how to improve flux wetting?

Discussion in 'Techniques' started by michl080, 11 June 2020.

  1. michl080

    michl080 Western Thunderer

    I am using 20% phosphoric acid as flux and love it after testing many other agents.
    However, I am not happy with the wetting capability. I tends to form local drops on brass, even if the brass has been degreased with acetone before. Technically, it has a high contact angle on brass.
    Is any of my fellow thunderers using a wetting agent with phosphoric acid to improve wetting?

    I have been testing ox bile, but the result looks ghastly. :eek:

    As some may know, I am not UK based, so brand names won't help much.

    thanks for your input.

  2. A few drops of isopropanol (ipa - but not the beer!) usually does it. Failing that try a few drops of screenwash or even washing-up liquid. You're basically trying to reduce the surface tension of the droplets.
    Tim Dubya, michl080 and simond like this.
  3. simond

    simond Western Thunderer

    worth looking at citric acid too, if you have not already tried it. I use it in saturated solution with water, and ipa added, approx 30% by volume. Im told that this might be too much ipa, but it does wet the brass.

    Tim Dubya, Deano747 and michl080 like this.
  4. michl080

    michl080 Western Thunderer


    I will try and report back.

  5. neaston

    neaston Active Member

    I think you can reduce the concentration by half. I've been using it at 10% for about fifty years with no problems at all.
    Never had a problem with wetting I just apply it along a short length of the joint with a bit of wire.
    simond likes this.
  6. SimonT

    SimonT Western Thunderer

    that is useful information as I have followed SimonD's path. I was amazed to find that citric acid flux worked to solder steel rail, so no more of the nasty Carr's Green. The only problem is that it leaves a sticky residue that needs to be washed off to do any cleaning up. Warm water a washes it off but a thinner solution might reduce the need for the trip to the sink.
    simond likes this.
  7. simond

    simond Western Thunderer



    Which concentration? Citric or alcohol?

    Sounds like a martini! Shaken or stirred, sir?

    P A D likes this.
  8. Deano747

    Deano747 Western Thunderer

    I have recently purchased some 45% phosphoric acid and by mixing 10ml acid to 25ml distilled water gives me 15% (14.7% to be precise) and I added 5% viakal as a wetting agent which may help prevent corrosion. Early days yet but seems to work ok so far. Will report back in due course.

    Regards, Deano.