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It is impossible to solder aluminum with only tin, tin does not "wet" aluminum surface because of the instantaneous reaction aluminum/oxygen on the surface of the base metal (aluminum). I suppose the problem is that the tin will be in contact with aluminum oxide and not with the metal.

It is known (in this industry field) that an alloy of Tin/Zinc can solder aluminum metal, this alloy will "stick" to the aluminum surface, so what about the role (interaction/reaction)of Zinc in the "wetting" of the passivated aluminum ?

EDIT: aluminum can be soldered by tin/zinc alloys https://www.belmontmetals.com/popular-uses-for-tin-zinc-solders/

here a specific product https://www.belmontmetals.com/product/60-aluminum-solder-tinzinc/

I can say from my experience that you can solder aluminum even with a homemade alloy of tin/zinc 70/30 or 80/20 using a normal gas torch in free air.

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    $\begingroup$ You should be more careful with capitalisation and spelling. BTW "It is known" was OK for Dothraki in Game of Thrones, but we like sources here ;) $\endgroup$
    – Mithoron
    Commented Oct 24, 2021 at 18:37
  • $\begingroup$ How do you know that an alloy zinc/tin, aluminum metal can be soldered ? Have you got a reference ? I thought aluminum can be soldered under argon and only this way. $\endgroup$
    – Maurice
    Commented Oct 24, 2021 at 18:39
  • $\begingroup$ ok I edited the question, hope that now it is more clear. I insert sources about factory made products (tin/zinc) that are used to solder aluminum in open air. $\endgroup$
    – gino
    Commented Oct 24, 2021 at 20:30
  • $\begingroup$ @Maurice, you use the right term "miracle", I want add that even the flux does nothing if you don't add zinc, i don't have found nothing on the net, but I suppose that Zinc can "capture" the oxide leaving the Tin a free aluminium surface to wet, in absence of air (because under the melted solder drop). If you perform a initial wetting with Tin/Zinc you can than solder with Tin only on that area. $\endgroup$
    – gino
    Commented Oct 24, 2021 at 20:56
  • $\begingroup$ Thank you for the added information.I don't have the slightest idea about how to explain this "miracle", which implies that the protecting alumina layer is removed. There may be some similarity with the action of mercury. If you dip an aluminum piece in a $\ce{HgCl2}$ solution, the following reaction occurs $$\ce{2 Al + 3 Hg^{2+} -> 2 Al^{3+} + 3 Hg}$$. The metallic mercury atoms make an alloy with the aluminum surface. And this alloy is no more protected by the alumina layer, So it reacts quickly with water producing $$\ce{2Al + 3 H2O -> Al2O3 + 3 H2}$$ $\endgroup$
    – Maurice
    Commented Oct 24, 2021 at 21:00

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I think the alloy is used to give a melting range ( 200 to 300 C) rather than a melting point ( 231 C) for pure tin. The zinc might reduce oxygen in the melt but I doubt it can breakdown aluminum oxide to clean the surface.

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  • $\begingroup$ Just speculatively, zinc being in the same group as mercury, could melted zinc have similar destroying effect to aluminium oxide as mercury? $\endgroup$
    – Poutnik
    Commented Mar 24, 2022 at 8:15
  • $\begingroup$ Interesting because zinc can cause liquid metal embrittlement of steels and stainless steels. $\endgroup$
    – blacksmith37
    Commented Mar 25, 2022 at 14:35

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