they even make aluminum screws - i have sheared off a lot of aluminum screws, assembling greenhouses...
    – JoSSte
    Apr 17, 2023 at 6:28
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    @JoSSte Because people buy them, and then destroy them and buy some more. It's like the perfect thing to sell to people: basically useless, but you'll end up buying lots of them.
    – Nelson
    Apr 17, 2023 at 7:54
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    @Huesmann I don't think I've ever lived in a building where the window frames had sacrificial anodes.
    – user253751
    Apr 17, 2023 at 14:01
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Without an electrolyte (salt), galvanic corrosion will be minimal. The aluminum windows on my house are attached with zinc screws. They are fine after 35 years. A small amount of rust, but they will likely hold for the life of the window.

If you live near salt water, different story, my sailboat had significant galvanic corrosion on the aluminum where the stainless brackets were attached with stainless screws.

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IF you don't want to use aluminum screws, galvanized is probably the next best option. Zinc and aluminum alloys are fairly close on the galvanic series, so the risk of corrosion should be minimal. Stainless steels are less compatible

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If you look at a metals compatability chart you can find the answer.

enter image description here

https://www.monarchmetal.com/wp-content/uploads/Galvanic-Corrosion_2.png

Looking at the chart your only sure option appears to be screws made of aluminum. Or screws not made of metal (plastic) or coated in plastic.

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Looking at it from another angle - use pop rivets. Even if they're going into a wooden surround, they'll hold, and since they're alu themselves, there's no corrosion problems. That's of course if you don't see them being removed too soon, but why would you?

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  • How would one use pop rivets to secure an aluminum window to a wood frame? Apr 17, 2023 at 16:10
  • @UnhandledExcepSean - drill holes in the wooden outer frame, assuming the alu frame is a fair fit inside, and pop through. Using screws would assume the alu is a snug fit anyhow.
    – Tim
    Apr 17, 2023 at 16:30
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Relative mass matters. There is much more aluminum than stainless steel in your setup. It would be much more problematic to have aluminum screws in stainless steel windows.

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Use normal screws and don't worry about galvanic corrosion. It is over-hyped risk as that is not an applicable risk for your scenario. Rainwater is a bad electrolyte and doesn't facilitate galvanic corrosion as good as your theoretical scenario. Any effort spent on overengineering just to mitigate a nearly non-existent risk could be better used to shield windows from other factors. I'd expect non-galvanic corrosion from bird excrements falling on the windows to be hundreds of times more destructive than having dissimilar metals. If it ain't broken, don't fix it.

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