Brazing/Soldering Galvanized Sheetmetal - techniques?

[jakeru]

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

[Tritium]

Silicon Bronze Rod is great for galvanized sheet metal (ERCuSi-A).

Thurmond

[jakeru]

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.

[Tritium]

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

[jakeru]

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.

[Tritium]

I think if it was me doing it I would use high temp RTV (the red stuff) between the cap and the flashing.

Thurmond

[Ray]

That red RTV works great on stuff like that.

[everlastsupport]

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.

[Trip59]

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.

[Tritium]

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

[Trip59]

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?

[Steve]

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.

[everlastsupport]

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.

We just added a few galvanized shelf bottoms in a metal box frame for a fellow (we have an NDA on the box and it is pretty smart what he is going to make $$$). It is a prototype.

We used the IMIG-200, drop volts down to 16 range (1/8" to 20 ga.) and wire speed really slow. It works quite well. We grind what we can to avoid the white smoke of course. Hit the heavy metal and melt into the sheet.

A spot welder would have been nice, but all can be done with a MIG or TIG. I doubt we (Everlast) will ever carry a sport welder and I never went for the HF one as it flexes like their vise grips, I played with the one on display. Sure it would have worked well probably. Vise grips and drill bits are not there strong point for sure.

There are a number of guys that now use the glues. I have seen it done and was going, heck I can do that.

Once you take anything down to the metal (steel) prep the heck out of it so the customer or next owner does not hate you.