Flux for Soldering Brass

Discussion in 'Fettling Forum' started by JonD, May 19, 2016.

  1. JonD

    JonD United Kingdom Subscriber

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    There are some good threads running at the moment about solder repairs to brass tanks, reattaching feet, pump tubes &c. See the one started by @Funfundfunfzig

    For soldering to be successful a good flux is vital. I have not found much mention of fluxes around the forum.

    I'm lazy and use electrical solder which has internal cores of "Rosin" flux. Don't know much about it but
    mostly it just works. There are syringes for dispensing a similar flux available. I think I should invest in one.

    Killed Hyrocholoric Acid has also been mentioned by @Jim Ashton, I wonder if this is what used to be used by plumbers etc? (Hydrochloric Acid neutralised by dissolving Zinc)

    I hope the chemists and materials scientists can help on this one please! We could do with a good treatise on the subject of fluxes for soldering. (Silver Soldering / brazing might come later? )
     
  2. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Hi, here are some soldering links:


    https://classiccampstoves.com/threads/soldering-instructions-fluxite-outfit-1930-50s.23229/

    I have used Bakers Fluid for tinning steel legs before attaching them to classic brass stoves, however Bakers fluid is not advised for non-ferrous metals so if you need a flux for using on brass with traditional lead/tin solders, resin fluxes such as Fluxite resin flux is useful.

    http://www.model-engineer.co.uk/forums/postings.asp?th=80659

     
    Last edited: May 19, 2016
  3. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Brazing with silver-based braze metal:

    http://homepage.ntlworld.com/bryan.rozier/silver_soldering/Silver_Soldering.html


    The reason that we recommended silver-based braze for some repairs on classic brass stoves is that it has a lower melting point than the braze used , for example, to make the complex burner in the factory.

    If you use the same braze for repair of the burner as was used in manufacture, it will likely fall apart, as you try to repair a single defect.

    Makers of metal jewellery use braze metals of a range of melting points, the first joints are made with the highest Melting point braze, the next joint with a lower M.Pt. braze and so on. In this way the current braze does not cause failure of earlier joints.

    1. A few more pieces of advice, when working with brass stoves:


      1. If working on a fuel tank, wash the tank out several times with water and detergent so that no flamable vapours remain. Dry thorougly with heat so no water residues remain anwhere near the area being repaired.

      2. Cleanliness is paramount when silver brazing.

      3. Use high silver-content silver brazing metal (such as from the Easyflo family of alloys). If possible obtain the now discouraged Cadmium bearing grades, as cadmium enhances the already excellent narrow gap-seeking property of silver braze metal. Make sure you have the correct specified flux for your chosen silver braze alloy.

      4. I would advise against filing a V-shaped notch in any crack you are trying to repair. These silver brazes like tight gaps and are not gap-fillers

      5. I believe that a narrow flame blowlamp using Propane, or even Butane/Propane mixture should be adequate for most stove repair jobs.


      Best Regards,
      Kerophile.

     
  4. JonD

    JonD United Kingdom Subscriber

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    Thanks Kerophile - already you triggered a memory which would not come back before.

    Bakers Fluid - that was the stuff my father always had around in a small can.
    I wondered if the Killed HCl+Zn amounted to that or is it different? I remember it being very
    thin consistency compared to the resin type fluxes he had around in small round tins.

    I wonder what makes it a bad choice for soldering brass? Maybe it's better for copper or even tin-plate?

    Come to think of it stoves have need to solder tin plate on occasion for wind shields and spirit cans.
    I wonder what fluxes are best suited to that?

    I have heard tallow and paraffin wax can be used for some things - but what? I'm sure it's a wide subject and I am eager to learn more.
    Thanks!
     
  5. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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  6. Jim Ashton

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    I also would like to know what detrimental effects the use of zinc chloride can have on brass!



    Jim.
     
  7. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Hi Jon, spot-on.:

    PS Bakers fluid = killed spirits = hydrochloric acid + zinc.

    I believe that this mixture is intended primarily for fluxing steel where the acid is needed to remove an adherent oxide film and also loose rust. The zinc in the mixture aids reaction between the cleaned steel and lead/tin solder once heat is applied to melt the solder.

    Residual Hydrochlorid acid, and zinc chloride are too aggresive for use with brass and would cause rapid corrosion.

    One of the main ingredient of Fluxite type flux pastes is tree resin or fats, with mixed salts, as I recall, primarily zinc chloride:

    https://www.silmid.com/MetaFiles/Silmid/b7/b734f476-aeec-4f64-895d-536efa2c2390.pdf

    Best Regards,
    Kerophile.
     
    Last edited by a moderator: May 20, 2016
  8. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Hi Jim, immersing brass objects in solutions containing zinc and chlorine ions, and allowing the brass to dry, results in the deposition of zinc chloride on the surface. If the brass is then heated, as it would be in a soldering operation, results in the formation of free chlorine gas which rapidly reacts with the zinc content of the brass causing it to corrode and, in the extreme, crumble:

    http://www2.uni-siegen.de/~pci/versuche/english/v44-1-10.html

    Best Regards,
    Kerophile.
     
  9. Jim Ashton

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    George, if you make it properly, there will be no Hydrochloric acid! I leave an excess of zinc in the HCL for two days, if no more zinc dissolves, there can be no more acid.


    Jim.
     
