soldering stainless steel

[John RutzenParticipant @johnrutzen76569]

Hi,

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.

[John RutzenParticipant @johnrutzen76569]

[David NobleParticipant @davidnoble71990]

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

David

[JAParticipant @ja]

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.

JA

[WearyParticipant @weary]

Phosphoric acid = easily available; even available on ebay.

Regards,

Phil

[Joseph Noci 1Participant @josephnoci1]

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 FarrParticipant @nicholasfarr14254]

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 RutzenParticipant @johnrutzen76569]

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 pgkParticipant @pgkpgk17461]

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 yetParticipant @notdoneityet]

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

[Clive HartlandParticipant @clivehartland94829]

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 RutzenParticipant @johnrutzen76569]

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

[JasonBModerator @jasonb]

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 FarrParticipant @nicholasfarr14254]

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.

[SteviegtrParticipant @steviegtr]

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.

[John RutzenParticipant @johnrutzen76569]

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

[John RutzenParticipant @johnrutzen76569]

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

[Keith HaleParticipant @keithhale68713]

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!

Keith

[StueeeeParticipant @stueeee]

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

[SillyOldDufferModerator @sillyoldduffer]

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 Builder5Participant @speedybuilder5]

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

[JAParticipant @ja]

John

I am sorry have not replied earlier but I have been out all day.

The phosphoric acid was concentrated. By now you have probably realised it is not a very strong acid and is relatively safe. It does corrode stainless steel as British Steel discovered when they tried to introduce stainless steel domestic water piping – they could not successful get clean (flux free) soldered fittings after assembly.

I have no comment on the strength of the joints I produced. I was making small boundary layer total pressure tubes that had a life of less than an hour.

JA

[John RutzenParticipant @johnrutzen76569]

Hi Thank you JA. The general consensus seems to be that soldering stainless is not easy at the best and impossible at worst. If I can get some phosphoric acid cheaply I'll try it but I don't think it's worth pursuing othethwise. I think a major problem with making a large assembly like a tender would be the poor heat conductivity which would lead to distortion. I made a steel guitar once and that was very difficult to solder all the seams – the thing bent every.way. Didn't matter too much, I wanted curves but it wouldn't suit a loco tender.

[SteviegtrParticipant @steviegtr]

Bakers fluid was bought through ebay & delivered. No problem.

[Keith HaleParticipant @keithhale68713]

General consensus?

Soldering stainless steel is no more difficult than soldering copper.

You just need the right materials. Beware the cheap products and bargains available on eBay. If they don't work, they are very expensive and make the job difficult or impossible!

I bet the royal mail don't know that acid is being sent through the post!

Best of luck with the bakers fluid.

Keith

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