Dissolving Braze Alloy with Acid

Dissolving braze alloys with acid is very dangerous and must be done with extreme caution.

Note: I have a lot of chemistry in both high school and college.  I have a chemistry lab at work with a dedicated fume hood. I have even been granted a couple of processes for chemical patents. Having said all this, I would much rather grind braze alloy off a part with a Dremel tool than use acid.

I find that a Dremel tool, with the little cutting disks on it, does a pretty good job of cutting off or grinding off braze alloy. 

However the question was about using acid to remove braze alloy.

WARNING: braze alloy is designed to be highly corrosion resistant.   To dissolve it you need very strong acids.

  • The acids required to dissolve braze alloy will burn you readily.
  • These acids will generate fumes as you pour them.
  • You must pour the acids very, very slowly and carefully into the water. If you pour the acid in too fast it will bump or flash boil and spray acid into your face.
  • When you pour the acid mixture over the braze alloy you will generate very strong fumes that will eat your lungs. If you get a whiff of them they will start eating the nice, soft skin inside your nose and mouth. You’ll be able to feel them eating at you for an hour or so

 

(I know everybody warned you about everything just to cover their rear ends and most of it is nonsense but this stuff really is dangerous.)

These acids are more likely to dissolve steel than braze alloy. I’ve tried using acid to clean up saw blades and other tools before I analyze them. Typically the steel dissolves first.

All my experiences been with the high silver braze alloys somewhere around the 40 to 60% range.

If I have to dissolve braze alloys with acid I use a 50% nitric acid and water mixture.  Be very, very, very careful how you mix this.  Very slowly and gently pour the concentrated nitric acid into the water. It is often suggested that you put a glass rod into the water and gently pour the acid down the rod. The rod will also help keep the water from boiling up.

When you pour the 50% nitric acid mixture over the parts it will create fumes.  Do this someplace with excellent ventilation.  Ideally you will have a dedicated fume hood as I do in the lab.  You can do this outside but you really have to watch out for breezes and shifts in the wind.

When you get done dissolving the braze alloy you will have some really nasty liquid waste and you will have to figure out how to dispose of that. You definitely do not want pour it down into a septic tank. And you really better check with your sewer utility department before you pour it down the drain.

 

There is another mixture called Aqua Regia.  Aqua Regia is Latin for Royal water and it has this name because it dissolves the royal metals which are gold and silver.

To make Aqua Regia you mix three parts concentrated nitric acid with one part concentrated hydrochloric acid. Sometimes people will try and sell you muriatic acid telling you it is the same thing as H hydrochloric acid.  This may or may not be true.

If you’re going to use Aqua Regia you must use a fresh mixture. 

The chemical process by which metals are dissolved involves the acid reacting with the metals and forming a salt. This salt forms a layer over the metals and prevents the acid from getting to the underlying metal.

You are only going to be able to dissolve so much metal before you have to take the parts out and remove the salt layer by sandblasting or something similar.

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