Solder flux recipe

[dpgibbons]

I read that the acid fluxes sold by Carrs and others for brass/nickel silver and for whitemetal soldering are no more than phosphoric acid of various concentrations, plus a surfactant. If that is so, does anyone have a recipe?

[Carnforth]

I just use diluted phosphoric acid without any other additives. A litre of 80% from the 'net, dilute to 10% for brass, nickel silver or white metal. enough to last a lifetime. works fine. HTH

Steve

 

[Izzy]

I have only ever used phosphoric acid -  a 500ml glass bottle bought from Boots back in the day they were a proper chemist - reduced/decanted down to about 20%. I believe some reduce it down to up to 10%. Anywhere between seems usable,  but of course the bigger reduction the less effective it will be. I still haven’t used up that from Boots, 2.5ltr goes a fair way.....

 

Izzy

[michl080]

Same here,

 

20% works perfect, but the fumes are pretty bad. 10% might be better in this aspect. I have been experimenting with surfactants, but couldn't find anything that improves wetting significantly.

 

Michael

[Gordon A]

I have reduced some phosphoric acid to about 12.5% using distilled water for use with brass and nickel silver.

 

Gordon A

[dpgibbons]

Thanks. I'll start at 20% strength for brass etc and 10% for whitemetal. I've just bought 1L of 95% phosphoric acid for about GBP15 so that will be enough for a lifetime supply of flux with plenty left over for cleaning rust off my RSU baseplate.

[JeffP]

The best I've ever had has a small amount of blue in it, which settles out over time.

Copper phosphate?

[dpgibbons]

Sounds right  - copper phosphate is a corrosion inhibitor

[jonhall]

On 23/05/2020 at 10:53, dpgibbons said:

Thanks. I'll start at 20% strength for brass etc and 10% for whitemetal. I've just bought 1L of 95% phosphoric acid for about GBP15 so that will be enough for a lifetime supply of flux with plenty left over for cleaning rust off my RSU baseplate.

I'd suggest a 12-15% range is about right - 95% phosphoric acid doesn't sound quite right - most lab grade 'conc' is about 85%, and remember the safety rules, gloves (without pinholes), glasses, keep out of reach of childern, and do the dilution in a well ventilated space (like the bottom of the garden) and add acid to water - conc phosphoric acid isn't very nice stuff. Please also label all the bottles correctly and don't re-use food or drink bottles for it.

 

Jon