Soldering stainless steel

I would like to solder some small pieces of stainless steel together. i have some a liquid Fluxite, and some paste flux as used by plumbers, will either be OK, or will i need some thing more active. John

Hi John, the key to joining stainless is the flux. When heated in air chromium oxides form which are very difficult to remove. I am afraid I wouldn't recommend either of those fluxes. I have to declare an interest as being from CuP Alloys, but we supply Stayclean soldering flux for stainless. Unfortunately this product cannot be sent via Royal mail so is quite expensive to cover the carriage charge.

Hi John,

I have had good success soldering approximately 1mm diameter 316 stainless pins into holes in copper using a drop of phosphoric acid as a flux and normal 60/40 tin/lead multicore soft solder. I got the phosphoric acid at the time from a chemical lab at work, but I see it is available on eBay at 85% as a rust remover. I don't know if there is a worry about the toxicity of fumes using phosphoric acid this way.

Andrew.

Edited By Andrew Entwistle on 16/09/2020 14:24:58

Edited By Andrew Entwistle on 16/09/2020 14:26:27

As AW says, Phosphoric acid - its what we used on the aircraft (away from the plane itself) down at Weybridge. JENOLITE is a "rust killing" product you can buy off the shelf that contains Phosphoric acid. I am sure there are more perfect products, but I usually have some of it on the shelf - Other products are available.

Does "Coke" (drink) still contain Phosphoric acid? Seem to remember it being used to clean Aluminininium castings.

Regards Ian.

Johnson matthey easy flow flux powder (mix with a small amount of water to form a paste, but only mix what you need)

Easy flo silver solder rods

Needs a higher temperature than normal plumber's solder.

I have had success with a flux sold as 'for lead-free solder' - but it helpfully does not explain how it is different. But it certainly works.

Cheers, Tim

There are numerous fluxes available "for lead free solders".

Very few are suitable for stainless steel. Those will remove chromium oxide. It's a chemistry/metallurgy thing!

Easyflo flux doesn't work particularly well unless the chromium content of the steel is low. That is why silver solder suppliers have a bespoke flux. Again it's a chemistry metallurgy thing.

Good luck with getting hold of easyflo silver solder. Its sale was banned years ago. HSE and Trading Standards still have a more than passing interest in its availability. I, like a lot of people, still have some but it's not for sale.

Using it is not illegal. Selling or giving it away is.

If you want more information, then get my book!

Keith

I suppose one could "steal" a rod of easyflo silver solder (Neither given or sold !!) ?