  10. Jim Ashton

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    I fully agree with your second reply, George, that is why I said you should wash it well after you have finished soldering.
     
  11. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Hi, Jim, you obviously take care to produce fully "killed" acid (This is really archane terminology).
    However, it is not the acid itself which has the potential to destroy brass, it is free chlorine produced when zinc chloride is decomposed by high temperatures during fluxing and soldering operations.

    Of course zinc chloride is also one of the active ingredients of Fluxite-type flux resins. However, they are working in the presence of liquid resins and fats, the temperatures could be slightly lower, and the Cl + brass reaction supressed.
    You may get away with the use of "killed acid" aka "Bakers Fluid" fluxes when soldering brass but if you do get a failure you now know why!
    Caveat Emptor

    Best Regards,
    Kerophile.
     
  12. JonD

    JonD United Kingdom Subscriber

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    Good stuff George - thank you very much! Just what I was after.
    I haven't read all the links yet but I will in time.
     
  13. Go Scout

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    Great, informative thread going on here. So many links to interesting related topics!

    Almost feel like tackling the split in my Primus 10's upstand!

    Thanks for sharing

    Baz
     
  14. Pillepalle3

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    Dear folks,

    I am having issues with soft-soldering, too, and hijack this thread for that reason (because it can be linked to the flux used).

    When trying to soft-solder brass I certainly clean the surfaces, apply soldering flux (grease-like), carefully apply heat with a small torch and put the tin-lead-solder on the brass. But almost always the molten solder remains like a drop/pearl of molten solder that does not spread into the joint, no matter, how much heat I apply. If there is excess heat the brass starts to corrode or melt :evil:, but the solder itself remains a drop.

    What am I doing wrong? Wrong flux? Wrong application of heat? :-k

    Silver-soldering in the meantime works fine for me, but for some joints soft-soldering should be sufficient. Unfortunately, I do not succeed with it ](*,).

    Any help is appreciated.

    Thanks & regards, Philipp
     
  15. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Hi,
    "In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder (e.g.) attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Additionally, flux allows solder to flow easily on the working piece rather than forming beads as it would otherwise."

    Therefore to summarise, flux has two main functions:

    1. To remove and prevent the re-growth of oxide films during the soldering operation.

    2. To alter the surface tension at the interface between the molten solder and the now oxide-free metal parts being joined. Once the surface tension forces have been overcome, the solder will "wet" the surface and flow readily.

    From your description, you have not reached this stage, so either the work is not hot enough, or the wrong flux has been used, failing to remove surface oxide, or unable to overcome surface tension.

    Trust this helps,
    Best Regards,
    Kerophile.
     
  16. Tony Press

    Tony Press Australia Subscriber

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    @kerophile

    That is an elegant explanation.

    Cheers

    Tony
     
  17. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Here are some earlier soft-soldering instructions:

    "1. You must have a good torch, with a narrow flame, that allows you to focus on the area you want to heat.

    2. You need to raise the temperature of the area you want to solder, in a controlled way by applying heat, testing if the solder has melted, and if not applying a little more heat, until you get a result.

    3. Home repairs were often done with Plumbers solder which does not all melt at a fixed temperature, but instead goes through a pasty stage, at which time the solder can be wire-brushed off a surface, or a part detached from the stove.

    4. Adjoining areas can be protected from some of the heat by wiring wet tissues or rags to the areas you want to protect. Wet tissues can also be inserted into pump tubes. You can also apply aluminium foil to feet to shield from direct torch flames.

    5. Make sure that all surfaces to be soldered are well prepared, fluxed, and tinned before attempting soldering attachments.

    6. If you use solder-cored electricians solder for repairs, this has two advantages: It melts at a fixed temperature rather than going through a pasty stage. Secondly it melts and then freezes at a slightly lower temperature than plumbers solder.

    7. Practice on an old stove tank, or with scraps of brass sheet to improve your skills, before attempting soldering work on a prized stove.

    I should emphasis, most of the fluxes we use for stove repairs are poisonous and corrosive. Handle and store them with care, always wear goggles and mask when soldering/brazing. Heat-proof gloves are also advised.

    Finally, please ensure that all the stove parts you have been working with, and working sufaces are fully cleaned with a damp sponge or other absorbent material after they have cooled down

    Good luck with your soldering.
    Best Regards,
    Kerophile.
     
    Last edited: May 20, 2016
  18. JonD

    JonD United Kingdom Subscriber

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    @Pillepalle3 your problem sounds like wrong flux or, as sometimes happens, do you actually have it too hot? If you burn the flux away before solder is applied balling and no adhesion is the usual result.

    Have you tried the electrical solder which contains flux cores? If the brass is properly clean and not overheated that never fails me.
     
  19. Jim Ashton

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    Yes, indeed, George, all these processes require care and, knowing the risks is a factor of whether you consider it is worth risking. I find Killed spirits (archaic but I was brought up to use the term) performs wonderfully, so I use it and recommend it. I just resoldered the riser on my new/ antique Valor stve, I had to remove the surplus with various grades of emery clat, then re-polish, but the result is
     
  20. NealXu

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    Hi....as per my knowledge fluxing steel where the acid is needed to remove an adherent oxide film and also loose rust. The zinc in the mixture aids reaction between the cleaned steel and lead/tin solder once heat is applied to melt the solder.