[abdulloh]

What's the best liquid flux for soldering copper onto stainless steel, HCl or   H₃PO₄,  of how many percent  concentration each ?
How's logical explanation on solder sticking process will actually work ?
Some suggest Phosphoric acid better, actually I tried both works, ...many thanks in advance !

[Enthalpy]

Hi abdulloh, welcome here!

Soldering is difficult on metals that have a protective oxide layer, for instance aluminium, stainless steel, nickel-chrome... The liquid added metal doesn't wet the parts.

I suppose that the successful fluxes spoil this oxide layer. Chlorine ions, as in HCL, are known to weaken the protective layer of stainless steel.

But not having tried myself on stainless steel, I can't tell more.

[billnotgatez]

Someone posted before I could finish this, but I am posting it anyway.

What's the best liquid flux for soldering copper onto stainless steel, HCl or   H₃PO₄,  of how many percent  concentration each ?
How's logical explanation on solder sticking process will actually work ?
Some suggest Phosphoric acid better, actually I tried both works, ...many thanks in advance !

This forum is a learning forum.
We often give hints on how to find the answers.

One technique is to use the a search engine on the Internet.

I got some interesting results using GOOGLE using the phrase

flux liquid for soldering copper onto stainless steel

GOOGLE also suggested I might use other phrases

soldering stainless steel to copper wire
copper to stainless steel pipe connection
soft soldering copper to stainless steel
flux for soldering stainless steel to copper

I got some more results using the phrase

H3PO4 soldering flux vs HCl soldering flux

Once you have done some research and you still are unclear as to an answer, you may come back here to discuss your results.

It is good that you have experimented already.

Additionally, your question is fairly general and may need some further information such as you application.
Is it electrical?
Is it plumbing?
Is it to do with a brewing system?
etc.
This will help any member who wishes to respond.

By the way there are other factors that might influence your decision .
For instance
The corrosive effect of the flux after the soldering is long done.

If by chance you have not done so
Please read the forum rules.
This is a forum policy.
Click on the link near the top center of the forum page.
Forum Rules: Read This Before Posting.
http://www.chemicalforums.com/index.php?topic=65859.0

[Borek]

There are also urea based fluxes for steel, I have used one by myself. My bet is that this is a well researched area.

I remember being told HCl is problematic as it promotes later corrosion, no idea if it is true. As I said, I am quite sure this was researched, soldering is an old, well established technique.

[Arkcon]

For just about any application, halides of any type in any form are contraindicated for exposure to steel.  I would like to see any reference to the use of HCl for any steel application.  I suspect you've misread something, or have found a deliberately false source.