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Thread: Brazing/Soldering Galvanized Sheetmetal - techniques?

  1. #1
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    Default Brazing/Soldering Galvanized Sheetmetal - techniques?

    Hey folks -

    I've got some galvanized steel sheetmetal flashing I'd like to join together. I've done some experimenting with techniques to join together some "test coupons", and I've managed to find some ways that do it better than others, but of course I'm wondering if there is a better way I haven't found yet.

    Wondering if anyone else here has any tips to share on how they would do it? What type of solder or braze would you use, flux, application techniques, etc. Any tips would be appreciated.

    PS - if anyone is interested, I could share what I discover about this as well. Thanks
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  2. #2
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    Silicon Bronze Rod is great for galvanized sheet metal (ERCuSi-A).

    Thurmond
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  3. #3
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    Thurmond, I think bronze alloy braze material may not be what I'm looking for, since you need to get it so hot ("red hot") before it melts. I believe that temperature would burn off (or at least, severely oxidize) the zinc galvanized plating. I was hoping to keep the plating in tact for this particular project, for corrosion resistance.

    I've noticed the galvanized coating actually melts, at somewhere above soldering temperature (of tin/lead solder.) When it melts, you can see it kind of start "shimmering in the wind", and for the most part it stays wetted out on the sheet. However, carelessly adding more heat, can result in it turning to a irreversible, dull grey, burned/oxidized look. This all happens at temperatures lower than "glowing hot".

    I found it was possible to join the galvanized sheet with potmetal (aluminum/zinc mix, 700-800F melting temp) filler rod, but it was difficult to accomplish without risking oxidiation of the zinc plating, at least using the air-propane torch I was using. The real problem may be it was difficult to get the potmetal rod to adhere very well, without mechanical scrubbing while molten. If I had an appropriate flux, perhaps it would work much better.

    I have one other brand/formulation of zinc/aluminum rod I could try, which may have higher % zinc content (and just a small amount of aluminum.) Can't help think it might would very well, if I had the right flux to go with it.
    (Edit: come to think of it, I have some pure zinc sheet, I could trim strips off of and experiment with using that for filler material)

    The flux I have (zinc chloride grease, common "old school" non water soluble plumber's type) says not to use it with aluminum, and indeed it didn't seem to be helpful with the potmetal filler rod. It seemed to just mostly burn, and make a big mess when I tried it.

    The flux on the other hand, seems to work real magic for getting tin/lead solder to wet out, and adhere well. So that's what I'm using. It's amazing how just adding in a little dollop of that flux onto a hot joint, even dirty with some oxidation (dull appearing), immediately followed by melting in some tin/lead solder, is adequate for making the solder wet out nice and shiny looking, and adhere extremely well.

    I ran out of tin/lead solder on my flashing project and the only replacement stuff I could find is lead-free, (apparently contains a wee bit of silver, with advertised mid 400's melting temp) is rather expensive -$15 / 8 oz. It has acid in the middle, AKA "acid core", which seems like it could be an added convenience. Hopefully I've got enough to finish my galvanized flashing project.

    I found a learning curve in getting the solder to flow where I wanted by orienting the workpiece so gravity moves it in your favor. Heat control with the air/propane has also been a learning curve, and a challenge, but with some experience I found it is possible to get one area molten with an adjacent area still solid. I did discover it was possible to 'tack weld' along an edge using small dabs of solder, even. (It was helpful to hold the sheets "pinched together" along a long joint!)

    The grease-based flux cleans up really nicely I found, by first wiping up the bulk excess when it is still warm (anywhere from "soft/gooey" to "runny"), then allowing work to cool, and using a solvent like brake parts cleaner to dissolve the "converted" darkened residue, wipes off with a rag/paper towel, leaving just the bright, shiny solder and zinc-plated parent metals.
    Last edited by jakeru; 08-06-2011 at 05:38 PM.
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  4. #4
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    I guess it depends on whether maximum strength of the joint is important. Personally I would tig braze it then flow a high zinc content solder over it to give corrosion resistance if I was looking for maximum strength, otherwise soldering it would be the preferred method. Since you have not told exactly what it is you are making and the material thickness, type of joint etc. it is hard to determine an exact process to use.

