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soldering stainless steel

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John Rutzen09/01/2020 08:24:15
160 forum posts
1 photos


I've recently acquired a lot of stainless steel 430 grade offcuts, 1mm thick. I've been trying to soft solder it. I had ideas of making a loco tender from it since the brass price is astronomical. The only flux that worked at all was baker's fluid and not very well at that. I've read that phosphoric acid is the stuff to use but I don't know where to get any. There are products from china on Ebay but it doesn't say what they are.

Has anyone any experience soft soldering this material?. Thanks.

David Noble09/01/2020 08:52:22
136 forum posts
5 photos

S26 is what I used to use. I think might have it.


JA09/01/2020 09:04:25
874 forum posts
49 photos

I spent many happy hours as a student soldering stainless hypodermic tube using softer solder, phosphoric acid and a gas cigarette lighter. I have not idea how easy it is to get phosphoric acid these days.


Weary09/01/2020 09:24:20
299 forum posts

Phosphoric acid = easily available; even available on ebay.



Joseph Noci 109/01/2020 09:56:10
595 forum posts
859 photos

Some liquid drain cleaners have a high phosphoric acid content - Might be available at your local hardware. Must admit I have never had success soft soldering SS of any grade. I have silver soldered a lot of SS with ease - made many an electric Slot Car chassis from SS Sheet and 'Piano-wire' - with moving pans so the car body could flex around the suspension, etc...

Nicholas Farr09/01/2020 10:21:54
2113 forum posts
1025 photos

Hi John, you need an etching flux to soft solder most stainless steels. I used to have to solder two pieces of stainless steel woven screen cloth about 1800mm long for a machine many years ago and had to use an acid flux to did it. Stunk like blazers and dissolved the soldering iron tip over a few years of use, although it was 45mm wide and 18mm thick.

Regards Nick.

Edited By Nicholas Farr on 09/01/2020 10:23:25

John Rutzen09/01/2020 10:22:05
160 forum posts
1 photos

Thanks for replies. JA have you any idea what concentration it was you used. I can get 81% on eBay easily. Tried the local builders merchant but they don't have it.

pgk pgk09/01/2020 10:26:56
1628 forum posts
286 photos

Just to add that phosphoric acid is available as a gel. Whether that woudl be suitable I don't know but allows easy controlled application. Halfords do a gel de-ruster which may be the same. Certainly available from a search for phosphoric acid gel and aslo for very small areas dental etching of enamel prior to fillings is done with the stuff applied through a blunt hypodermic type needle tube.

not done it yet09/01/2020 11:38:28
4149 forum posts
15 photos

I think I might use ‘alumite’ rods or perhaps mig/tig the parts.

Clive Hartland09/01/2020 13:16:17
2514 forum posts
40 photos

I have found that soft soldering on thin s/steel makes weak joints. Having to go back over it later. Etching flux essential, take care it will burn you!

John Rutzen09/01/2020 13:17:39
160 forum posts
1 photos

Thanks for more replies. Nick do you know the name of the etching flux you used?

JasonB09/01/2020 13:19:30
17276 forum posts
1859 photos
1 articles

Or just buy flux made for the job such as this one that CuP do by Harris or Carrs Brown

Edited By JasonB on 09/01/2020 13:25:15

Nicholas Farr09/01/2020 14:43:18
2113 forum posts
1025 photos

Hi John, I can't remember the actual product name, but it was supplied by the Eutectic company who now go under the name Castolin Eutectic Ltd. I believe. It was a white runny sort of paste, which you would apply by brush and it was pretty aggressive and like Clive has said, it will burn on your skin, may not be allowed to market it these days, as it was 30 or more years ago since I last used any. But as Jason has pointed out in his link, CuP are likely to have something.

Regards Nick.

Steviegtr09/01/2020 15:08:13
784 forum posts
168 photos

Is this what you are looking for. Bought this on e-bay a few weeks ago to repair a antique coal scuttle. It was made of spun steel though. Bakers fluid

John Rutzen09/01/2020 15:27:35
160 forum posts
1 photos

Thanks That's Bakers fluid which I have. It does work to a certain extent but it isn't certain.

John Rutzen09/01/2020 15:31:49
160 forum posts
1 photos

There's a problem getting these aggressive fluxes because they won't send them to Northern Ireland!

CuP Alloys 109/01/2020 15:37:17
226 forum posts

Bakers no. 3 is a diluted"general purpose" flux for soft soldering copper/brass/mild steel.

Not for stainless steel.

More info available via M Mouse on eBay or any related supplier!

Alternatively get the right stuff first time and all the information you may need from CuP. It will be cheaper at the end of the day.

The bakers fluid cannot be sent by post (ask royal mail)so collect it from CuP at ally pally.

Not going to the exhibition, CuP can send the flux as a cored wire. The solder has a low melting point and an excellent colour match to the steel.

Again check this out with Mr Mouse.

CuP will ensure that you are successful. Declaration of interest - I used to own the company!


Stueeee09/01/2020 16:57:20
44 forum posts

I used a flux called A8 which I bought from Solder Connection on their advice after struggling to solder stainless with the usual acid based fluxes. Using this flux made the job akin to soldering brass. Do the work in a very well ventilated area though as the fumes are really horrible.

Edited By Stueeee on 09/01/2020 16:57:48

SillyOldDuffer09/01/2020 18:06:01
5323 forum posts
1089 photos
Posted by John Rutzen on 09/01/2020 15:31:49:

There's a problem getting these aggressive fluxes because they won't send them to Northern Ireland!

That's down to the Carrier - not all of them are insured against spillage, especially when the package might go by air. Somebody like DHL is more likely to take it than Royal Mail.

Fussing about with carriers might be too much bother for suppliers as well - doesn't take much extra admin to eat all the profit.

Speedy Builder509/01/2020 18:28:21
1925 forum posts
132 photos

Jenolite rust remover - its phosphoric acid. (well it used to be).

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