Copper Silver solder chemical question

I'm assembling some machined copper parts for my rocket project. I'm using silver solder. If I heat the parts hot enough to make the solder flow then
the parts get a scaly dark almost black coating.
On the exterior of the parts I can wire brish this away, but in the interior passages I have some very fine (0.010") passages I don't want to get clogged.
Does anyone know of a chemical method for cleaning this?
Does anyone know how to prevent it from forming?
I'm using a MAPP torch in normal air, I've Contemplated getting a sand blasting box and converting it to an argon purge box, then using an oxy acelelne torch to heat it, as the box has no Oxygen.. I could also maybe put it in a purge box and heat with a TIG torch.
Would the purge box help?
Am I better off just finding a furnace brazing shop?
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Other than removing the scale with some mechanical means such as the sandblast or wire wheel that you mentioned , I don't know , would suspect that there is some acid or chemical that should work , am wondering if you used the proper flux and heat to attempt to silver solder your part , the black that you describe would indicate a lack of silver flux on the part before you attempted to solder , you will need enough heat to bring the copper part up to a slight red color , if you don't have enough heat it will take forever to get the temp. up and you will eventfully lose the effect of the flux. Good Luck

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Phil wrote:

I always heard you should use sulfuric, not hydrochloric (muriatic) for copper-bearing workpieces, to clean soldering flux.
GWE
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On May 19, 6:01 pm, snipped-for-privacy@Rasdoc.com wrote:

Not sure if you really want to hear this, but... for our science lab, we used a vacuum chamber and induction heater to do that kind of thing, if it was critical. Silfos type silver solder is relatively low temp, and is used on copper for refrigeration and the like, but the result usually has copper oxide. Higher temperature silver solders and long heat times just turn copper to oxide.
For some processes, there are 'anti-flux' coatings that don't wick the melted material and which seal the surface; kind of a removable paint. Maybe there's one for copper at brazing temperature?
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wrote:

lab,
relatively
the
The easiest one to come by and most effective that I've found is Tippex typing correction fluid. The chalky deposit takes the heat and the silver solder is effectively stopped by it.
AWEM
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