    Thurmond
    Miller Bobcat 3 Phase,
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    I'm building a sheetmetal base for a new chimney cap for my house. Sheetmetal thickness is in the area of .010"-.015". It will sit on top of the brick chimney, and will have a cast in place concrete cap poured on top of it. The completely waterproof sheetmetal base will be another layer of water protection. I am going to put some drip edge flashing pieces around the perimeter (and the cap will overhang over the brick a little bit.) All the sheetmetal joints are lap joints, although there were a few spots where I had to fill some gap, and I also filled some screw holes (where I had some self-piercing sheetmetal screws I used to temporarily hold the sheetmetal pieces together.)

    I haven't had any luck getting my aluminum-zinc brazing rod to stick to bare steel. It just sits on the surface of bare steel, but has no or very weak bond. I wonder if I am just missing a special flux that would allow it to wet out and adhere better? I have a high zinc content, "cold galvanized" spray paint I used before, but unlike a solid metal coating, it's not as durable. (Little impact strength; easy to scratch it off.) That is why I have tried to preserve the zinc plating by soldering. If I knew how to reliably reapply a zinc plating or similar, it would be really good to know.
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  6. #6
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    I think if it was me doing it I would use high temp RTV (the red stuff) between the cap and the flashing.

    Thurmond
    Miller Bobcat 3 Phase,
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    26 series gas cooled TIG torch, setup for quick connect to Bobcat.
    17 series gas cooled Tig Torch for Low Amp Solar Tig (Direct Solar Panel Powered Tig welding)
    Hobart Handler 187 Mig / Fluxcore
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  7. #7

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    Or mig weld it and spray it with cold galvanizing.
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  8. #8
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    That red RTV works great on stuff like that.
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  9. #9

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    The RTV is great stuff for sure.

    The red is higher heat than the black, I use a lot of the black. My guess would be 600+ degrees on the red stuff. It would be quick for sure. And it skins pretty fast.
    Mike R.
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  10. #10
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    Back when I did chimney work for a living, most of the caps were formed, then mechanically (locked seam) or weld (spot welder) assembled. There was always a mastic/sealant involved, none were welded water tight.

    I've done duct type work where seams were spot welded, if memory serves, they were bonded but the galvanizing wasn't destroyed.

    Are you trying to seal it or just fasten it?

    http://www.harborfreight.com/115-vol...:referralID=NA

    Similar to what I've used, though the ones I used were 240 or 440 volt units.
    Trip Bauer
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  11. #11
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    jakeru has a nice tig welder (and soon to have a PowerTig 250EXT if I am not mistaken since he won the last contest) that will do spot welds as will most tig welders in skilled hands even without the spot timer.

    Thurmond
    Miller Bobcat 3 Phase,
    Miller Suitcase X-Treme 12VS wire feeder for the Bobcat with M-25 300A .045" gun / Bernard 400A 5/64" wire mig gun .
    26 series gas cooled TIG torch, setup for quick connect to Bobcat.
    17 series gas cooled Tig Torch for Low Amp Solar Tig (Direct Solar Panel Powered Tig welding)
    Hobart Handler 187 Mig / Fluxcore
    EVERLAST PowerUltra 205P
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  12. #12
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    I posed the spot welder option, as the arc/fusion would be metal to metal, the tig would melt through the outside of one to the inside, melting the galvanizing on the outsides instead of just the insides... no?
    Trip Bauer
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  13. #13
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    I got the project all done guys, it's ready for the rain.

    The sheetmetal is buried under 4" or so of concrete chimney cap slab. It turned out really nice. Dare I say, the best looking chimney cap in the neighborhood.

