ncscott

So I did my first silver brazing on a rack and am left with black flux residue.
Does the black mean I need more flux next time?
How should I clean it off? According the Harris (the manufacturer) water and acid pickle. I don't want to use industrial acid so would vinegar help?
Any tips would be appreciated.
Thanks,
Scott

Nessism

Black usually means you need more flux or maybe better flame control.
To remove silver brazing flux hot water typically gets it. Submerge the part in the hottest water you can manage and let the part dwell for a while. The black may not come off but the flux will.

unterhausen

I have an ultrasonic cleaner with a heater. Before I got that, I went to the thrift store and bought the biggest salad bowl I could get. And then I would fill it with hot water from the tap. I wonder if a little trisodium phosphate would help the cleaning.

The black is carbon from the tubing. But if you don't burn off the flux, the flux will be like watery icing when you're done. There is almost always some carbon on the areas of tubing that got hot but didn't see any flux.

Now I'm hungry for some cake

Andrew R Stewart 

Agreed that black usually means either no flux at that spot or overheated flux. First is simple to correct, apply flux for more tube length then you think, if you tack and braze later add flux before you complete the joint. The second takes practice, heat control. Andy

dsaul

Hot water will get the remaining flux off. The black will need to be mechanically removed with a file or sandpaper.

unterhausen

I decided to add some acid to my ultrasonic cleaner. "Concrete prep," it's dilute phosphoric acid. I used much less than the normal concentration. I'll report back on the results. Hopefully I can get it neutralized okay. They recommend painting within 48 hours, but that's not going to happen.


I'm pretty sure the recommendation for pickling is because they are addressing industrial users. Back in the steel days, Trek had an elaborate chemical setup which had a very dilute acid to remove flux, just like Harris recommends. It worked great. I'm reasonably sure it was phosphate based.

The electrician fell into that tank and thought he was going to dissolve. Well, his clothes were not going to survive a washing, but he was fine. That's the only time I ever used one of those chemical showers. The tank was heated and had a fairly thin plastic cover which wasn't strong enough to support a person's weight. The electrician thought it would support his weight.

unterhausen

I didn't really like it and now I have to clean out my ultrasonic cleaner

JohnDThompson 

As others have mentioned, hot water should get the flux off. A stiff wire brush will help with any stubborn bits and also often removes black from burned flux. Practice will help with avoiding the black in the future.