soldering stainless - silver vs 50/50

I botched up the edge of a stainless steel darkroom sink with a saw. Whoops. The cut into the edge is cosmetic, but really really ugly. I
called the manufacturer of the sink to see how to best patch the cut which is a tad over 1/6" wide and about 5/8" long. It passes though a folded edge of what seems to be 3 layers of 24 guage stainless. I belive it's 304 but am not 100% sure, and forgot to ask.
I was told, yes it can be soldered and to use 50/50 lead/tin solder, any really active fluxes for stainless and do not use a torch as it will oxidize the metal.
Harris makes a liquid flux that seems to be fine for stainless. The issue is it seems people really like silver solder for stainless. Even the old timer at the local welding store seemed baffled by using 50/50 on stainless.
Obviously both can and do work. Any ideas on why one might use a silver solder vs the 50/50 stuff? Harris makes something called Stay-Brite solder which appears to be something close to or the same as silver solder or potable water stuff. Is it the same, or can plain lead free plumbing solder be used on stainless?
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On 7/08/2015 4:48 AM, Cydrome Leader wrote:

You gunna need a torch for silver solder . Best call Harris and see what they recommend to use.
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On Thursday, August 6, 2015 at 3:18:09 PM UTC-4, Cydrome Leader wrote:

Take all my comments wit a grain of salt. I have no experience using 50-50 solder on stainless.
I suspoct 50-50 solder would oxidize with time and not be a good color match.
Real hard silver solder will be harder and a better color match, but a lot more expensive.
I think if I were going to try the repair, I would get some stainless steel and beat it into a shape that would fill in the cut. And then solder that in place, And sand as necessary to blend everything together.
I would use real silver solder , but then I already have some on hand. Second choice would be the stay brite. Notice the name implies that it stays shiny.
I would probably slather on flux and use a propane torch because I do not think I have a soldering iron that is big enough.
Dan
Actually I would make a piece to fill in the cut and then use my arc welder with stainless rod to put everything together. But I have assumed you do not have a welder handy.
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On Thu, 6 Aug 2015 18:07:14 -0700 (PDT), " snipped-for-privacy@krl.org"

I haven't had great luck soldering stainless, although I haven't tried for 20 years. I once had some great flux that one of our advertisers gave me as a sample, and I could do it with that. I have two soldering irons over 300 W and the stainless was thin -- probably 0.020". That was lead/tin solder, which I still have and use. Stainless has lower thermal conductivity than carbon steel and it's actually a lot easier to heat to soldering temperature as a result.
So I don't have nuch help to offer on that score. However, I have a caution: The OP said this is a darkroom sink. If he's processing film the traditional way, he'll be using hypo (fixer), and that attacks silver metal as well as silver halide.
How long it may take, I don't know, but hypo is ammonium thiosulfate. You probably could look up its rate of activity with silver.
'Just a caution.
--
Ed Huntress



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I'd do some research. Maybe lead silver solder would be best. There is nickel and other metals in SS. Depends on the Alloy of the SS.
Martin
On 8/6/2015 8:42 PM, Ed Huntress wrote:

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Never thought about the fixer eating silver from silver bearing solder. The guy that made the sink has been making these for decades and knows what he's doing.
It is true the "factory" solder doesn't match the color of the stainless at all and is dark grey/nearly black. but it has has in fact lasted what I suspect to be decades already, and I've seen a bunch of the sinks made by that company.
There is cracking of the solder in other areas that do not actually get wet, and I was assured that they can be touched up and reflowed if I'm familiar with soldering.
The maker was sort of vague about the iron he uses other than it's copper and gas heated. I wasn't able to find out if it's a torch with a copper head or just a block of copper he heats with a separate torch.
I ordered the harris liquid flux and a roll of 50-50. Will get a fresh stainless brush and report back in a week or so, or when I can dig up a 200 or maybe 300 watt iron.
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On Friday, August 7, 2015 at 10:59:32 AM UTC-7, Cydrome Leader wrote:

So, it's soft-solder. If you apply flux anywhere that gets wet, it might matter how you passivate afterward (even stainless is attacked by some photographic chemicals, if you don't treat the surface - passivate it). Any effective solder flux will damage the existing passivation.
Oxidizing acid (dilute nitric, or maybe citric) is typically used to passivate stainless steel.
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Sounds like a recycle pit is the best bet. Martin
On 8/7/2015 12:59 PM, Cydrome Leader wrote:

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Nope. This thing will get patched up.

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wrote:

Would a botched soldering job interfere with a proper TIG repair?
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Jim Wilkins wrote:

YES - unless you grind out ALL of the lead or whatever you used .
--
Snag



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On Thu, 6 Aug 2015 18:07:14 -0700 (PDT), " snipped-for-privacy@krl.org"

I don't have the details of what was used but a year or so ago we had a stainless gutter installed on one edge of the roof. The sheet metal guys showed up with two big - maybe 5 lb. or heavier - soldering coppers and a small LPG tank with a burner on the top. They heated up the coppers and pro ceded to solder the gutter sections together so it was leak proof.
While I didn't watch them closely it appeared that they were soldering just as they would with galvanized iron. Smeared some paste on it put the copper on it and in a moment ran a bead of solder.
To the best of my knowledge silver solder doesn't melt at temperatures you can achieve with copper so it must have been some sort of soft solder.
--
cheers,

John B.
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There are special fluxes for soft-soldering stainless steel. These fluxes contain phosphoric acid.
There are also a number of standard soft solder (meaning it melts below 800 F) varieties for work on stainless steel.
Plumbing supply houses usually carry these fluxes and solders.
Do not try to hard-solder stainless steel sinks unless they are made of a low-carbon stainless steel alloy that will not be ruined if heated to a red heat.
The term "silver solder" is ambiguous. There are silver-bearing hard and soft solders. The key is the temperature needed to solder.
Joe Gwinn
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