    I am new to concrete work, this was my first concrete pouring (but I enlisted the help of a skilled old hand at it; my old man.) I'll have to post some pictures of the end product (with the formwork removed and everything) if anyone is interested in seeing. Here are some before the formwork was removed.

    I ended up spraying the cold galvanized over the flashing after all (and it got harder than I thought it would, after a few days baking in the hot sun, which I was happy about. To scratch off with fingernail, had to be pretty aggressive):
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    Here is the flashing installed, with formwork around it, before the concrete pouring:
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    And after the concrete pour:
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    I hope it fixes the pesky water infiltration problem around the chimney I was having last couple winters. Last summer I re-flashed the roof around the chimney, but it leaked again last winter, and there was efflourescence (whisish powder) out of the brick as well, all indicating moisture problems. So this summer I repointed all the exposed brick, and re-did the chimney cap. If that doesn't solve it, there must be some problem with one of my flat roof seams. Part of the joys of owning a "retro modern" 1950's era home I guess.
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  14. #14
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    Nice work! How did you ultimately join the sheet metal? Out of curiosity, why the concrete?
    Trip Bauer
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  15. #15

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    Looks good Jake. It's not gonna blow off the roof for sure.

    Your Dad can form up a nice slab, but I would not be drinking on the roof Was that varnish or something you were putting on the form?
    Mike R.
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    Mike - That's motor oil. Keeps the concrete from adhering to the wood form. It worked like the charm - the wood form practically fell right off.

    Yeah it would take a major hurricane to blow off that chimney cap. hehe.

    Trip59 -
    Regarding why the concrete, it is the material of choice for chimney caps; much more long lasting than a mortar chimney cap. Apparently the lime in the mortar gives it a reduced life expectancy. However, it is the stuff of choice for something to put between the bricks.

    But were you thinking I'd make the chimney cap out of stainless steel or something? Now that would be a metalworker's chimney cap. Actually probably wouldn't have been much more work compared to the way I ended up doing it, with the fully waterproof flashing underneath the cap and all.

    How I joined the flashing: I joined the sheetmetal in the middle around the flat flashing piece to the piece that goes around the flue liner with solder as I have shown in the pictures. The first batch of solder nice ol' good tin/lead without flux core, and the second batch of solder lead-free with flux core. For joining the sheetmetal around the outer edge (there is a drip edge piece along the edges of the cap, I used silicone caulk to join that. Waterproofing is not as important there. Anything that leaks out there is just going to run down along the drip edge anyway.
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  17. #17
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    Always amazes me when I don't realize there are more than two ways to do things I did chimney cleaning and repair in north TX, and have done a couple to help folks out here in FL. No mortar or cement over the cap, all of them have been stainless, copper or galvanized, attached to the chimney by screws or tapcons.
    Trip Bauer
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  18. #18

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    I just I would have cheaped out and went with galvanized sheet. Sprayed the tar out of it and screwed it down. That is how mine is now, and one year, a hurricane got it loose, but left enough for me to work with. Only time I have had to mess with it. New roof a couple years ago, and no snow here.
    Mike R.
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  19. #19
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    I took some pics of the completed chimney cap from below (didn't get up on the roof):
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    The drip edge under the concrete cap turned out really good.
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  20. #20

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    Any time I mess with galvanized I spot weld. Those spot welders that Harbor Freight sells are pretty good for what they are. I hate welding with arc or gas galvanized as it pops and smokes and is generally a PITA. The Spot welder just hits it with one small area and it holds like nothing else. Now I have the Powertig 250EX, I am looking at using it for spot welding too on galvanized.

    I have a friend who is a body man and he often uses galvanized sheet for car body repair. He uses glue to hold it down and in almost 15 years, the cars he has fixed for rust have never rusted again according to him. He also rustproofs the daylights out of the repair areas on the back side which also helps. He says that gluing is way better than welding for body repair as the technology has gotten better and there is no heat distortion to deal with.
    Last edited by Steve; 10-03-2011 at 08:07 PM.
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