Removing oxides formed during fillet brazing

As a fillet brazing frame builder, I often find myself having to clean up oxidized metal after the brazing process. I’m not sure how much of this is oxide formation is due to technique (e.g. heat control), the fact that I only use paste flux (no gas fluxer), or perhaps it’s just a normal part of the process and I can relax. Here’s what I’m talking about: the black areas around the braze. (This is not flux, by the way - the flux has been completely soaked off at this point.)

I’ve been doing this for ten years now and most of my brazed joints come out a lot cleaner than this, but it’s a good visual example of what I’m talking about. It’s difficult and time consuming to mechanically clean this hard oxide layer off down to bare metal. I’d be curious to hear any of your thoughts on this occurrence and how you deal with it.

HOWEVER, I seem to have stumbled upon something very interesting recently and I wonder if anyone here can explain or corroborate my experience. I was soaking my most recent frame in hot water, maybe 120F or so (to be clear, somewhat acidic water due to the dissolved flux content) and I started brushing at the oxidized spots with a brass brush (warm frame in warm water). The oxide layer came off easily after only a few strokes with the brass brush! I had a stainless bristle brush nearby, so I grabbed it thinking: harder bristles - this will be even faster, but no… the brass bristle took the spots off while the stainless brush did not. This got me thinking: is the black oxide layer cupric oxide? Is it reacting with the copper bristles in a chemical way? What is even happening here?

I am thinking of setting up a few experiments to compare different acids and different abrasives in a more controlled way to see what I learn. It kind of blew my mind that a brass brush would make quick work of a spot that would take much more time with a strip of 80-grit aluminum oxide shop roll.


I find these black spots when I’ve either gone too hot or didn’t use enough flux. Mostly when I come at the edges of the headset or BB shell or when I take too much time or go back to fix a fillet. But I do find that at these black spots the fillet has more chance of bubbles, so I try to avoid it as much as possible.
You can see here that at the bottom of the headtube that I went to hot.

I don’t know how much flux you use, but I tend to overdo it as I am not using gasflux. This is how much flux I have left before soaking:


Very interesting… I have no idea, but it seems plausible. I wonder if someone else can repeat this result.


Michiel, thanks for sharing your experience. It’s likely that in some cases I’m getting the joint too hot. Regarding flux, I have perhaps been watering my flux down too much. I tend to use a thin coat of flux for bronze brazing, but maybe I am going too thin so I’m not getting enough protection from the flux. I will try using more flux and being hyper aware of overheating.

I allways felt like that black stuff is burnt flux rather than oxidized steel. It only appears when you get a bit to hot. Anyways water doesn,t like to clean it off nicely, but what I discovered, is that the trick with adding citric acid to your soak tank, not only makes removing flux a breeze but it allso disolves these black spots, leaving you with less work. Light sandblasting would be a good fit allso, I think.


I am most certainly not a brazing expert.

The aftermath of my joins look way different to those. I get almost none of that thin black stuff around the brazed fillet.

Could it be your fuel gas? Acetylene right? It burns sooty as if the mix ain’t spot on, and even then soot gets all over the workshop. Maybe some sort of carbon spot caked on.

Maybe the flame mix is too fuel heavy? Too much heat? but it could very well be perfectly fine.

I use LPG-oxygen.


Thanks. I’ve read somewhere else about citric acid - will have to try that. How much do you add per volume of water?

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Yes, I use an oxygen/acetylene setup. I’m always careful to adjust the torch to a neutral flame before fillet brazing with bronze and a carburizing flame for brazing bosses etc. with silver. I’ve never tried another fuel gas, but your results are interesting.


I am interested to see where this goes. If there’s a better way, we’ll all use it at some point.

I have had the best luck with a little water on the sandpaper.
Could the difference in how well each brush worked, be from water left on the surface from the soak?

Even though the question is how to get the black stuff off, it may be worth mentioning that like the others I have found that a thicker layer of flux helps me prevent this.
I used to use aerosol carburetor cleaner to clean before flux but I found that it leaves enough residue to chase off the flux and then I’ll get the same type of black crud in the parts with less/thinner flux. Carburetor cleaner is popular with some welding crowds so I figure others may use it…


You’ll unfortunately have to figure this out yourself as I’ve tried it only once and probably used way to much acid, having plans to go down with it on next tries.